TECHNICAL REPORT
R-60
First Version: July, 2020
Last Revision: Mar, 2021

1

On Pearl’s Hierarchy andthe Foundations of Causal
Inference
Elias Bareinboim†
Juan D. Correa†

EB@CS.COLUMBIA.EDU
JDCORREA@CS.COLUMBIA.EDU† Columbia University

New York, NY 10027, USA

Duligur Ibeling‡
Thomas Icard‡

DULIGUR@STANFORD.EDU
ICARD@STANFORD.EDU‡ Stanford University

Stanford, CA 94305, USA
Abstract. Cause and effect relationships play a central role in how we perceive and makesense of the world around us, how we act upon it, and ultimately, how we understand ourselves. Almost two decades ago, computer scientist Judea Pearl made a breakthrough inunderstanding causality by discovering and systematically studying the “Ladder of Causation” [Pearl and Mackenzie 2018], a framework that highlights the distinct roles of seeing,
doing, and imagining. In honor of this landmark discovery, we name this the Pearl Causal
Hierarchy (PCH). In this chapter, we develop a novel and comprehensive treatment of the
PCH through two complementary lenses, one logical-probabilistic and another inferentialgraphical. Following Pearl’s own presentation of the hierarchy, we begin by showing how thePCH organically emerges from a well-specified collection of causal mechanisms (a structuralcausal model, or SCM). We then turn to the logical lens. Our first result, the Causal Hierarchy
Theorem (CHT), demonstrates that the three layers of the hierarchy almost always separate
in a measure-theoretic sense. Roughly speaking, the CHT says that data at one layer virtually always underdetermines information at higher layers. Since in most practical settings
the scientist does not have access to the precise form of the underlying causal mechanisms

1

2

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference– only to data generated by them with respect to some of PCH’s layers – this motivates us
to study inferences within the PCH through the graphical lens. Specifically, we explore a setof methods known as causal inference that enable inferences bridging PCH’s layers given a
partial specification of the SCM. For instance, one may want to infer what would happen had
an intervention been performed in the environment (second-layer statement) when only passive observations (first-layer data) are available. We introduce a family of graphical models
that allows the scientist to represent such a partial specification of the SCM in a cognitively
meaningful and parsimonious way. Finally, we investigate an inferential system known as docalculus, showing how it can be sufficient, and in many cases necessary, to allow inferences
across PCH’s layers. We believe that connecting with the essential dimensions of human experience as delineated by the PCH is a critical step towards creating the next generation of AI
systems that will be safe, robust, human-compatible, and aligned with the social good.

1.1

Introduction
Causal information is deemed highly valuable and desirable along many dimensions of thehuman endeavor, including in science, engineering, business, and law. The ability to learn,process, and leverage causal information is arguably a distinctive feature of homo sapiens
when compared to other species, perhaps one of the hallmarks of human intelligence [Pennand Povinelli 2007]. Pearl argued for the centrality of causal reasoning eloquently in his mostrecent book, for instance [Pearl and Mackenzie 2018, p. 1]: “Some tens of thousands of years
ago, humans began to realize that certain things cause other things and that tinkering withthe former can change the latter... From this discovery came organized societies, then towns
and cities, and eventually the science and technology-based civilization we enjoy today. All
because we asked a simple question: Why?”At an intuitive level, the capacity for processing causal information is central to humancognition from early development, playing a critical role in higher-level cognition, allowing
us to plan a course of action, to assign credit, to determine blame and responsibility, and to
generalize across changing conditions. More personally, it allows us to understand ourselves,to interact with others, and to make sense of the world around us. Among the first tasks
confronting an infant is to discover what kinds of objects are in the world and how thoseobjects are causally related to one another. The past several decades of work in developmental
psychology have uncovered striking ways in which children explore an unknown world inmuch the same manner as a scientist would [Gopnik 2012]. They ask and answer “What if?”and “Why?” questions [Buchsbaum et al. 2012], use data to formulate causal hypotheses, andeven test those hypotheses by actively performing interventions on the environment [Gopnik
et al. 2004]. By adulthood, our causal knowledge forms the very cement that holds ourunderstanding of the world together [Danks 2014, Sloman and Lagnado 2015].

1.1 Introduction 3

In a more systematic fashion, causality plays a central role on how we probe the physicalworld around us and ultimately understand Nature. Standard scientific methodology is built
around the idea of combining observations and experiments (more on their distinction lateron) and formulating hypotheses about unobserved causal mechanisms, submitting thesehypotheses to further observation and experimentation in a continual process of refinement[Machamer et al. 2000, Salmon 1984, Woodward 2002, 2003]. In modern molecular biology,
for example, scientists can explain the synthesis of proteins by first identifying the criticalmolecular components involved – DNA, mRNA, tRNA, rRNA, codons, amino acids – andputting them together in a series of steps that lead from initial transcription of DNA into
mRNA, all the way down to the final protein folding. In a similar vein but a rather different
context, economists have been able to predict and explain macro-level consumer behavior
using models of individual choice behavior over a lifetime, as a function of other relevant
variables like income, assets, and interest rates (see, e.g., [Deaton 1992] for a classic example).
Causal explanations like these purport to be more than mere descriptions, or summariesof the observed data. By breaking down a phenomenon into modular components, anddescribing how they interact to produce an emergent behavior or a final product [Simon 1953],
scientists seek to uncover the underlying data-generating processes, or features thereof. When
successful, it allows one to infer what would or could happen under various hypothetical(counter-to-fact) suppositions, going beyond the limited observations (i.e., data) afforded up tothat point. For instance, biologists are able to predict the effect that bacterial or viral pathogens
might have on otherwise normal protein pathways, while economists may predict what effecthigher interest rates would have on consumption and economic activity. Practically speaking,mechanistic knowledge of this sort can often support cleaner and more surgical interventions,which has the potential to allow one to bring about desired states of affairs [Woodward 2003],
whether social, economic, or political.
Given the centrality of causation throughout so many aspects of human experience, wewould naturally like to have a formal framework for encoding and reasoning with cause andeffect relationships. Interestingly, the 20th century saw other instances in which an intuitive,
ordinary concept underwent mathematical formalization, before then entering engineering
practice. As an especially notable example, it may be surprising to readers outside computerscience and related disciplines to learn that the notion of computation itself was only semiformally understood up until the 1920s. Following the seminal work of mathematician andphilosopher Alan Turing, among others, multiple breakthroughs ensued, including the very
emergence of the modern computer, passing through the theory and foundations of computerscience, and culminating in the rich and varied technological advances we enjoy today.
We feel it is appropriate in this special edition dedicated to Judea Pearl, a Turing awardee
himself, to recognize a similar historical development in the discipline of causality. The
subject was studied in a semi-formal way for centuries [Hume 1739, 1748, Mackie 1980,
von Wright 1971], to cite a few prominent references, and Pearl, his collaborators, and4

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

SCM(Unobserved Nature)(
X ← fX (Ux )Unobserved
CausalMechanisms

Y← fY (X,Uy )L3 CounterfactualP(Ux ,Uy )Observed
Phenomena

L2 Interventional

P(X,Y )

P(Y |do(X))

P(Yx |x0 , y0 )

L1

L2

L3

(a)

L1 Associational(b)

Figure 1.1: (a) Collection of causal mechanisms (or SCM) generating certain observed phenomena (qualitatively different probability distributions). (b) PCH’s containment structure.

many others helped to understand and formalize this notion. In fact, following this precise
mathematization, we now see a blossoming of algorithmic developments and rapid expansion
towards applications.1
What was the crucial development that spawned such dramatic progress on this centuriesold problem? One critical insight, tracing back at least to the British empiricist philosophers, isthat the causal mechanisms behind a system under investigation are not generally observable,
but they do produce observable traces (“data,” in modern terminology).2 That is, “reality”
and the data generated by it are fundamentally distinct. This dichotomy has been prominent
at least since Pearl’s seminal Biometrika paper [Pearl 1995], and received central status andcomprehensive treatment in his longer treatise [Pearl 2000, 2009]. This insight naturally leads
to two practical desiderata for any proper framework for causal inference, namely:
1. The causal mechanisms underlying the phenomenon under investigation should be accounted for – indeed, formalized – in the analysis.
2. This collection of mechanisms (even if mostly unobservable) should be formally tied toits output: the generated phenomena and corresponding datasets.
1 It lies outside the scope of this chapter to pursue a detailed historical account, and we refer readers to [Pearl andMackenzie 2018] for additional context.
2 For instance, Locke famously argued that when we observe data, we cannot “so much as guess, much less know,
their manner of production” [Locke 1690, Essay IV]. Hume maintained a similarly skeptical stance, stating that“nature has kept us at a great distance from all her secrets, and has afforded only the knowledge of a few superficial
qualities of objects; while she conceals from us those powers and principles, on which the influence of these objects
entirely depends” [Hume 1748, §4.16]. See [de Pierris 2015] for discussion.

1.1 Introduction 5This intuitive picture is illustrated in Fig. 1.1(a). One of the main goals of this chapter is tomake this distinction crisp and unambiguous, translating these two desiderata into a formal
framework, and uncovering its consequences for the practice of causal inference.
Regarding the first requirement, the underlying reality (“ground truth”) that is our putative
target can be naturally represented as a collection of causal mechanisms in the form of a mathematical object called a structural causal model (SCM) [Pearl 1995, 2000], to be introduced in
Section 1.2. In many practical settings, it may be challenging, even impossible, to determine
the specific form of the underlying causal mechanisms, especially when high-dimensional,
complex phenomena are involved and humans are present in the loop.3 Nevertheless, we ordinarily presume that these causal mechanisms are there regardless of our practical ability to
discover their form, shape, and specific details.
Regarding the second requirement, Pearl further noted something very basic and fundamental, namely, that each collection of causal mechanisms (i.e., SCM) induces a causal hierarchy (or “ladder of causation”), which highlights qualitatively different aspects of the underlying reality. We fondly name this the Pearl Causal Hierarchy (PCH, for short), for he was thefirst to identify and study it systematically [Pearl 1995, 2000, Pearl and Mackenzie 2018]. The
hierarchy consists of three layers (or “rungs”) encoding different concepts: the associational,
the interventional, and the counterfactual, corresponding roughly to the ordinary human activities of seeing, doing, and imagining, respectively [Pearl and Mackenzie 2018, Chapter
1]. Knowledge at each layer allows reasoning about different classes of causal concepts, or“queries.” Layer 1 deals with purely “observational”, factual information. Layer 2 encodes
information about what would happen, hypothetically speaking, were some intervention tobe performed, viz. effects of actions. Finally, Layer 3 involves queries about what would
have happened, counterfactually speaking, had some intervention been performed, given thatsomething else in fact occurred (possibly conflicting with the hypothetical intervention). The
hierarchy establishes a useful classification of concepts that might be relevant for a giventask, thereby also classifying formal frameworks in terms of the questions that they are able
to represent, and ideally answer.
Roadmap of the Chapter. Against this background, we start in Sec. 1.2 by showing
how the Pearl Causal Hierarchy naturally emerges from a structural causal model, formallycharacterizing the layers by means of symbolic logical languages, each of which receives astraightforward interpretation in an SCM. Thus, as soon as one admits that a domain of interest
can be represented by an SCM (whether or not we, as an epistemological matter, know muchabout it), the hierarchy of causal concepts already exists.4 In Sec. 1.3, we prove that the PCH
3 At the same time, many of the natural sciences, most prominently physics, chemistry, will often purport to determine

the underlying causal mechanisms quite precisely.
4 This is despite skepticism that has been expressed in the literature about meaningfulness of one layer of the hierarchy
or another; cf., e.g., Maudlin 2019 on Layer 2, and Dawid 2000 on Layer 3.

6Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference(a) SCM(Unobserved Nature)(
X ← fX (Ux )Y← fY (X,Uy )(c)Structural Constraints(Graphical Model)(L1 ) Associational(L2 ) Interventional(L3 ) CounterfactualP(Ux ,Uy )

(b)
PCH

(L1 ) P(X,Y )(d) Data(L2 ) P(Y |do(X)) (L3 ) P(Yx |x0 , y0 )(e) Query

Figure 1.2: Schema depicting building blocks of canonical causal inferences – on the top, theSCM itself, i.e., the unobserved collection of mechanisms and underlying uncertainty (a); on
the bottom, different probability distributions forming PCH’s layers (b); on the right, structural
constraints entailed by the SCM (c). Example of possible input dataset (d) and query (e).

is strict for almost-all SCMs (Thm. 1), in a technical sense of ‘almost-all’ (Fig. 1.1(b)).5 It
follows (Corollary 1) that it is generically impossible to draw higher-layer inferences using
only lower-layer information, a result known informally in the field under the familiar adage:“no causes-in, no causes-out” [Cartwright 1989]. This first part of the chapter does not directly
address the practice of causal inference; rather, it formally establishes a general motivation forcausal inference from a logical perspective.
In the second part of the chapter (Sec. 1.4), we acknowledge that in many practicalsettings our ability to interact with (observe and experiment on) the phenomenon of interest
is modest at best, and inducing a reasonable, fully specified SCM (Fig. 1.2(a)) is essentiallyhopeless.6 Virtually all approaches to causal inference, therefore, set for themselves a more
restricted target, operating under the less stringent condition that only partial knowledgeof the underlying SCM is available. The problem of causal inference is thus to perform
inferences across layers of the hierarchy (Fig. 1.2(b)) from a partial understanding of the
5 Hierarchies abound in logic and computer science, particularly those pertaining to computational resources, promi-

nent examples being the Chomsky-Schützenberger hierarchy [Chomsky 1959] and its probabilistic variant (see [Icard
2020]), or the polynomial time complexity hierarchy [Stockmeyer 1977]. Such hierarchies delimit what can be computed given various bounds on computational resources. Perhaps surprisingly, the Pearl hierarchy is orthogonal tothese hierarchies. If one’s representation language is only capable of encoding queries at a given layer, no amount oftime or space for computation – and no amount of data either – will allow making inferences at higher layers.
6 Of course, if we have been able to induce the structural mechanisms themselves – as may be feasible in some of thesciences, e.g., molecular biology or Newtonian physics – we can simply “read off” any causal information we likeby computing it directly or, for instance, by simulating the corresponding mechanisms.

1.2 Structural Causal Models and the Causal Hierarchy 7

SCM (Fig. 1.2(c)). Technically speaking, if one has layer-1 type of data (Fig. 1.2(d)), e.g.,collected through random sampling, and aims to infer the effect of a new intervention (layer-2
type of query, (Fig. 1.2(e)), we show that the problem is not always solvable. In words, there
is not enough information about the SCM encoded in the dataset (coming from the realized
world) so that one can learn how the system would react when submitted to a new intervention(a still unrealized, hypothetical world).
Departing from these impossibility results, we develop a framework that can parsimoniously and efficiently encode knowledge (viz. structural constraints) necessary to perform
this general class of inferences. In particular, we move beyond layer 1-type constraints (conditional independences) and investigate structural constraints that live in Layer 2 (Fig. 1.2(e)).
In particular, we use these constraints to define a new family of graphical models called Causal
Bayesian Networks (CBNs), which are comprised of a pair, a graphical model and a collection of observational and interventional distributions. We present a constructive definition of
CBNs that naturally emerges from an SCM, as well as one that is purely empirical. This
treatment generalizes existing characterizations [Bareinboim et al. 2012, Pearl 2000] to thesemi-Markovian setting and allows for the existence of unobserved confounders. Against thisbackdrop, we provide a novel proof of do-calculus [Pearl 1995] based strictly on layer 2 semantics. We then show how the graphical structure bridges the layers of the PCH; one may
be able to draw inferences at a higher layer given a combination of partial knowledge of theunderlying structural model, in the form of a causal graph, and data at lower layers.
Finally, in Sec. 1.5, we conclude summarizing our main contributions and putting thiswork into the broader context of AI and data science. In particular, we outline some of
the ways that progress toward the goal of developing safe, robust, explainable, and humancompatible artificial systems will be greatly amplified by further appreciation of both theinherent limitations and the exciting possibilities afforded by the study of Pearl’s Hierarchy.
Notation. We now introduce the notation used throughout this chapter. Single randomvariables are denoted by (non-boldface) uppercase letters X and the range (or possible
values) of X is written as Val(X). Lowercase x denotes a particular element in this range,x ∈ Val(X). Boldfaced uppercase X denotes a collection of variables, Val(X) their possible
joint values, and boldfaced lowercase x a particular joint realization x ∈ Val(X). For example,
two independent fair coin flips are represented by X = {X1 , X2 }, Val(X1 ) = Val(X2 ) = {0, 1},
Val(X) = {(0, 0), . . . , (1, 1)}, with P(x1 ) = P(x2 ) = ∑x2 P(x1 , x2 ) = ∑x(X1 )=x1 P(x) = 1/2.

1.2Structural Causal Models and the Causal Hierarchy
We build on the language of Structural Causal Models (SCMs) to describe the collection of
mechanisms underpinning a phenomenon of interest. Essentially any causal inference can beseen as an inquiry about these mechanisms or their properties, in some way or another. We
will generally dispense with the distinction between the underlying system and its SCM.

8

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

Layer(Symbolic)TypicalActivity

TypicalQuestion

ExampleMachine Learning

L1

Associational
P(y|x)

SeeingWhat is?
How would seeing
X change my beliefin Y ?

What does a symptom tell us about thedisease?

Supervised /Unsupervised
Learning

L2

Interventional
P(y|do(x), c)

Doing

What if?
What if I do X?

What if I take aspirin,
will my headache be
cured?

Reinforcement
Learning

L3

CounterfactualP(yx |x0 , y0 )Imagining

Why?
What if I had acted
differently?

Was it the aspirinthat stopped myheadache?

Table 1.1: Pearl’s Causal Hierarchy.

Each SCM naturally defines a qualitative hierarchy of concepts, described as the “ladderof causation” in [Pearl and Mackenzie 2018], which we have been calling the Pearl Causal
Hierarchy, or PCH (Fig. 1.1). Following Pearl’s presentation, we label the layers (or rungs,
or levels) of the hierarchy associational, interventional, and counterfactual. The conceptsof each layer can be described in a formal language and correspond to distinct notions
within human cognition. Each of these allows one to articulate with mathematical precisionqualitatively different types of question regarding the observed variables of the underlying
system; for some examples, see Table 1.1.
SCMs provide a flexible formalism for data-generating models, subsuming virtually all of
the previous frameworks in the literature. In the sequel, we formally define SCMs and thenshow how a fully specified model underpins the concepts in the PCH.
Definition 1 (Structural Causal Model (SCM)). A structural causal model M is a 4-tuple
hU, V, F , P(U)i, where• U is a set of background variables, also called exogenous variables, that are determinedby factors outside the model;• V is a set {V1 ,V2 , . . . ,Vn } of variables, called endogenous, that are determined by othervariables in the model – that is, variables in U ∪ V;
• F is a set of functions { f1 , f2 , . . . , fn } such that each fi is a mapping from (the respective

domains of) Ui ∪ Pai to Vi , where Ui ⊆ U, Pai ⊆ V \ Vi , and the entire set F forms amapping from U to V. That is, for i = 1, . . . , n, each fi ∈ F is such thatvi ← fi (pai , ui ),(1.1)

1.2 Structural Causal Models and the Causal Hierarchy 9i.e., it assigns a value to Vi that depends on (the values of) a select set of variables in
U ∪ V; and• P(U) is a probability function defined over the domain of U.

Each structural causal model can be seen as partitioning the variables involved in thephenomenon into sets of exogenous (unobserved) and endogenous (observed) variables,
respectively, U and V. The exogenous ones are determined “outside” of the model and theirassociated probability distribution, P(U), represents a summary of the state of the world
outside the phenomenon of interest. In many settings, these variables represent the units
involved in the phenomenon, which correspond to elements of the population under study,
for instance, patients, students, customers. Naturally, their randomness (encoded in P(U))
induces variations in the endogenous set V.
Inside the model, the value of each endogenous variable Vi is determined by a causal
process, vi ← fi (pai , ui ), that maps the exogenous factors Ui and a set of endogenous variables
Pai (so called parents) to Vi . These causal processes – or mechanisms – are assumed to beinvariant unless explicitly intervened on (as defined later in the section).7 Together with thebackground factors, they represent the data-generating process according to which Natureassigns values to the endogenous variables in the study.
Henceforth, we assume that V and its domain is finite8 and that all models are recursive(i.e., acyclic).9 A structural model is Markovian if the exogenous parent sets Ui ,U j are
independent whenever i 6= j. In the treatment provided here, we allow for the sharing ofexogenous parents and we allow for arbitrary dependences among the exogenous variables,
which means that, in general, the SCM need not to be Markovian. This wider class of models
is called semi-Markovian. For concreteness, we provide a simple SCM next.
Example 1. Consider a game of chance described through the SCM M 1 = hU = {U1 ,U2 },V = {X,Y }, F , P(U1 ,U2 )i, whereX ← U +U
1
2
F =
,
(1.2)
Y ← U1 −U2and P(Ui = k) = 1/6, i = 1, 2, k = 1, . . . , 6. In other words, this structural model represents
the setting in which two dice are rolled but only the sum (X) and the difference (Y ) of their
7 It is possible to conceive an SCM as a “a high-level abstraction of an underlying system of differential equations”[Schölkopf 2019], which under relatively mild conditions is attainable [Rubenstein et al. 2017].
8 In most of the literature the set V is assumed to be finite; however, the axiomatic characterization of SCMs can beextended to the infinitary setting in a very natural way [Ibeling and Icard 2019].
9 An SCM M is said to be recursive if there exists a “temporal” order over the functions in F such that for every
pair fi , f j ∈ F , if fi < f j in the order, we have that fi does not have V j as an argument. In particular, this implies thatchoosing a unit u uniquely fixes the values of all variables in V. For Y ⊆ V, we write Y(u) to denote the solution of Y
given unit u. For a more comprehensive discussion, see [Galles and Pearl 1998, Halpern 1998] and [Halpern 2000].

10Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

values is observed. Here, the domains of X and Y are, respectively, Val(X) = {2, . . . , 12} andVal(Y ) = {−5, . . . , 0, . . . , 5}.
Pearl Hierarchy, Layer 1 – Seeing
Layer 1 of the hierarchy (Table 1.1) captures the notion of “seeing,” that is, observing a certain
phenomenon unfold, and perhaps making inferences about it. For instance, if we observea certain symptom, how will this change our belief in the disease? An SCM gives naturalvaluations for quantities of this kind (cf. Eq. (7.2) in [Pearl 2000]), as shown next.
Definition 2 (Layer 1 Valuation – “Observing”). An SCM M = hU, V, F , P(U)i defines ajoint probability distribution PM (V) such that for each Y ⊆ V:10
PM (y) =

∑

P(u),

(1.3)

{u | Y(u)=y}

where Y(u) is the solution for Y after evaluating F with U = u.

In words, the procedure dictated by Eq. (1.3) can be described as follows:
1. For each unit U = u, Nature evaluates F following a valid order (i.e., any variable in thel.h.s. is evaluated after the ones in the r.h.s.)11 , and
2. The probability mass P(U = u) is accumulated for each instantiation U = u consistent
with the event Y = y.
This evaluation is graphically depicted in Fig. 1.3(i), which represents a mapping fromthe external and unobserved state of the system (distributed as P(U)), to an observablestate (distributed as P(V)). For concreteness, let us consider Example 1 again. Let the dice(exogenous variables) be hU1 = 1,U2 = 1i, then V = {X,Y } attain their values through F as
X = 1 + 1 = 2 and Y = 1 − 1 = 0. Since P(U1 = 1,U2 = 1) = 1/36 and hU1 = 1,U2 = 1i isthe only configuration capable of producing the observed behavior hX = 2,Y = 0i, it follows
that P(X = 2,Y = 0) = 1/36. More interestingly, consider the different dice (exogenous)
configurations hU1 ,U2 i = {h1, 1i, h2, 2i, h3, 3i, h4, 4i, h5, 5i, h6, 6i}, which are all compatible
with hY = 0i. Since each of the U’s realization happens with probability 1/36, the event of thedifference between the first and second dice being zero (Y = 0) occurs with probability 1/6.
Finally, what is the probability of the difference of the two dice being zero (Y = 0) if we knowthat their sum is two, i.e., P(Y = 0 | X = 2)? The answer is one since the only event compatible
with hX = 2,Y = 0i is hU1 = 1,U2 = 1i. Without any evidence, the event (Y = 0) happens
with probability 1/6, yet if we know that X = 2, the event becomes certain (probability 1). In
fact, X and Y become deterministically related.
10 We will typically omit the superscript on PM whenever there is no room for confusion, thus using P for both the

distribution P(U) on exogenous variables and the distributions P(Y) on endogenous variables induced by the SCM.
11 Here, we deliberately invoke the entity “Nature” as the evaluator of the SCM to emphasize the separation between
the modeler/agent and the underlying dynamics of the system, which is almost invariably unknown to them.

1.2 Structural Causal Models and the Causal Hierarchy 11(i)Observational(ii)Interventional(iii)
Counterfactual
P(U)

(a)

External state

P(U)

P(U)

(b)

TransformationFFx(c) Induced Distribution

P(Y)P(Yx )

Fx

···

Fw

P(Yx , . . . , Zw )Figure 1.3: Given an SCM’s initial state (i.e., population) (a), we show the different functionaltransformations (b) and the corresponding induced distribution (c) of each layer of thehierarchy. (i) represents the transformation (i.e., F ) from the natural state of the system (P(U))
to an observational world, (ii) to an interventional world (i.e., with modified mechanisms Fx ),and (iii) to multiple counterfactual worlds (i.e., with multiple modified mechanisms).

Many tasks throughout data sciences can be seen as evaluating the probability of certain
events occurring. For instance, expressions such as P(Y | X), with X, Y ⊆ V, are naturally
probabilistic reflecting the uncertainty we may have about the world. In the context of modern
machine learning, for example, one could observe a certain collection of pixels, or features,with the goal of predicting whether it contains a dog or a cat. Consider a slightly more involved
example that appears in the context of medical decision-making.
Example 2. The SCM M 2 = hV = {X,Y, Z}, U = {Ur ,Ux ,Uy ,Uz }, F = { fx , fy , fz }, P(Ur ,Ux ,Uy ,Uz )i, where F will be specified below. The endogenous variables V represent, respectively, a certain treatment X (e.g., drug), an outcome Y (survival), and the presence or notof a symptom Z (hypertension). The exogenous variable Ur represents whether the person
has a certain natural resistance to the disease, and Ux ,Uy ,Uz are sources of variations outside
the model affecting X,Y, Z, respectively. In this population, units with resistance (Ur = 1)
are likely to survive (Y = 1) regardless of the treatment received. Whenever the symptom ispresent (Z = 1), physicians try to counter it by prescribing this drug (X = 1). While the treatment (X = 1) helps resistant patients (with Ur = 1), it worsens the situation for those who are
not resistant (Ur = 0). The form of the underlying causal mechanisms is:Z ← 1{Ur =1,Uz =1}

F = X ← 1{Z=1,Ux =1} + 1{Z=0,Ux =0}Y← 1{X=1,Ur =1} + 1{X=0,Ur =1,Uy =1} + 1{X=0,Ur =0,Uy =0}

.

(1.4)

12

1
2
3
4
5
6
7
8

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

Ur

Uz

Ux

Uy

Z

X

Y

P(u)

0
0
0
0
0
0
0
0

0
0
0
0
1
1
1
1

0
0
1
1
0
0
1
1

0
1
0
1
0
1
0
1

0
0
0
0
0
0
0
0

1
1
0
0
1
1
0
0

0
0
1
0
0
0
1
0

0.001125
0.002625
0.0101250.023625
0.021375
0.049875
0.192375
0.448875

9
10
11
12
13
14
15
16

Ur

Uz

Ux

Uy

Z

X

Y

P(u)

1
1
1
1
1
1
1
1

0
0
0
0
1
1
1
1

0
0
1
1
0
0
1
1

0
1
0
1
0
1
0
1

0
0
0
0
1
1
1
1

1
1
0
0
0
0
1
1

1
1
0
1
0
1
1
1

0.000375
0.000875
0.003375
0.007875
0.007125
0.016625
0.064125
0.149625

Table 1.2: Mapping of events in the space of U to V in the context of Example 2.

Finally, all the exogenous variables are binary with P(Ur = 1) = 0.25, P(Uz = 1) = 0.95,
P(Ux = 1) = 0.9, and P(Uy = 1) = 0.7.
Recall that Def. 2 (Eq. 1.3) induces a mapping between P(U) and P(V). In this example,
each entry of Table 1.2 corresponds to an event in the space of U and the corresponding
realization of V according to the functions in F .
Using this mapping and Def. 2, a query P(Y = 1 | X = 1) can be evaluated from M as:P(Y = 1, X = 1) ∑{u|Y (u)=1,X(u)=1} P(u)=P(X = 1)∑{u|X(u)=1} P(u)0.215
= 0.7414,=
0.29

P(Y = 1 | X = 1) =(1.5)

which is just the ratio between the sum of the probabilities of the events in the space of U
consistent with the events hY = 1, X = 1i and hX = 1i.This means that the probability ofsurvival given that one took the drug is higher than chance. Similarly, one could obtain otherprobabilistic expressions such as P(Y = 1 | X = 0) = 0.3197 or P(Z = 1) = 0.2375. One may
be tempted to believe at this point that the drug has a positive effect upon comparing theprobabilities P(Y = 1 | X = 0) and P(Y = 1 | X = 1). We shall discuss this issue next.
Pearl Hierarchy, Layer 2 – Doing
Layer 2 of the hierarchy (Table 1.1) allows one to represent the notion of “doing”, thatis, intervening (acting) in the world to bring about some state of affairs. For instance, if aphysician gives a drug to her patient, would the headache be cured? A modification of an
SCM gives natural valuations for quantities of this kind, as defined next.
Definition 3 (Submodel – “Interventional SCM”). Let M be a causal model, X a set ofvariables in V, and x a particular realization of X. A submodel Mx of M is the causal model

Mx = hU, V, Fx , P(U)i,(1.6)

1.2 Structural Causal Models and the Causal Hierarchy 13

where

Fx = { fi : Vi ∈/ X} ∪ {X ← x}.

(1.7)


In words, performing an external intervention (or action) is modelled through the replacement of the original (natural) mechanisms associated with some variables X with a constantx, which is represented by the do-operator.12,13 The impact of the intervention on an outcome
variable Y is called potential response (cf. Def. (7.1.4) in [Pearl 2000]):
Definition 4 (Potential Response). Let X and Y be two sets of variables in V, and u be a unit.
The potential response Yx (u) is defined as the solution for Y of the set of equations Fx withrespect to SCM M (for short, YMx (u)). That is, Yx (u) = YMx (u).
An SCM gives valuation for interventional quantities (Eq. 7.3, [Pearl 2000]) as follows:
Definition 5 (Layer 2 Valuation – “Intervening”). An SCM M = hU, V, F , P(U)i induces afamily of joint distributions over V, one for each intervention x. For each Y ⊆ V:
PM (yx ) =

∑

P(u).

(1.8)

{u|Yx (u)=y}The evaluation implied by Eq. 1.8 can be described as the following process (Fig. 1.3(ii)):
1. Replace the mechanism of each X ∈ X with the corresponding constants x generating Fx(Eq. 1.7), which induces a submodel Mx (of M );2. For each unit U = u, Nature evaluates Fx following a valid order (where any variable in
the l.h.s. is evaluated after the ones in the r.h.s.), and3. The probability mass P(U = u) is then accumulated for each instantiation U = u consistent with the event Yx = y (i.e., Y in the submodel Mx ).
The potential response expresses causal effects, and over a probabilistic setting it induces
random variables. Specifically, Yx denotes a random variable induced by averaging the potential response Yx (u) over all u according to P(U).14 Further, note that this procedure disconnects X from any other source of “natural” variation when it follows the original function fx
12 The idea of representing intervention through the modification of equations in a structural system appears to havefirst emerged in the context of Econometrics, see [Haavelmo 1943], and [Marschak 1950, Simon 1953]. It was thenmade more explicit and called “wiping out” by [Strotz and Wold 1960]; see also [Fisher 1970] and [Sobel 1990].
13 Pearl credits his realization on the connection of this operation with graphical models to a lecture of Peter Spirtes
at the International Congress of Philosophy of Science (Uppsala, Sweden, 1991), in his words [Pearl 2000, pp. 104]:“In one of his slides, Peter illustrated how a causal diagram would change when a variable is manipulated. To me,that slide of Spirtes’s – when combined with the deterministic structural equations – was the key to unfolding themanipulative account of causation (...)”. To avoid confusion, we note that the sense of manipulation here is notintended to be reductive, e.g., see [Pearl 2018b, 2019, Woodward 2016].
14 The notation Y (u) is borrowed from the potential-outcome framework of [Neyman 1923] and [Rubin 1974]. See
x[Pearl 2000, § 7.4.4] for a more detailed comparison; see also [Pearl and Bareinboim 2019].

14

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference(e.g., the observed (Pax ) or unobserved (Ux ) parents). This means that the variations of Y inthis world would be due to changes in X (say, from 0 to 1) that occurred externally, from outside the modeled system.15 This, in turn, guarantees that they will be causal. To see why, notethat all variations of X that may have an effect on Y can only be realized through variables ofwhich X is an argument, since X itself is a constant, not affected by other variables. Indeed,
the notion of average causal effect can be formally written as E(YX=1 ) − E(YX=0 ).16
The distribution P(Yx ) defined in Eq. (1.8) is often written P(Y | do(x)), and we henceforth
adopt this notation in the context of PCH’s second layer.17 By convention, the do(x) maps
over the entire formula in the conditional case, i.e.:P(Y | do(x), z) = P(Yx | zx ) =P(Yx , zx ) P(Y, z | do(x))=,
P(zx )
P(z | do(x))(1.9)

where the last expression is manifestly L2 . Further, in accordance with the semantics of theintervention do(x), it is clear that the distribution of the intervened variables must satisfy aproperty called effectiveness, which we define explicitly next:
Definition 6 (Effectiveness). A joint interventional distribution P(v|do(x)) is said to satisfy
effectiveness if for every Vi ∈ X,
P(vi | do(x)) = 1 if vi is consistent with x and 0 otherwise.

(1.10)
In words, if a variable X is fixed to x by intervention, X = x must be observed withprobability one. This is a technical condition and reflects the probabilistic meaning of hardinterventions.18 (For a comparison against Bayesian conditioning, see [Pearl 2017].)
Example 3 (Example 1 continued). Let us consider the same dice game but now the observer
decides to misreport the sum of the two dice as 2, which can be written as submodel MX=2 :X ← 2
FX=2 =,(1.11)
Y ← U1 −U2 ,while P(U) remains invariant. It is immediate to see that YX=2 (u1 , u2 ) is the same as Y (u1 , u2 );in words, misreporting the sum of the two dice will of course not change their difference. This,
15 For a discussion of what it means for these changes to arise “from outside” the system, see, e.g., [Woodward 2003,

2016]. Of course, in many settings this simply means the intervention is performed deliberately by an agent outside
the system, for example, in reinforcement learning [Sutton and Barto 2018].
16 This difference and the corresponding expected values are sometimes taken as the definition of “causal effect”, see[Rosenbaum and Rubin 1983]. In the structural account of causation pursued here, this quantity is not a primitive but
derivable from the SCM, as all others within the PCH. To witness, note YX=1 ← fY (1, εY ) when do(X = 1).
17 This allows researchers to use the syntax to immediately distinguish statements that are amenable to some sort ofexperimentation, at least in principle, from other counterfactuals that may be empirically unrealizable.
18 For the sake of presentation, we discuss the class of atomic interventions even though there are more general
within the SCM framework, including soft, conditional, stochastic [Correa and Bareinboim 2020, Pearl 2000, Ch. 4].

1.2 Structural Causal Models and the Causal Hierarchy 15

in turn, entails the following probabilistic invariance,
P(Y = 0 | do(X = 2)) = P(Y = 0).

(1.12)

In fact, the distribution of Y when X is fixed to two remains the same as before (i.e.,P(Y = 0|do(X = 2)) = 1/6). We saw in the first part of the example that knowing thatthe sum was two meant that, with probability one, their difference had to be zero (i.e.,P(Y = 0|X = 2) = 1). On the other hand, intervening on X will not change Y ’s distribution(Eq. 1.12); as we say, X does not have a causal effect on Y .
Example 4 (Example 2 continued). Consider now that a public health official performs anintervention by giving the treatment to all patients regardless of the symptom (Z). This meansthat the function fX would be replaced by the constant 1. In words, patients do not have an
option of deciding their own treatment, but are compelled to take the specific drug.19 This isrepresented through the new modified set of mechanisms,Z ← 1{Ur =1,Uz =1}

FX=1 = X ← 1



Y

,

(1.13)← 1{X=1,Ur =1} + 1{X=0,Ur =1,Uy =1} + 1{X=0,Ur =0,Uy =0}

and where the distribution of exogenous variables remains the same. Note that the potential
response YX=1 (u) represents the survival of patient u had they been treated, while the randomvariable YX=1 describes the average population survival had everyone been given the treatment. Notice that for those patients who naturally received treatment (X ← fx (U) = 1), thenatural outcome Y (u) is equal to YX=1 (u). For this intervened model, YX=1 (u) is equal to 1 inevery event where Ur = 1, regardless of Uz ,Ux , and Uy . Then
P(Y =1|do(X=1)) =

∑

P(u)

(1.14)∑P(ur ) = P(Ur =1) = 0.25.

(1.15){u|YX=1 (u)=1}={ur |YX=1 (ur )=1}

Similarly, one can evaluate P(Y =1|do(X=0)), which is equal to 0.4. This may be surprisingsince from the perspective of Layer 1, P(Y = 1|X = 1) − P(Y = 1|X = 0) = 0.43 > 0, whichappear to suggest that taking the drug is helpful, having a positive effect on recovery. On theother hand, interventionally speaking, P(Y = 1|do(X = 1)) − P(Y = 1|do(X = 0)) = −0.15 <0, which means that the drug has a negative (average) effect in the population.

19 This physical procedure is the very basis for the discipline of experimental design [Fisher 1936], which is realized

through randomization of the treatment assignment in a sample of the population. In practice, the function of X,
fX , is replaced with an alternative source of randomness that is uncorrelated with any other variable in the system.
This procedure is pervasive in modern society, for example, in randomized controlled trials (RCTs) when drugs areevaluated for their efficacy, or in A/B experiments when products are tested by internet companies.

16

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

The evaluation of an interventional distribution is a function of the modified system Mx
that reflects Fx , which follows from the replacement of X, as illustrated in Fig. 1.3(ii). Even
though computing observational and interventional distributions is immediate from a fully
specified SCM, the distinction between Layer 1 (seeing) and Layer 2 (doing) will be a central
topic in causal inference, as discussed more substantively in Section 1.4.
Pearl Hierarchy, Layer 3 – Imagining counterfactual worlds
Layer 3 of the hierarchy (Table 1.1) allows operationalizing the notion of “imagination”(and the closely related activities of retrospection, introspection, and other forms of “modal”
reasoning), that is, thinking about alternative ways the world could be, including ways thatmight conflict with how the world, in fact, currently is. For instance, if the patient took theaspirin and the headache was cured, would the headache still be gone had they not taken the
drug? Or, if an individual ended up getting a great promotion, would this still be the case
had they not earned a PhD? What if they had a different gender? Obviously, in this world,
the person has a particular gender, has a PhD, and ended up getting the promotion, so wewould need a way of conceiving and grounding these alternative possibilities to evaluate
such scenarios. In fact, no experiment in the world (Layer 2) will be sufficient to answer
this type of question, despite their ubiquity in human discourse, cognition, and decisionmaking. Fortunately, the meaning of every term in the counterfactual layer (L3 ) can be directly
determined from a fully specified structural causal model, as described in the sequel:
Definition 7 (Layer 3 Valuation). An SCM M = hU, V, F , P(U)i induces a family of joint
distributions over counterfactual events Yx , . . . , Zw , for any Y, Z, . . . , X, W ⊆ V:
PM (yx , . . . , zw ) =

∑

P(u).

(1.16){u | Yx (u)=y,..., Zw (u)=z}
Note that the l.h.s. of Eq. (1.16) contains variables with different subscripts, which, syntactically, encode different counterfactual “worlds.” The evaluation implied by this equation
can be described as the following process (as shown in Fig. 1.3(iii)):
1. For each set of subscripts relative to each set of variables (e.g., X, ..., W for Y, ..., Z,
respectively), replace the corresponding mechanisms with the appropriate constants andgenerate Fx , ..., Fw (Eq. 1.7), creating submodels Mx , ..., Mw (of M );2. For each unit U = u, Nature evaluates the modified mechanisms (e.g., Fx , ..., Fw )following a valid order (i.e., any variable in the l.h.s. is evaluated after the ones in the
r.h.s.) to obtain the potential responses of the observables, and3. The probability mass P(U = u) is then accumulated for each instantiation U = u that is
consistent with the events over the counterfactual variables – for instance, Yx = y, ...,
Zw = z, i.e., Y = y, ..., Z = z in the submodels Mx , ..., Mw , respectively.

1.2 Structural Causal Models and the Causal Hierarchy 17

Example 5 (Example 2 continued). Since there is a group of patients who did not receive
the treatment and died (X = 0,Y = 0), one may wonder whether these patients would havebeen alive (Y = 1) had they been given the treatment (X = 1). In the language of Layer 3, thisquestion is written as P(YX=1 = 1 | X = 0,Y = 0). This is a non-trivial question since these
individuals did not take the drug and are already deceased in the actual world (as displayed
after the conditioning bar, X = 0,Y = 0); the question is about an unrealized world and how
these patients would have reacted had they been submitted to a different course of action(formally written before the conditioning bar, YX=1 = 1). In other words, did they die becauseof the lack of treatment? Or would this fatal unfolding of events happen regardless of thetreatment? Unfortunately, there is no conceivable experiment in which we could draw samples
from P(YX=1 = 1|X = 0,Y = 0), since these patients cannot be resuscitated and submitted tothe alternative condition. This is the very essence of counterfactuals.
For simplicity, note that P(YX=1 = 1|X = 0,Y = 0) can be written as the ratio P(YX=1 =1, X = 0,Y = 0)/P(X = 0,Y = 0), where the denominator is trivially obtainable since it
only involves observational probabilities (about one specific world, the factual one). The
numerator, P(YX=1 = 1, X = 0,Y = 0), refers to two different worlds and cannot be written in
the languages of layers 1 and 2 since they do not allow for probability expressions involving
more than one subscript (each encoding a different world). This means we need to climb up
to the third layer in order to formally specify the quantity of interest. Using the procedure
dictated in Eq. 1.16, we obtain
P(YX=1 =1, X=0,Y =0) ∑{u|YX=1 (u)=1,X(u)=0,Y (u)=0} P(u)=P(X=0,Y =0)∑{u|X(u)=0,Y (u)=0} P(u)0.0105= 0.0217.
(1.17)=
0.483

P(YX=1 =1|X=0,Y =0) =This evaluation is shown step by step in [Bareinboim et al. 2020a, Appendix E.1], but
we emphasize here that the expression in the numerator involves evaluating multiple worlds
simultaneously (in this case, one factual and one related to intervention do(X = 1)), as
illustrated in Fig. 1.3(iii). The conclusion following from this counterfactual analysis isclear: even if we had given the treatment to everyone who did not survive, only around
2% would have survived. In other words, the drug would not have prevented their death.
Another aspect of this situation worth examining is whether the treatment would have been
harmful for those who did not get it and still survived, formally written in layer 3 languageas P(YX=1 = 1|X = 0,Y = 1). Following the same procedure, we find that this quantity is
0.1079, which means that about 90% of such people would have died had they been given thetreatment. While a uniform policy over the entire population would be catastrophic (as shownin Example 4), the physicians knew what they were doing in this case and were effective inchoosing the treatment for the patients who could benefit more from it.
18

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

The probability of necessity, P(y0 x0 | x, y), encodes how a disease (Y ) is “attributable” toa particular exposure (X), interpreted counterfactually as “the probability that disease would
not have occurred in the absence of exposure, given that disease and exposure did in factoccur.” Conversely, the probability of sufficiency, P(yx | x0 , y0 ), captures how a certain exposure might impact the healthy population, counterfactually, “whether a healthy unexposed
individual would have contracted the disease had they been exposed.” There are many othercounterfactual quantities implied by a structural model, for example, the previous two quantities can be combined to form the probability of necessity and sufficiency (PNS) [Pearl 2000,
Ch. 9], counterfactually written as P(yx , y0x0 ). The PNS encodes the extent to which a certain
treatment to a particular outcome would be both necessary and sufficient, that is, the probability that Y would respond to X in both of the ways described above. This quantity addressesa quintessential “why” question, where one wants to understand what caused a given event.
Still in the purview of Layer 3, some critical applications demand that counterfactuals be
nested inside other counterfactuals. For instance, consider the quantity Yx, Mx0 that represents
the counterfactual value of Y had X been x, and M had whatever value it would have taken
had X been x0 . In words, for Y the value of X is x, while for M the value of X is x0 . This
type of nested counterfactual allows us to write contrasts such as P[Yx, Mx − Yx, Mx0 ], the socalled indirect effect on Y when X changes from x0 to x [Pearl 2001]. The use of nested
counterfactuals led to a natural and general treatment of direct and indirect effects, including
a precise understanding of their relationship in non-linear systems, epitomized through what
is known as the mediation formula [Pearl 2012, VanderWeele 2015].20 Overall, counterfactual
statements are central to our ability to explain how and why certain events come about
in the world. Indeed, explanations have practical ramifications for credit assignment, thedetermination of blame and responsibility, the analysis of biases and unfairness in decisionmaking, and, more broadly, the systematic understanding of the world around us.

1.3

Pearl Hierarchy – A Logical Perspective
We have thus far learned that each layer of the PCH corresponds to a different intuitive notion
in human cognition: seeing, acting, and imagining. Table 1.1 presents characteristic questions
associated with each of the layers. Layer 1 concerns questions like, “How likely is Y giventhat I have observed X?” In Layer 2 we can ask hypothetical (“conditional”) questions suchas, “How likely would Y be if one were to make it the case that X?” Layer 3 takes us evenfurther, allowing questions like, “Given that I observed X and Y , how likely would Y havebeen if X 0 had been true instead of X?”
What does the difference among these questions amount to, given that an SCM answers allof them? Implicit in our presentation was a series of increasingly complex symbolic languages
20 This result can be generalized to disentangle X − Y variations of any nature, including direct, indirect, and also

spurious [Zhang and Bareinboim 2018c]. This treatment led to the explanation formula [Zhang and Bareinboim
2018a,b], which has implications for fairness analysis and attribution in the context of non-manipulable variables.

1.3 Pearl Hierarchy – A Logical Perspective 19(Defs. 2, 5, 7). Each type of question above can be phrased in one of these languages, theanalysis of which reveals a logical perspective on PCH. We begin our analysis by isolating
the syntax of these systems explicitly. We define three languages L1 , L2 , L3 , each basedon polynomials built over basic probability terms P(α), the only difference among them
being which terms P(α) we allow: as we go up in the PCH, we allow increasingly complex
expressions α to appear in the probability terms. In particular, L1 is just a familiar probabilistic
logic (see, e.g., [Fagin et al. 1990]).
Definition 8 (Symbolic Languages L1 , L2 , L3 ). Let variables V be given and X, Y, Z ⊆ V.
Each language Li , i = 1, 2, 3, consists of (Boolean combinations of) inequalities between
polynomials over terms P(α), where P(α) is an Li term, defined as follows:

L1 terms are those of the form P(Y = y), encoding the probability that Y take on values y;
L2 terms additionally include probabilities of conditional expressions, P(Yx = y), giving
the probability that variables Y would take on values y, were X to have values x;

L3 terms encode probabilities over conjunctions of conditional (that is, L2 ) expressions,
P(Yx =y, . . . , Zw =z), symbolizing the joint probability that all of these conditional statements hold simultaneously.

For concreteness, a typical L1 sentence might be P(X = 1,Y = 1) = P(X = 1)P(Y = 1).
The L1 conjunction over all such combinations
P(X = 1,Y = 1) = P(X = 1)P(Y = 1) ∧ P(X = 1,Y = 0) = P(X = 1)P(Y = 0)
∧ P(X = 0,Y = 1) = P(X = 0)P(Y = 1) ∧ P(X = 0,Y = 0) = P(X = 0)P(Y = 0) (1.18)
would express that X and Y are probabilistically independent if X and Y are binary variables.
Of course, we would ordinarily write this simply as P(X,Y ) = P(X)P(Y ).
In L2 we have sentences like P(YX=1 = 1) = 3/4, which intuitively expresses that theprobability of Y taking on value 1 were X to take on value 1 is 3/4.21 As before, we could
also write this as P(Y = 1 | do(X = 1)) = 3/4. Finally, L3 allows statements about joint
probabilities over conditional terms with possibly inconsistent subscripts (also known asantecedents in logic). For instance, P(yx , y0x0 ) ≥ P(y | x) − P(y | x0 ) is a statement expressing alower bound on the probability of necessity and sufficiency.22
21 These “conditional” expressions such as Y
X=1 = 1 are familiar from the literature in conditional logic. In David
Lewis’s early work on counterfactual conditionals, YX=1 = 1 would have been written X = 1  Y = 1 (see[Lewis 1973]). More recently, some authors have used notation from dynamic logic, [X = 1]Y = 1, with the sameinterpretation over SCMs (see, e.g., [Halpern 2000]). For more discussion on the connection between the present
SCM-based interpretation and Lewis’s “system-of-spheres” interpretation, we refer readers to [Pearl 2000, § 7.4.17.4.3] and [Briggs 2012, Halpern 2013, Zhang 2013]. A third interpretation is over (probabilistic) “simulation”
programs, which under suitable conditions are equivalent to SCMs – see [Ibeling and Icard 2018, 2019, 2020].
22 For details of this bound and the assumptions guaranteeing it, see [Pearl 2009, Thm. 9.2.10]. Formally speaking,statements such as this one involving conditional probabilities are shorthand for polynomial inequalities; in this casethe polynomial inequality is P(yx , y0x0 )P(x)P(x0 ) + P(x0 , y)P(x) ≥ P(x, y)P(x0 ).

20

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

Def. 8 gives the formal structure (syntax) of L1 , L2 , L3 , but not their interpretation ormeaning (semantics). In fact, we have already specified their meaning in SCMs via Defs.
2, 5, 7. Specifically, let Ω denote the set of all SCMs over endogenous variables V. Then eachM ∈ Ω assigns a real number to P(α) for all α at each layer, namely the value PM (α) ∈ [0, 1].
Given such numbers, arithmetic and logic suffice to finish evaluating these languages. Thusin each SCM M , every sentence of our languages, such as (1.18), comes out true or false.23
At this stage, we are ready to formally define the Pearl Causal Hierarchy:
Definition 9 (Pearl Causal Hierarchy). Let M ∗ be a fully specified SCM. The collection ofobservational, interventional, and counterfactual distributions induced by M ∗ , as delineatedby languages L1 , L2 , L3 (syntax) and following Defs. 2, 5, 7 (semantics), is called the Pearl
Causal Hierarchy (PCH, for short).

In summary, as soon as we have an SCM, the PCH is thereby well-defined, in the sensethat this SCM provides valuations for any conceivable quantity in these languages L1 , L2 , L3(of associations, interventions, and counterfactuals, respectively). It therefore makes sense to
ask about properties of the hierarchy for any given SCM, as well as for the class Ω of all
SCMs. One substantive question is whether the PCH can be shown strict.
If we take L1 terms to involve a tacit empty intervention, i.e., that P(y) means P(y∅ ), thenthe formal syntax of this series of languages clearly forms a strict hierarchy L1 ( L2 ( L3 :there are patently L2 terms that do not appear in L1 (e.g., P(yx )), and L3 terms that do notappear in L2 (e.g., P(yx , y0x0 )). One has the impression that each layer of the Pearl hierarchy
is somehow richer or more expressive than those below it, capable of encoding information
about an underlying ground truth that surpasses what lower layers can possibly express. Is
this an illusion, the mere appearance of complexity, or are the concepts expressed by thelayers in some way fundamentally distinct?24 The sense of strictness that we would like tounderstand concerns the fundamental issue of logical expressiveness. If each language did not
expressively exceed its predecessors, then in some sense, our talk of causation and imagination
would be no more than mere figures of speech, being fully reducible to lower layers.
What would it mean for the layers of the hierarchy not to be distinct? Toward clarifying
this, let us call the set of all layer i (Li ) statements that come out true according to someM ∈ Ω the Li -theory of M . We shall write M ∼i M 0 for M , M 0 ∈ Ω to mean that their Li 23 Building on the classic axiomatization for (finite) deterministic SCMs [Galles and Pearl 1998, Halpern 2000], theprobabilistic logical languages L1 , L2 , L3 were axiomatized over probabilistic SCMs in [Ibeling and Icard 2020]. The
work presented in this chapter – including Def. 9 and Thm. 1 (below) – can be cast in axiomatic terms, although theseresults do not depend in any direct way on questions of axiomatization.
24 As a rough analogy, consider the ordinary concepts of ‘cardinality of the integers,’ ‘cardinality of the rational
numbers,’ and ‘cardinality of the real numbers.’ One’s first intuition may be that these are three distinct notions, andmoreover that they form a kind of hierarchy: there are strictly more rational numbers than integers, and strictly more
real numbers than rational numbers. Of course, in this instance the intuition can be vindicated in the second case but
dismissed as an illusion in the first. (See, e.g., Cantorian arguments from any basic textbook in logic or CS.)

1.3 Pearl Hierarchy – A Logical Perspective 21

theories coincide, i.e., that M , M 0 agree on all layer i statements. Intuitively, M ∼i M 0 saysthat M and M 0 are indistinguishable given knowledge only of Li .
For the remainder of this section assume that the true data-generating process M ∗ is fixed.
Suppose we had that M ∗ ∼2 M implies M ∗ ∼3 M for any other SCM M ∈ Ω; that is, any
SCM M which agrees with M ∗ on all L2 valuations also agrees on all of the L3 valuations.25
This would mean that the collection of L2 facts fully determines all of the L3 facts. Morecolloquially, if this happens, it means that we can answer any L3 question – including any
counterfactual question, e.g., the exact value of P(yx | y0x0 ) – merely from L2 information. For
instance, simply construct any SCM M with the right L2 valuation (i.e., such that M ∼2 M ∗ )and read off the L3 facts from M .26 In this case it would not matter that M is not the true
data-generating process, as any differences would not be visible even at L3 . This can happen
in exceptional circumstances, for instance, if the functional relationship is deterministic:
Example 6. Consider the SCM M ∗ = hU = {U}, V = {X,Y }, F , P(U)i, whereX ← UF =,Y ← X(1.19)and U is distributed as a fair coin flip. Then, there is no model M that agrees with M ∗ on all
L1 and L2 valuations, but disagrees on L3 . To see why, consider any model M with structuralfunctions fX , fY . We must have fY (x, u) = x for any unit u, or else the L2 -probability P(yx )differs between M , M ∗ . The function fX is also easily determined by the L1 requirement that
P(x) match between M , M ∗ . This is enough to determine all L3 quantities.
An additional motivation for understanding when layers of the PCH might collapse
comes from the observation that, at least in some notable cases, adding syntactic complexity
does not genuinely increase expressivity. As an example, we could extend the language
L3 to allow more complex expressions. We discussed nested counterfactuals earlier in thischapter (Section 1.2), namely, statements such as P(Yx, Zx0 ), which can also be given a naturalinterpretation in SCMs. Such notions are of significant interest, but it can be shown that any
such statement is systematically reducible to a layer 3 statement. (See [Bareinboim et al.
2020a] Appendix B for details.) That is, for any statement ϕ involving nested counterfactual
expressions, there is an L3 statement ψ such that ϕ and ψ hold in exactly the same models.27
Such a result shows that adding nested counterfactuals, while providing a useful shorthand,
would not allow us to say anything about the world above and beyond what we can say in L3 .
25 For readers familiar with causal inference, this can be seen as a generalization of the notion of identifiability (e.g.,see [Pearl 2000, Def. 3.2.3]), where P represents all quantities in layer i, Q all quantities in layer j, and the set offeatures FM is left unrestricted (all in the notation of [Pearl 2000]). This more relaxed notion has a long history inmathematical logic, viz. Padoa’s method in the theory of definability [Beth 1956].
26 Alternatively, given the completeness results in [Ibeling and Icard 2020], one could axiomatically derive any L
3statement from appropriate L2 statements.
27 In logic we would say that nested counterfactuals are thus definable in L (see, e.g., [Beth 1956]).
3

22

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

Does something similar happen with Layers 1, 2, 3 themselves? How often might an L3 -theory
completely reduce to an L2 -theory, or an L2 -theory reduce to an L1 -theory?
In light of the foregoing, we can say exactly what it means for the PCH to collapse in agiven SCM M ∗ . Note that the quantification here is over the class of all SCMs in Ω, that is,
all SCMs with the same set of endogenous (i.e., observable) variables as M ∗ :Definition 10 (Collapse relative to M ∗ ). Layer j of the causal hierarchy collapses to Layer i,with i < j, relative to M ∗ ∈ Ω if M ∗ ∼i M implies that M ∗ ∼ j M for all M ∈ Ω. 28
In the Example 6 above, we would say that Layer 3 collapses to Layer 2. The significanceof the possibility of collapse cannot be overstated. To the extent that Layer 2 collapses to Layer
1, this would imply that we can draw all possible causal conclusions from mere correlations.
Likewise, if Layer 3 collapses to Layer 2, this means that we could make statements about
any counterfactual merely by conducting controlled experiments. Our main result can then be
stated (first, informally) as:
Theorem 1. [Causal Hierarchy Theorem (CHT), informal version] The PCH almost nevercollapses. That is, for almost any SCM, the layers of the hierarchy remain distinct.

What does almost-never mean? Here is an analogy. Suppose (fully specified) SCMs are
drawn at random from Ω. Then, the probability that we draw an SCM relative to which PCH
collapses is 0. This holds regardless of the distribution on SCMs, so long as it is smooth.
The CHT thus says in a general manner that there will typically be causal questions that
one cannot answer with knowledge and/or data restricted to a lower layer in the hierarchy.29
In fact, this can be seen as the formal grounding for the intuition behind the PCH as discussed
in [Pearl and Mackenzie 2018, Ch. 1]:
Corollary 1. To answer questions at Layer i, one needs knowledge at Layer i or higher.

With this intuitive understanding of the CHT, we now state the formal version and offer anoutline of the main arguments used in the proof. In order to state the theorem, note that ∼3
is an equivalence relation on Ω, inducing L3 -equivalence classes of SCMs. Under a suitableencoding, this space of equivalence classes can be seen as a convex subset of [0, 1]K , for K ∈ N.
This means we can put a natural (uniform) measure on the space of (equivalence classes) of
28 Equivalently, there does not exist M ∈ Ω such that M ∗ ∼ M but M ∗ 6∼ M . In other words, every layer j queryij
can be answered with suitable layer i data.
29 The CHT is not to be understood as a general impossibility result for causal inferences, quite the contrary, as will
become clear in the rest of this chapter. In fact, some of the most celebrated results in the field are precisely about
conditions under which these inferences are allowed from lower layer data together with minimal assumptions about
the underlying SCM. For instance, the investigation of the next section will be on conditions that could allow causal
inferences from lower level data combined with graphical assumptions of the underlying SCM; see, e.g., [Bareinboim
and Pearl 2016]. Another common thread in the literature is structural learning: adopting arguably mild assumptionsof minimality (e.g., faithfulness) one can often discover fragments of the underlying causal diagram (Layer 2) fromobservational data (Layer 1) [Peters et al. 2017, Spirtes et al. 2001, Zhang 2008].

1.3 Pearl Hierarchy – A Logical Perspective 23

SCMs. The theorem then states (for the complete proof and further details, we refer readers
to [Bareinboim et al. 2020a, Appendix A]):
Theorem 1. [Causal Hierarchy Theorem (CHT), formal version] With respect to theLebesgue measure over (a suitable encoding of L3 -equivalence classes of) SCMs, the subset
in which any PCH collapse occurs is measure zero.
It bears emphasis that the CHT is a theory-neutral result, in the sense that it makes
only minimal assumptions and only presupposes the existence of a temporal ordering of thestructural mechanisms – an assumption made to obtain unique valuations via Defs. 2, 5, 7.
In the remainder of this section, we would like to discuss the basic idea behind the CHT
proof. There are essentially two parts to the argument, one showing that L2 almost nevercollapses to L1 , the second showing that L3 almost never collapses to L2 . (It easily follows
that neither collapse occurs, almost always.) In both parts it suffices to identify some simple
property of SCMs that we can show is typical, and moreover sufficient to ensure non-collapse.
In fact, Layer 2 never collapses to Layer 1: for any SCM M ∗ there is always another
SCM M with the same L1 -theory but a different L2 -theory. In case there is any non-trivial
dependence in M ∗ , we can construct a second model M with a single exogenous variable
U and all endogenous variables depending only on U, such that M ∗ ∼1 M (cf. [Suppes and
Zanotti 1981]). On the other hand, if M ∗ has no variable depending on any other, it is possible
to induce such a dependence that nonetheless does not show up at Layer 1. (For full details of
the argument see [Bareinboim et al. 2020a, Appendix A]).
The case of Layers 2 and 3 is slightly more subtle. The reason is that adding or removing
arguments in the underlying functional relationships usually changes the corresponding causal
effect. Here we need to show that the equations of the true M ∗ can be perturbed in a way thatit does not affect any L2 facts but does change some joint probabilities over combinationsof potential responses. It turns out there are many ways to accomplish this goal. Examples
7 and 8 below demonstrate two quite different strategies. However, for the CHT we need asystematic method. One possibility – again, informally speaking – is to take two exogenous
variable settings that witness two different values for some potential response, and swap thesevalues with some sufficiently small probability (see [Bareinboim et al. 2020a, Lemma 3]). For
this to work, essentially all we need is for there to be at least some non-trivial probabilistic
relationship between variables. This property is quite obviously typical of SCMs. We illustrate
this method with our running Example 2 (Example 9 below).
Turning now to these examples, we start with a variation of a classic construction presented by Pearl himself [Pearl 2009, §1.4.4]. The example has been used to demonstrate theinadequacy of (causal) Bayesian networks (discussed further in the next section) for encoding
counterfactual information. Here we use it to illustrate a more abstract lesson, namely, thatknowing the values of higher layer expressions generically requires knowing progressively
more about the underlying SCM (Corollary 1).

24

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

Example 7. Let M ∗ = hU = {U1 ,U2 }, V = {X,Y }, F ∗ , P(U)i, whereX ← U
1F∗=.
Y ← U2(1.20)and U1 ,U2 are binary with P(U1 = 1) = P(U2 = 1) = 1/2. Let the variable X represent whether∗
the patient received treatment and Y whether they recovered. Evidently, PM (x, y) = 1/4 forall values of X,Y . In particular X,Y are independent. Now, suppose that we just observed∗
samples from PM and were confident, statistically speaking, that X,Y are probabilisticallyindependent. Would we be justified in concluding that X has no causal effect on Y ? If theactual mechanism happened to be M ∗ , then this would certainly be the case. However, thislayer 1 data is equally consistent with other SCMs in which Y depends strongly on X. Let M
be just like M ∗ , except with mechanisms:
X ← 1U1 =U2F =.
(1.21)
Y ← U1 + 1X=1,U =0,U =1
1

M∗

2

M∗

Then P (X,Y ) = PM (X,Y ), yet P (Y = 1 | do(X = 1)) = 1/2 since X does not affect Y
in M ∗ , while PM (Y = 1 | do(X = 1)) = 3/4. If M were the actual mechanisms, assigning
the treatment would actually improve the chance of survival. Thus, just as one cannot infer
causation from correlation, one cannot always expect to infer correlation from causation.
Having internalized this lesson that correlation and causation are distinct, one might
perform a randomized controlled trial and discover that all causal effects in this setting
trivialize – in particular, P(Y | do(X)) = P(Y ) – the treatment does not affect the chance ofsurvival at all. Suppose we observe patient S, who took the treatment and died. We might welllike to know whether S’s death occurred because of the treatment, in spite of the treatment,or regardless of the treatment. This is a quintessentially counterfactual question: given that S
took the treatment and died, what is the probability that S would have survived had they not
been treated? We write this as P(YX=0 = 1 | X = 1,Y = 0), as discussed in Example 4. Can
we infer anything about this expression from layer 2 information (in this case, that all causal
effects trivialize)? We cannot, as shown by other variations of M ∗ , say M 0 such that
X ← U
1
F0=.
(1.22)
Y ← XU2 + (1 − X)(1 −U2 )Like M , this model reveals a dependence of Y on X. However, this is not at all visible at Layer
1 or at Layer 2; all causal effects trivialize in M 0 as well – see Table 1.3. The dependence∗
only becomes visible at Layer 3. In M ∗ , we have PM (YX=0 = 1 | X = 1,Y = 0) = 0, whereas
0in M 0 we have the exact opposite pattern, PM (YX=0 = 1 | X = 1,Y = 0) = 1. These twomodels thus make diametrically opposed predictions about whether S would have survived
had they not taken the treatment. In other words, the best explanation for S’s death may be1.3 Pearl Hierarchy – A Logical Perspective 25

M∗
U1
0
0
1
1

U2
0
1
0
1

X
0
0
1
1

Y
0
1
0
1

YX=0
0
1
0
1

M0
YX=1
0
1
0
1

Y
1
0
0
1

YX=0
1
0
1
0

YX=1
0
1
0
1

P(u)
1/4
1/4
1/4
1/4

Table 1.3: Counterfactual evaluation of M ∗ and M 0 in Example 7.

completely different depending on whether the world is like M ∗ or M 0 . In M ∗ , S would havedied anyway, while in M 0 , S would actually have survived, if only they had not been given
the treatment. Needless to say, such matters can be of fundamental importance for criticalpractical questions, such as determining who or what is to blame for S’s death.

The CHT tells us that the failure of collapse witnessed in Example 7 is typical. However,
it is worth seeing further examples to appreciate the many ways we can take an SCM M ∗ andfind an alternative SCM M that agrees at all lower layers but disagrees at higher layers.
Example 8 (Example 1 continued). Let SCM M ∗ = M 1 from Example 1, and consider
another model M such that Y ← (U1 − U2 ) as before, but now X ← (2U1 − Y ). It is then∗easy to see that PM (X,Y ) = PM (X,Y ), that is, M ∗ ∼1 M . However, at Layer 2 we have∗∗1
PM (X = 10 | do(Y = 0)) = PM (X = 10) = 12
6= 16 = PM (X = 10 | do(Y = 0)). There is no∗causal relationship between X and Y in M , while Y exerts a clear influence on X in M , onethat is simply not visible at Layer 1.
In a similar vein, we can find another SCM M 0 with M ∗ ∼2 M 0 but M ∗ 6∼3 M 0 . In
0 , . . . ,U 0 , . . . ,U 0 , with each U distributed as
M 0 , let us add new exogenous variables U−5
k
0
5
∗
M
P (X | (U1 −U2 ) 6= k), and let:X ← 1Y =(U1 −U2 ) (U1 +U2 ) + ∑−5≤k≤5 1Y =k6=(U1 −U2 )Uk
F0=.
(1.23)
Y ← U1 −U20∗Then, M ∗ ∼1 M 0 – in fact, M ∗ ∼2 M 0 , since PM (XY =k ) = PM (X, (U1 − U2 ) = k) +∗∗∗∗PM (X, (U1 − U2 ) 6= k) = PM (X) = PM (XY =k ). However, e.g., PM (XY =0 = 7 | Y = 5) =∗0∗
PM (X = 7 | U1 = 6,U2 = 1) = 1 6= 51 = PM (X = 7 | U1 6= U2 ) = PM (XY =0 = 7 | Y = 5). 
Finally, for this last illustration we see two quite different patterns. To show that Layer
2 does not collapse to Layer 1 we actually eliminate the functional dependence of one
variable on another – all probabilistic dependence patterns are due to common causes. Moreinterestingly, we employ a very general method to show that Layer 3 does not collapse toLayer 2, whose efficacy is proven systematically in [Bareinboim et al. 2020a, Lemma 3].

26

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

Example 9 (Example 2 continued). For the SCM M ∗ = M 2 of Example 2, consider anothermodel M with the equation for Y replaced by a new equation Y ← 1{Ur =1,Ux =1,Uz =1} +1{Ur =1,Ux =0,Uz =0} + 1{Ur =1,Ux =0,Uy =1,Uz =1} + 1{Ur =1,Ux =1,Uy =1,Uz =0} + 1{Ur =1,Ux =1,Uy =0} , andeverything else unchanged. It is then easy to check that M ∗ ∼1 M . However, Y now no longer
shows a functional dependence on X: the probabilistic dependence of Y on X is due to the∗∗common causes Ux ,Uz ,Ur . While in Example 4 we saw that PM (Y | X) 6= PM (Y | do(X)),
here we have PM (Y | X) = PM (Y | do(X)). In other words, even though X does exert a causal
influence on Y (assuming M ∗ is the true data-generating process), we would not be able toinfer this from observational data alone.
To show that Layer 3 does not collapse to Layer 2, consider a third model M 0 , in which
X,Y, Z all share one exogenous parent U, with Val(U) = {0, 1}4 ∪ {u∗1 , u∗2 }. The probabilityof a quadruple hur , uz , ux , uy i in this model is simply given by the product from model M ∗— P(Ur = ur ) · P(Uz = uz ) · P(Ux = ux ) · P(Uy = uy ) — as calculated explicitly in Table 1.2,
with one exception: for the two quadruples, h1, 1, 1, 0i and h1, 1, 0, 0i, we subtract ε = .005
from these probabilities, and redistribute the remaining mass so that u∗1 and u∗2 each receive
probability ε. This produces a proper distribution P0 (U). We will continue write, e.g., Ur = u
simply to mean that U 6= u∗1 , u∗2 and the first coordinate of U is u, and similarly for Uz ,Ux ,Uy .
The causal mechanisms can now be written:
Z ← 1{Ur =1,Uz =1} + 1U∈{u∗1 ,u∗2 }0. (1.24)
F = X ← 1{Z=1,Ux =1} + 1{Z=0,Ux =0} + 1U=u∗2Y ← 1{X=1,Ur =1} +1{X=0,Ur =1,Uy =1} +1{X=0,Ur =0,Uy =0} +1{X=1,U∈{u∗1 ,u∗2 }}∗0

To check that the joint distributions PM (X,Y, Z) and PM (X,Y, Z) are the same, note thatthe two models coincide at all exogenous settings with the exception of the two quadruples
h1, 1, 1, 0i and h1, 1, 0, 0i. In the first we have Z = X = Y = 1 (recall Table 1.2), and the εloss in probability for this possibility is corrected by the fact that X(u∗2 ) = Y (u∗2 ) = Z(u∗2 ) = 1
and P0 (u∗2 ) = ε. Similarly for h1, 1, 0, 0i and the state Z = 1, X = Y = 0, which results whenU = u∗1 . To show that M ∗ ∼2 M 0 is also straightforward.
However, consider the L3 expression YZ=1 =1,YZ=0 =1, which says that the patient would
survive no matter whether hypertension was induced or prevented. For both exogenous
settings h1, 1, 1, 0i and h1, 1, 0, 0i, this expression is false, yet in setting u∗2 the expression
0∗is true. Hence, PM (YZ=1 = 1,YZ=0 = 1) = PM (YZ=1 = 1,YZ=0 = 1) + ε.

Whereas Example 6 shows that collapse of the layers is possible if M ∗ is exceptional,
the CHT shows that this is the exception indeed. Typical cases are like Examples 7, 8, 9,each showing a different way of perturbing an SCM to obtain a second SCM revealing noncollapse. As these examples demonstrate, a typical data-generating process M ∗ encodes richinformation at all three layers, and even small changes to the mechanisms in M ∗ can have

1.4 Pearl Hierarchy – A Graphical Perspective

27

substantial impact on quantities across the hierarchy. Critically, such differences will often be
visible only at higher layers in the PCH.
The lesson learned from the CHT is clear – since the layers of PCH come apart in thegeneric case and one cannot make inferences at one layer given knowledge at lower layers(e.g., using observational data to make interventional claims), some additional assumptions
are logically necessary if one wants in general to do causal inference.

1.4

Pearl Hierarchy – A Graphical Perspective
All conceivable quantities from any layer of the PCH – associational, interventional, andcounterfactual – are immediately computable once the fully specified SCM is known. Unfortunately, we usually cannot determine the structural model at this level of precision in mostpractical settings, and the CHT severely curtails the ability to “climb up” the PCH via lowerlevel data. Still, learning about cause and effect relationships is arguably one of the main goals
found throughout the empirical sciences. How might we progress?
The recognition that there are mechanisms underlying the phenomena of interest, but that
we usually cannot determine them precisely, gives rise to the discipline of causal inference[Pearl 2000]. Virtually every approach to causal inference works under the stringent condition
that only partial knowledge of the underlying SCM is available. In particular, our goal here isto understand the conditions under which valid causal claims can be made (following thesemantics given by the SCM) when the data scientist does not have direct access to theunderlying causal mechanisms (the SCM itself), but only some features and data thereof. For
instance, one typical task is to determine the effect of an intervention – what would happen
with Y were X to be intervened on and set to x, P(Y | do(X = x)) – from observational data,
P(X,Y ). This constitutes a cross-layer inference where the goal is to use data from layer L1 to
try to make an inference about an L2 quantity, given a partial specification of the underlying
SCM (see Fig. 1.4(a-d)).
In this section, we investigate the question of what type of causal knowledge could be (1)intuitively meaningful, (2) possibly available, and (3) powerful enough to encode constraints
that would allow cross-layer inferences, as if the SCM were itself available. A key observation
useful to answer this question is that each SCM imprints specific “marks” on the distributions
it generates, depicted generically in the schema in Fig. 1.4(d) as structural constraints.

1.4.1

Causal Inference via L1 -constraints
Following this discussion, a first attempt to solve our task could be to leverage L1 -constraints,
those imprinted on the observed L1 data by the unknown SCM, to make inferences about
the target L2 -quantity. This is especially appealing considering that L1 data is often readily
available. The signature type of constraint for L1 distributions is known as conditional
independence, and Bayesian Networks (BNs) are among the most prominent formal models

28

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

Graphical Models(a) SCM(Unobserved Nature)

(d)
(
X

← fX (Ux )Y← fY (X,Uy )Structural
Constraints(L1 ) Bayesian Networks(L2 ) Causal Bayesian Networks(L3 ) Functional Networks

P(Ux ,Uy )(L1 ) P(Y, X)(L2 ) P(Y |do(x))(b) Data(c) Query

Figure 1.4: Example of Prototypical Causal Inference – on top the SCM itself, representing
the unobserved collection of mechanisms and corresponding uncertainty (a); on the bottom,
the different probability distributions entailed by the model (b,c); on the right side, thegraphical model representing the specific constraints of the SCM (d).

used to encode this type of knowledge.30 The example below shows that L1 constraints (andBNs) alone are insufficient to support causal reasoning in general.
Example 10. Let M 1 and M 2 be two SCMs such that V = {X, Z,Y }, U = {Ux ,Uz ,Uy }, andthe structural mechanisms are, respectively,X ← Z ⊕Ux
X ← Ux(1.25)
F2 = Z ← Y ⊕Uz ,F1 = Z ← X ⊕Uz ,






Y ← Z ⊕Uy
Y ← Uywhere ⊕ is the logical xor operator. Further, the distributions of the exogenous variables are
P1 (Ux = 1) = P2 (Uy = 1) = 1/2, P1 (Uz = 1) = P2 (Ux = 1) = a, and P1 (Uy = 1) = P2 (Uz =1) = b, for some a, b ∈ (0, 1). It is immediate to see (via Def. 2 and Eq. (1.3)) that both models
generate the same observational distribution,
P1,2 (X = 0, Z = 0,Y = 0) = P1,2 (X = 1, Z = 1,Y = 1) = (1 − a)(1 − b)/2,
P1,2 (X = 0, Z = 0,Y = 1) = P1,2 (X = 1, Z = 1,Y = 0) = (1 − a)b/2,
30 This formalism is especially effective for encoding conditional independences in a parsimonious way, capable ofspanning the (exponentially many) constraints implied by it. This encoding leads to efficiency gains in the context
of probabilistic reasoning in terms of both computational and sample complexity. Due to space constraints, we referreaders to [Bareinboim et al. 2020a, Appendix C], where a more detailed account of the semantics and constructionof L1 -models is provided. Understanding the relationship between a fully specified SCM and its corresponding BNshould be helpful for comprehending the nature of the constraints and characteristic models of the other layers.

1.4 Pearl Hierarchy – A Graphical Perspective

Structural Causal Models(Unobserved)

M∗
X

Z

Y

29

Bayesian Network

G∗
G0

M0

(a) G 1X

Z(b) G 2Y

P0(Y|do(x))
P∗(Y|do(x))P∗(V)=P0(V)Observational(L1 ) Distributions
Data?

Interventional(L2 ) Distributions
Query

(c)

Figure 1.5: Two causal diagrams encoding knowledge about the causal mechanisms governing
three observable variables X, Z and Y . In (a) X is an argument to fZ , and Z an argument to fY .
In (b) the opposite is true. In (c), schema representing the impossibility of identifying causal
queries from L1 data, constraints, and graphical models.

P1,2 (X = 0, Z = 1,Y = 1) = P1,2 (X = 1, Z = 0,Y = 0) = a(1 − b)/2,
P1,2 (X = 0, Z = 1,Y = 0) = P1,2 (X = 1, Z = 0,Y = 1) = ab/2.

(1.26)

We further compute the effect of the intervention do(x) (via Def. 5 and Eq. 1.8),
P1 (Y = 1 | do(X = 1)) = ab + (1 − a)(1 − b),
P2 (Y = 1 | do(X = 1)) = 1/2,

(1.27)

which are different for most values a, b. The models M 1 and M 2 naturally induce BNs G 1
and G 2 , respectively, see Figs. 1.5(a) and (b).31 In terms of L1 -constraints, G 1 and G 2 both
imply that X is independent of Y given Z (for short, X ⊥⊥ Y | Z) and nothing more.32 Thismeans that G 1 and G 2 are equivalent through the lens of L1 , while the original M 1 and M 2
generate different answers to L2 queries, as shown in Eqs. (1.27).
The main takeaway from the example is that, from only the distribution P(V) and thequalitative (conditional independence) constraints implied by it, it is impossible to tell whether
the underlying reality corresponds to M 1 , M 2 , or any other SCM inducing the same P(V).
And yet, each such model could entail a different causal effect. This suggests that, in general,
31 This construction follows from the order in which the functions are determined in the SCM, systematized inDef. 24([Bareinboim et al. 2020a, Appendix C]). This procedure is guaranteed to produce BNs that are compatible
with the independence constraints implied by the SCM in L1 ([Bareinboim et al. 2020a, Thm. 8, Appendix C]).
32 We refer readers to [Bareinboim et al. 2020a, Appendix C, Definition 23], for more details on a criterion called
d-separation [Pearl 1988], which is the tool used for reading these constraints off from the graphical model.

30

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

causal inference cannot be carried out with mere L1 objects – the observational distribution,
its constraints, and corresponding models (Bayesian networks). This result can be seen as agraphical instantiation of Corollary 1 and is schematically summarized in Fig. 1.5(c).

It is sometimes believed in the literature that Bayesian networks might provide a basis forcausal reasoning, but as this example suggests, such an assumption is unfounded even forthe simplest models. Pearl himself acknowledged this impossibility, which was one of the
motivating factors for his journey towards the principles and additional constraints needed
to support causal inference; in his own words [Pearl 2018a, pp. 7]: “I had been under the
impression that probabilities could express every aspect of human knowledge, even causality,
but they can’t. After I realized this, my research shifted completely to a new direction, as
I tried to understand how to represent causal knowledge and draw causal conclusions.” Hefurther hints at the importance of causal Bayesian networks, the very topic of the next section:“In retrospect, fighting for the acceptance of Bayesian networks was a picnic – no, a luxury
cruise! – compared with the fight I had to go through for causal Bayesian networks.”
1.4.2

Causal Inference via L2 -constraints – Markovian Causal Bayesian Networks
Having witnessed the impossibility of performing causal inference from L1 constraints,
we come back to the original question – what kind of structural constraints (Fig. 1.4(d))
imprinted by the underlying SCM could license causal inferences? To answer this question, it
is instructive to compare more closely the effect of an intervention do(X = 1) in the two SCMs
2
from Example 10. First, note that the function fY does not depend on X in the submodel MX=1
(constructed following Def. 3), so, probabilistically, Y will not depend on X. This implies thefollowing relationship between distributions,
P2 (Y = 1 | do(X = 1)) = P(Y = 1),(1.28)

1 , and (ii) Y responds
In contrast, note that (i) fY does take into account the value of X in MX=1
(or varies) in the same way when X takes a particular value, be it naturally (as in M 1 ) or due
1 ). These facts can be formally written as
to an intervention (as in MX=1P1 (Y = 1 | do(X = 1)) = P(Y = 1 | X = 1).

(1.29)The exact computation of Eqs. (1.28) and (1.29) follows immediately from Defs. 2 and 5. Remarkably, the intuition behind these equalities does not arise from the particular form of theunderlying functions, the exogenous variables, or their distribution, but from structural properties of the model. In particular, they are determined by qualitative functional dependencesamong the variables: what variable is an argument to the function of the other.
Technically, these equalities can be seen as constraints (but not mere conditional independences) and can further be pieced together, and given a graphical interpretation. Consider
again the Eq. (1.28) as an example, which says that variable X does not have an effect on

1.4 Pearl Hierarchy – A Graphical Perspective31

Y (doing X does not change the marginal distribution of Y ), which graphically would entailthat X is not an ancestor of Y in G 2 . While true in M 2 , it certainly does not hold in M 1 ,nor, consequently, in G 1 . Even though G 1 and G 2 are graphically equivalent with respect to
L1 , and could be used interchangeably for probabilistic reasoning, they are, interventionally
speaking, very distinct objects.
These constraints encode one of the fundamental intuitions we have about causality,
namely, the asymmetry that a cause may change its effect but not the other way around. Our
goal henceforth will be to systematically incorporate these constraints into a new family ofgraphical models with arrows carrying causal meaning and supporting L2 -types of inferences.
First, we introduce a procedure that returns a new graphical model following the intuition
behind the constraints discussed so far, and then show how it relates to the collection ofinterventional distributions (L2 -valuations) entailed by the SCM.
Definition 11 (Causal Diagram (Markovian Models)). Consider a Markovian SCM M =hU, V, F , P(U)i. Then, G is said to be a causal diagram (of M ) if constructed as follows:(1) add a vertex for every endogenous variable in the set V,(2) add an edge (V j → Vi ) for every Vi ∈ V if V j appears as an argument of fi ∈ F .

The procedure encapsulated in Def. 11 is central to the elicitation of the knowledge
necessary to perform causal inference (Fig. 1.4(d)). Intuitively, G has an arrow from A to
B (A → B) if B “listens” to the value of A; functionally, A appears as an argument of themechanism of B. The importance of this notion has been emphasized in the literature by Pearl:“This listening metaphor encapsulates the entire knowledge that a causal network conveys;
the rest can be derived, sometimes by leveraging data.” [Pearl and Mackenzie 2018, pp. 129].
This construction produces a coarsening of the underlying SCM such that the arguments ofthe functions are preserved while their particular forms are discarded.33,34
The assumptions that the causal diagram encodes about the SCM impose constraints
not only over the L1 -distribution P but also over all the interventional (L2 ) distributions as
encapsulated in the following definition [Bareinboim et al. 2012].
Definition 12 (Causal Bayesian Network (CBN - Markovian)). Let P∗ be the collection of allinterventional distributions P(V | do(x)), X ⊆ V, x ∈ Val(X), including the null intervention,
P(V), where V is the set of observed variables. A directed acyclic graph G is called a Causal
Bayesian Network for P∗ if for all X ⊆ V, the following conditions hold:(i) [Markovian] P(V | do(x)) is Markov relative to G .
33 Given the lack of constraints over the form and shape of the underlying functions and distribution of the exogenous

variables, these models are usually called non-parametric in the causal inference literature.
34 Here is where we depart ways with the tradition in, e.g., the physical sciences, of focusing on the precise functional
relationships among variables, and move to a more relaxed approach where only the qualitative relationships between
them are invoked; obviously, the former is more powerful than the latter, even though not always realizable.

32

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference(ii) [Missing-link] For every Vi ∈ V, Vi ∈/ X such that there is no arrow from X to Vi in G :P(vi | do(pai ), do(x)) = P(vi | do(pai )).

(1.30)(iii) [Parents do/see] For every Vi ∈ V, Vi ∈/ X:
P(vi | do(x), do(pai )) = P(vi | do(x), pai ).

(1.31)
The first condition requires the graph to be Markov relative35 to every interventional distribution P(V | do(X = x)) that holds if every variable is independent of its non-descendants
given its parents.36 The second condition, missing-link, encapsulates the type of constraint
exemplified by Eq. (1.28): after fixing the parents of a variable by intervention, the corresponding function should be insensitive to any other intervention elsewhere in the system. In
words, the parents Pai interventionally shield Vi from interventions (do(X)) on other variables.
Finally, the third condition, parents do/see, encodes the intuition behind Eq. (1.29): whether
the function fi takes the value of its arguments following an intervention (do(Pai = pai )) orby observation (conditioned on Pai = pai ), the same behavior for Vi is observed.37
Some observations follow immediately from these conditions. First, and perhaps not
surprisingly, a CBN encodes stronger assumptions about the world than a BN. In fact,
all the content of a BN is encapsulated in condition (i) of a CBN (Def. 12) with respect
to the observational (null-intervention) distribution P(V) (L1 ). A CBN encodes additional
constraints on interventional distributions (L2 ) beyond conditional independence, involving
different interventions such as those represented ii conditions (ii) and (iii).
Second, readers familiar with graphical models will be quick to point out that the knowledge encoded in these models is not in the presence, but in the absence of the arrows; eachmissing arrow makes a claim about a certain type of invariance. In the context of Bayesian
networks (L1 ), each missing arrow corresponds to a conditional independence, a probabilistic
type of invariance.38 On the other hand, each missing arrow in a CBN represents an L2 -type
constraint, for example, the lack of a direct effect, as encoded in Def. 12 through cond. (ii).
This new family of constraints closes a long-standing semantic gap, from a graphical model’s
perspective, rendering the causal interpretation of the graphical model totally unambiguous.
Before proving that this graphical model encapsulates all the probabilistic and causal
constraints required for reasoning in L2 , we show next that the L2 -empirical content of an
35 This notion is also known in the literature as compatibility or i-mapness [Koller and Friedman 2009, Pearl 1988],

which is usually encoded in the decomposition of P(v) as ∏i P(vi |pai ) in the Markovian case.
36 In some accounts of causation, this condition is known as the causal Markov condition (CMC), and it usuallyphrased in terms of “causal” parents. We invite the reader to check that conditions (ii), (iii) are in no way implied by(i). One could in fact see Def. 12 as offering a precise characterization of what CMC formally means.
37 Conds. (ii) and (iii) are equivalent to a condition called modularity in [Pearl 2000, pp. 24], i.e., P(v |do(x), pa ) =i
i
P(vi |pai ). These definitions were shown equivalent to the truncated product [Bareinboim et al. 2012].
38 One can show that there always exists a separator, in the d-separation sense, between non-adjacent nodes.

1.4 Pearl Hierarchy – A Graphical Perspective

33

SCM – i.e., the collection of observational and interventional distributions (Def. 5) – indeed
matches the content of the CBN (Def. 11), as defined above.
Theorem 2. [L2 -Connection – SCM-CBN (Markovian)] The causal diagram G induced by
the SCM M (following the constructive procedure in Def. 11) is a CBN for PM∗ – the collectionof observational and experimental distributions induced by M .
As this result demonstrates, CBNs serve as proxies for SCMs in terms of the observed
L2 distributions. 39 In practice, whenever the SCM is not fully known and the collection ofinterventional distributions is not available, this duality suggests that a CBN can act as a basis
for causal reasoning.40
To ground this point, we go back to our task of inferring the interventional distribution,
P(Y | do(X=x)), from a combination of the observational distribution, P(V), and the qualitative knowledge of the SCM encoded in the causal diagram G . A remarkable result that
holds in Markovian models is that causal inference is always possible, i.e., any interventional
distribution is computable from L1 -data.
Theorem 3 (Truncated Factorization Product (Markovian)). Let the graphical model G be
a CBN for the set of interventional distributions P∗ . For any X ⊆ V, the interventional (L2 )distribution P(V | do(x)) is identifiable through the truncated factorization product, namely,
P(v | do(x)) =∏ P(vi | pai ){i|Vi ∈X}/.
X=x

(1.32)
In other words, the interventional distribution in the l.h.s. of Eq. (1.32) can be expressedas the product given in the r.h.s. involving only L1 -quantities, where the factors relative tothe intervened variables are removed, hence the name truncated factorization product (see[Pearl 2000, Eq. 1.37]).41 Obviously, any marginal distribution of interest can be obtained bysumming out the irrelevant factors, including the causal effect of X on Y .
Corollary 2 (Back-door Criterion (Markovian)). In Markovian models (i.e., models without
unobserved confounding), for any treatment X and outcome Y, the interventional distribution
39 It can also be shown that there is an SCM inducing P and G for any CBN (G , P ) of a Markovian model.
∗∗
40 In fact, one could take an axiomatic view of the CBNs and consider alternative ways for satisfying their conditions,detached from the structural semantics. It is conceivable that other constructions that do not use the “listening” (i.e.,functional) metaphor as a building block, the signature of structural models, may also entail the same conditions of a
CBN, therefore, supporting valid cross-layer (causal) inferences. Due to space constraints, we present Algorithm 10
in [Bareinboim et al. 2020a], which constitutes one such alternative supporting a purely experimental construction of
a CBN. This procedure can be seen as the probabilistic-empirical counterpart of the SCM-functional Def. 11; see alsoThm. 10 in the corresponding Appendix. The same type of experimental construction can be explored in the context
of structure learning [Kocaoglu et al. 2017]; see also [Ghassami et al. 2018, Kocaoglu et al. 2019].
41 The truncated formula is also known as the “manipulation theorem” [Spirtes et al. 1993] or G-computation formula[Robins 1986, pp. 1423]. For further details, we refer readers to [Pearl 2000, § 3.6.4].

34

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

Structural Causal Models(Unobserved)

M∗

M∗

G

P∗(Y|do(x)) =P∗(V)=P0(V)Observational(L1 ) Distributions
Data

Structural Causal Models(Unobserved)

Causal Bayesian Network

M0

G

P0(Y|do(x))
P∗(Y|do(x))P∗(V)=P0(V)P0(Y|do(x))?

Causal Bayesian Network

M0

Interventional(L2 ) Distributions
Query

Observational(L1 ) Distributions
Data

(a)

?

Interventional(L2 ) Distributions
Query

(b)

Figure 1.6: Two identifiability scenarios. In some settings, the causal graph is sufficient forsolving this task (a), while in others the problem is unsolvable (b).

P(Y | do(x)) is always identifiable and given by the expression
P(Y | do(x)) = ∑ P(Y | x, z)P(z),(1.33)zwhere Z is the set of all variables not affected by the action X (non-descendants of X).



The importance of this result stems from the fact that the estimand in Eq. (1.33), calledadjustment, is generally used in many fields – viz., averaging the conditional probability ofY given X by the margins of the covariates. This corollary specifies a condition on its causal
validity; in words, if the set of covariates Z is constituted by all pre-treatment variables andthe model is Markovian (all relevant sources of variations are measured), adjusting for thesevariables will lead to the causal effect.42
1.4.3

Causal Inference via L2 -constraints – Semi-Markovian Causal Bayes Networks
The treatment provided for the Markovian case turned out to be simple and elegant, yetsurprisingly powerful. The causal graph is a perfect surrogate for the SCM in the sense thatall L2 quantities (causal effects) are computable from L1 -type of data (observational) andthe constraints in G . A “model-theoretic” way of understanding this result is summarized
in Fig. 1.6(a), which shows that all the SCMs that induce the same causal diagram andgenerate the same observational distribution will also generate the same set of experimental
distributions, immediately computable via the truncated product (Thm. 3). This is a quite
remarkable result since we moved from a model based on L1 -structural constraints (e.g., a
Bayes net) such that no causal inference was permitted, to a model encoding L2 -constraints(a Causal Bayes net) such that any conceivable cross-layer inference is immediately allowed.
42 A condition known as conditional ignorability follows, i.e., Y ⊥x ⊥ X | Z [Imbens and Rubin 2015, Rosenbaum andRubin 1983], whenever these conditions of the corollary are satisfied; see discussion in [Pearl 2000, § 11.3.2].

X

Y

(a)

X

Y

(b)

1.4 Pearl Hierarchy – A Graphical Perspective

35

X

Y

Y

(c)

X

Y

(d)

X

(e)

Figure 1.7: The diagram in (a) implies that neither X nor Y is an argument to the functionof the other. In (b, c) one endogenous variable causes the other. In (d) there is no causal
relationship yet the functions share exogenous arguments, as encoded through the bidirected
arrow. In (e) both types of influence are encoded.

In light of these results, one may be tempted to surmise that causal inference is a solved
problem. This could not be farther from the truth, unfortunately. The assumption that all the
relevant factors about the phenomenon under investigation are measured and represented in
the causal diagram (i.e., Markovianity holds) is often too stringent, and violated in most realworld scenarios. This means that the aforementioned results are usually not applicable inpractice. Departing from this observation, our goal is to understand the principles that allow
cross-layer inferences when the Markov condition does not hold, which entails incorporating
unobserved confounders as a building block of L2 -graphical models. We start by investigating
the reasons the machinery developed so far is insufficient to accommodate such cases.
Example 11 (Example 1 revisited). Recall the two-dice game where the endogenous variables
X and Y (the sum and difference of two dice, respectively) do not functionally depend on eachother, despite their strong association. One could attempt to model such a setting with thegraphical structure shown in Fig. 1.7(a), somewhat naively, trying to avoid a directed arrow
between X and Y . As previously noted, if the sum of the dice is equal to two (X = 2), one
could, with probability one, infer that the two dice obtained the same value (Y = 0). The
hypothesized graphical model, however, forces the two variables to be independent, which
would rule out the possibility of performing such an inference.
Upon recognition of such impropriety, one could reconsider adding an arrow from X to Y
(or Y to X) so as to leverage the valuable information shared across the observed variables,
as shown in Fig. 1.7(b). We previously learned, on the other hand, that reporting that the sumof the dice is 2 does not change their difference, formally, P(Y | do(X = 2)) = P(Y ) must
hold in this setting (Eq. 1.12). Obviously, this would be violated were the world to mirror thisgraphical structure. To witness, consider the alternative SCM M 0 , similar to the construction
discussed in Example 8, where the function for X is identical and Y ← (X − 2U2 ). We canverify that P(X,Y ) is the same as in M 1 , while the causal effect of X on Y is non-zero.
The recognition that certain dependencies among endogenous variables cannot be explained by other variables inside the model (but also cannot be ignored) led Pearl to introduce a new type of arrow to account for these relationships. The new arrows are dashed and36

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

bidirected. In the example above, variables X and Y are correlated due to the existence of two
common exogenous variables, {U1 ,U2 }, which are arguments of both fX and fY . We will usually refer to these variables as Uxy since, a priori, we will not know, neither want to assume,
their particular form, dimensionality, or distribution. This new type of arrow will allow for
the probabilistic dependence between them, (X ⊥6 ⊥ Y ), while being neutral with respect to theirinterventional invariance. That is, it would accept constraints such as P(Y |do(X)) = P(Y ) andP(X|do(Y )) = P(X). See Fig. 1.7(d) for a graphical example.43
In practice, some variables may be related through both sources of variations – one
exogenous, not explained by the variables in the model, and another endogenous, causally
explained by the relationships between the variables in the model, as shown in Fig. 1.7(e). Dueto the unobserved confounder Uxy , the equality P(Y | do(x)) = P(Y | x) will not, in general,
hold in this model. In words, Y ’s distribution will be different depending on whether weobserve X = x or intervene and do(X = x). Fundamentally, this will translate into a violationof the constraint encoded in Eq. (1.29) and, more generally, in cond. (iii) of the definition ofMarkovian CBNs (Def. 12).
Our goal henceforth will be to cope with the complexity arising due to violations ofMarkovianity. One particular implication of these violations is the widening of the empiricalcontent carried by the CBN versus its underlying SCM, as shown in the next example.
Example 12. Consider two SCMs M ∗ and M 0 such that V = {X,Y }, U = {Uy0 ,Uy1 ,Uxy },the structural mechanisms are:
X ← Uxy
F∗=,(1.34)
Y ← (Uy ∧ X) ∨ (Uy ∧ ¬X) ∨ (Uxy ⊕ X)
10

X ← Uxy
F0=.
(1.35)
Y ← (Uy ∧ X) ∨ (Uy ∧ ¬X) ∨ ((Uxy ⊕ X) ∧Uy )1

0

1

The exogenous distributions of both models, P∗ (U) and P0 (U), are the same and given by
P(Uxy = 1) = 1/2, P(Uy0 = 1) = 4/5, P(Uy1 = 1) = 1/4, and they both follow the diagram
shown in Fig. 1.7(e). It is easy to verify that both models induce the same P(V), whileP∗ (Y = 1 | do(X = 1)) = 5/8 6= 1/4 = P0 (Y = 1 | do(X = 1)).
Remarkably, this is our first encounter with a situation in which a causal diagram – encoding all the L2 -structural invariances of the underlying SCM M ∗ – is too weak, incapable ofanswering the intended cross-layer inference – computing P(Y | do(x)) from the correspond43 This constitutes a subtle distinction between L and L views of the underlying SCM. If we consider an SCM
12such that X and Y are not causally related but share an unobserved common cause, they will appear in L1 as

Fig. 1.7(a) (or with the arrow reversed), since no prediction about the effect of interventions could be made, while inL2 this unobserved confounder needs to be encoded through a bidirected arrow, such as in Fig. 1.7(d). The existenceof minimal i-maps and the obedience of the corresponding blankets is somewhat more subtle, however, which is
discussed in more detail in [Bareinboim et al. 2020a, Example 16 in Appendix C].

1.4 Pearl Hierarchy – A Graphical Perspective

37

ing L1 -distribution, P(X,Y ). There exists at least one other SCM M 0 (Eq. 1.35) that shares
the same set of structural features, in the form of the constraints encoded in the causal diagram, but generates a different answer for the causal effect. This scenario is summarized inFig. 1.6(b). In words, one cannot commit and make a claim about the target effect since there
are multiple, unobserved SCMs compatible with the given diagram and observational data.
Whenever the causal effect is not uniquely computable from the constraints embedded in
the graphical model, we say that it is non-identifiable from G (to be formally defined lateron). More generally, we would like to understand under what conditions an interventional
distribution can be computed from the observational one, given the structural constraints
encoded in the causal diagram. First, we supplement the Markovian construction of CBNs,
given in Def. 11, to formally account for the existence of unobserved confounders.
Definition 13 (Causal Diagram (Semi-Markovian Models)). Consider an SCM M =hU, V, F , P(U)i. Then, G is said to be a causal diagram (of M ) if constructed as follows:(1) add a vertex for every endogenous variable in the set V,(2) add an edge (V j → Vi ) for every Vi ,V j ∈ V if V j appears as an argument of fi ∈ F .
(3) add a bidirected edge (V j L9999KVi ) for every Vi ,V j ∈ V if the corresponding Ui ,U j ⊂ U
are correlated or the corresponding functions fi , f j share some U ∈ U as an argument.

Following this procedure, each SCM M induces a unique causal diagram. Furthermore,each bidirected arrow encodes unobserved confounding in G . They indicate correlation
between the unobserved parents of the endogenous variables at the endpoints of such edges.
1.4.3.1

Revisiting Locality in Semi-Markovian Models
Graphical models provide a transparent and systematic way of encoding structural constraints
about the underlying SCM (Fig. 1.4(d)). In practice, these constraints follows from theautonomy of the structural mechanisms [Aldrich 1989, Pearl 2000], which materializes aslocal relationships in the causal diagram. In Markovian models, these local constraints appear
in the form of family relationships, for example, (1) each variable Vi is independent of its
non-descendants given its parents Pai , or (2) each variable is invariant to interventions inother variables once its parents are held constant (following Def. 12). The local nature ofthese relations leads to a parsimonious factorization of the joint probability distribution, andtranslates into desirable sample and computational complexity properties. In Fig. 1.5(a), forexample, we can see that Y ⊥⊥ NDescy | Pay , where the non-descendants of Y is {X} and theparent set is {Z}. This implies, for example, that if we know the value of Z, the value of X is(probabilistically) irrelevant for computing the likelihood of Y . Further, once we intervene on
Z, the variable Y is invariant to interventions on X, i.e., P(y | do(x, z)) = P(y | do(z)), ∀x, y, z.
On the other hand, the family relations in semi-Markovian models are less well-behaved
and the boundaries of influence among the variables are usually less local. To witness, consider Fig. 1.8(a), and note that, where Pad = {B,C} and the remaining NDescd = {A, F}, D ⊥⊥38

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

A

A
C

F

B

C
D

E

(a) graph with four C-components.

F

B

D

E

(b) graph under intervention do(c).

Figure 1.8: Causal diagram with bidirected arrows and its mutilated counterpart under do(c).

NDescd | Pad does not hold since D and A are connected through the open path D L9999K B ←A. We introduce below a construct called confounded component [Tian and Pearl 2002b] torestore and help to make sense of modularity in these models.
Definition 14 (Confounded Component). Let {C1 , C2 , . . . Ck } be a partition over the set ofvariables V, where Ci is said to be a confounded component (for short, C-component) of G if
for every Vi ,V j ∈ Ci there exists a path made entirely of bidirected edges between Vi and V j in
G and Ci is maximal.
This construct represents clusters of variables that share the same exogenous variations
regardless of their directed connections. The causal diagram in Fig. 1.8(a) has two bidirectededges indicating the presence of unobserved confounders affecting the pairs (B, D) and (C, E),
and contains four C-components, namely, C1 = {A}, C2 = {B, D}, C3 = {C, E} and C4 ={F}. Similarly, each causal diagram in Figs. 1.7(a-c) contains two C-components, C1 = {X}and C2 = {Y }, while each in Figs. 1.7(d-e) contains one C-component, C1 = {X,Y }.
Our goal is to understand the boundaries of influence among variables in semi-Markovian
models since the parents of a node no longer shield it from its non-descendants, and thiscondition is a basic building block in the construction of Markovian models. Consider again
the graph in Fig. 1.8(a) and the node E and its only parent D. If we condition on D, E will not
be independent of its non-descendants in the graph. Obviously, E is automatically connected
to its bidirected neighbors, so it cannot be separated from C. Further, upon conditioning on theparent D, the collider through C is opened up since D is its descendant (i.e., EL9999KC ← A
carries correlation given D). In this case, the ancestors and descendants of C also becomecorrelated with E, which is now connected to every other variable in the graph (A, F, B).
Further, note that by conditioning on C itself, its descendants will be independent of E but
its ancestors and ancestors’ descendants will still be connected. In this graph, E is connected
to all other nodes upon conditioning on its observed parent D and C-component neighbor C,
i.e., A, B, F. Then, we also need to condition on the parents of C (i.e., A) to render its otherancestors and their descendants (i.e., F) independent of E.
Putting these observations together, for each endogenous variable Vi , we need to condition
on its parents, the variables in the same C-component that precede it, and the parents of the1.4 Pearl Hierarchy – A Graphical Perspective

39

latter so as to shield Vi from the other non-descendants in the graph. Such a maximal set isformally defined as Pa+
i as follows. Let < be a topological order V1 , . . . ,Vn of the variables V
44
in G , and let G (Vi ) be the subgraph of G composed only of variables in V1 , . . . ,Vi . Given
X ⊆ V, let Pa1 (X) = X ∪ {Pa(X) : X ∈ X}; further, let C(Vi ) be the C-component of Vi
1
in G (Vi ). Then define Pa+
i = Pa ({V ∈ C(Vi ) : V ≤ Vi }) \ {Vi }. For instance, in Fig. 1.8(a),++Pae = {D,C, A} and Pad = {B,C, A}.
Akin to the concept of Markov relative, a causal diagram also imposes factorization
constraints over the observational distribution in semi-Markovian CBNs, as shown next.
Definition 15 (Semi-Markov Relative). A distribution P is said to be semi-Markov relative toa graph G if for any topological order < of G , P factorizes as
P(v) = ∏ P(vi | pa+
i ),(1.36)Vi ∈V

where Pa+
i is defined using <.

Not only is the joint observational distribution related to a causal graph, but so are the L2 distributions P(· | do(x)) under an intervention do(X = x). The corresponding graph is GX ,where the incoming arrows towards X are cut, and the semi-Markovian factorization isPx (v) = ∏ Px (vi | pax+i ),(1.37)Vi ∈Vx+where Pax+
i is constructed as Pai but according to GX .

Example 13 (Factorization implied by the semi-Markov condition). Let P(A, B,C, D, E, F)
be a distribution semi-Markov relative to the diagram G in Fig. 1.8(a). One topological orderof G is A < B < C < D < E < F, which implies that
P(a, b, c, d, e, f ) = P(a)P(b|a)P(c|a)P(d|b, c, a)P(e|d, c, a)P( f |a).
In contrast, an application of the chain rule yields:
P(a, b, c, d, e, f ) = P(a)P(b|a)P(c|b, a)P(d|b, c, a)P(e|d, c, b, a)P( f |e, d, c, b, a).
A comparison of the two previous factorizations highlights some of the independence
constraints implied by the semi-Markov condition, for instance, (C ⊥⊥ B | A), (E ⊥⊥B |D,C, A) and (F ⊥⊥ E, D,C, B | A). The same applies to interventional distributions. First, let
Pc (A, B,C, D, E, F) be semi-Markov relative to GC (Fig. 1.8(b)). Then, note that Pc (A, B,C, D, E, F)
factorizes as Pc (a) Pc (b|a)Pc (c) Pc (d|b, c, a)Pc (e|d)Pc ( f |a). This distribution satisfies thesame conditional independence constraints as P(A, B,C, D, E, F), but also additional ones
such as (E ⊥⊥ A | D). This last constraint holds true since (CL9999KE) is absent in GC . The
C+
extended parents in both distributions are Pa+
e = {A,C, D} and Pae = {D}, respectively. 
44 That is, an order on the nodes (endogenous variables) V such that if V

j → Vi ∈ G , then V j < Vi .

40

1.4.3.2

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

CBNs with Latent Variables – Putting all the pieces together
The constructive procedure described in Def. 13 produces a coarsening of the underlying
SCM such that (1) the arguments of the functions are preserved while their particular forms
are discarded, and (2) the relationships between the exogenous variables are preserved while
their precise distribution is discarded.45 The pair (G , P∗ ) consisting of a causal diagram G ,constructed through such a procedure, and the collection of interventional (L2 ) distributions,P∗ , will be called a Causal Bayesian Network (CBN) if it satisfies the definition below. This
substitutes for Def. 12 in semi-Markovian models, and is similar to the way that constraints
on a (observational) probability distribution (viz. conditional independencies) are captured
by graphical constraints in a Bayesian network and the additional missing-link and do-see
constraints are encoded in the Markov-CBNs (Def. 12).
Definition 16 (Causal Bayesian Network (CBN - Semi-Markovian)). Let P∗ be the collectionof all interventional distributions P(V | do(x)), X ⊆ V, x ∈ Val(X), including the null intervention, P(V), where V is the set of observed variables. A graphical model with directed andbidirected edges G is a Causal Bayesian Network for P∗ if for every intervention do(X = x),
X ⊆ V, the following conditions hold:(i) [Semi-Markovian] P(V | do(x)) is semi-Markov relative to GX .
(ii) [Missing directed-link] For every Vi ∈ V \ X, W ⊆ V \ (Pax+i ∪ X ∪ {Vi }):x+
P(vi | do(x), pax+i , do(w)) = P(vi | do(x), pai ),(1.38)(iii) [Missing bidirected-link] For every Vi ∈ V \ X, let Pax+i be partitioned into two sets ofcu
confounded and unconfounded parents, Pai and Pai in GX . Then
P(vi | do(x), paci , do(paui )) = P(vi | do(x), paci , paui ).

(1.39)
The first condition requires each interventional distribution to factorize in a semiMarkovian fashion relative to the corresponding interventional graph GX , as discussed in
Example 13. The remaining conditions give semantics for the missing directed and bidirected
links in the model, which encode the lack of direct effect and of unobserved confounders
between the corresponding variables, respectively. Specifically, the missing directed-link
condition (ii) states that under any intervention do(X = x), conditioning on the set ofaugmented parents Pax+renders Vi invariant to an intervention on other variables W –iin words, W has no direct effect on Vi . For instance, note that for Vi = D in Fig. 1.8(a),P(d | do( f , e), b, c, a) = P(d | b, c, a) as well as P(d | do(b, c), do(a, f , e)) = P(d | do(b, c)).
Further, the missing bidirected-link condition relaxes the stringent parents do/see condition in Markovian CBNs (Def. 12(iii)). Note that the do/see condition does not hold due
45 Given the lack of constraints over the form and shape of the underlying functions and distribution of the exogenous

variables, it is possible to non-parametrically write one in terms of the other.

1.4 Pearl Hierarchy – A Graphical Perspective

CBN
P(V) → P(Y | do(x)) ∈ P∗

MarkovianSemi-Markovian

AlwaysSometimes

41

BN
Never

Figure 1.9: Summary of when we can identify a causal query from the observational L1 distribution and different types of graphical models compatible with the underlying phenomena.

to the unobserved correlation between certain endogenous variables, for instance, bothP(d | do(b)) = P(d | b) and P(e | do(d)) = P(e | d) do not hold in Fig. 1.8(a).46 Still, given
the set of extended parents of Vi , observations and interventions on parents not connected via
a bidirected path (i.e., Paui ) yield the same distribution. For instance, P(e | do(a, d), c) = P(e |a, d, c), where Paue = {A, D}, Pace = {C}; also, P(d | do(b, a, c)) = P(d | do(b), a, c), where
Paud = {A,C}, Pacd = {B}. There exists no unobserved confounding in Markovian models, so
Paui = Pai , which means that the condition is enforced for all parents.
Finally, the causal diagram G constructed from the SCM and the set of interventional
distributions P∗ can be formally connected through the following result:
Theorem 4. [L2 -Connection – SCM-CBN (Semi-Markovian)] The causal diagram G inducedby the SCM M (following the constructive procedure in Def. 13) is a CBN for PM∗ .
One could take an axiomatic view of CBNs and consider alternative constructions thatsatisfy their conditions, detached from the structural semantics (similarly to the Markoviancase). We provide in [Bareinboim et al. 2020a, Appendix D, Algorithm 2] a procedure called
C ONSTRUCT CBN that constitutes such an alternative. It can be seen as the experimentalstochastic counterpart of the SCM-functional Def. 13 (see also Thm. 10). We show in thenext section that CBNs can act as a basis for causal inference regardless of their underlying
generating model.
1.4.3.3

Cross-layer inferences through CBNs with Latent Variables
The causal diagram associated with a CBN will sometimes be a proper surrogate for the
SCM, and allow one to compute the effect of interventions as if the fully specified SCM
were available. Unfortunately, in some other cases, it will be insufficient, as evident from thediscussion in Example 12. This dichotomy is summarized in Fig. 1.9. We introduce next thenotion of identifiability [Pearl 2000, pp.77] to more visibly capture each of these instances.
Definition 17 (Effect Identifiability). The causal effect of an action do(X=x) on a set ofvariables Y given a set of observations on variables Z = z, P(Y | do(x), z), is said to be
46 To see why this is the case in the last expression, first let U be any exogenous argument to f . Now note thatDdP(e|do(d)) does not depend on Ud , while P(e|d) does due to the path Ud → D ← CL9999KE.

42

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

identifiable from P and G if for every two models M (1) and M (2) with causal diagram G ,P(1) (v)=P(2) (v)>0 implies P(1) (Y | do(x), z)=P(2) (Y | do(x), z).
This formalizes the very natural type of cross-layer inference we have discussed in Fig. 1.4,
namely: given qualitative assumptions encoded in the causal diagram G , one would like toestablish whether the interventional distribution (L2 -quantity), P(Y | do(x), z) is inferablefrom the observational one (L1 -data). We introduce next a set of inference rules known as
do-calculus [Pearl 1995] developed to answer this question.47,48
Theorem 5. [Do-Calculus] Let G be a CBN for P∗ , then P∗ satisfies the Do-Calculus rules
according to G . Namely, for any disjoint sets X, Y, Z, W ⊆ V the following three rules hold:
Rule 1 P(y | do(x), z, w) = P(y | do(x), w)

if (Y ⊥
⊥ Z | X, W) in GX . (1.40)

Rule 2 P(y | do(x), do(z), w) = P(y | do(x), z, w)

if (Y ⊥
⊥ Z | X, W) in GXZ . (1.41)

Rule 3 P(y | do(x), do(z), w) = P(y | do(x), w)

if (Y ⊥
⊥ Z | X, W) in GXZ(W) , (1.42)

where a graph GXZ is obtained from G by removing the arrows incoming to X and outgoingfrom Z, and Z(W) is the set of Z-nodes non-ancestors of W in the corresponding graph. 
These rules can be seen as a tool that allows one to navigate in the space of interventional
distributions, jumping across unrealized worlds, and licensed by the invariances encodedin the causal graph. Specifically, rule 1 can be seen as an extension of the d-separation
criterion49 for reading conditional independences under a fixed intervention do(X = x) fromthe graph denoted GX . Furthermore, rules 2 and 3 entail constraints among distributions under
different interventions. Rule 2 permits the exchange of a do(z) operator with an observation of
Z = z, capturing situations when intervening and observing Z influence the set of variables Y
indistinguishably. Rule 3 licenses the removal or addition of an intervention from a probability
expression, recognizing situations where do(z) has no effect whatsoever on Y. A moredetailed discussion of do-calculus can be found in [Pearl 2000, Ch. 3].50
We have previously shown that in some very simple settings causal inference is unattainable with only L1 -data, and that causal knowledge conveniently encoded in the form of a
47 The do-calculus can be seen as an inference engine that allows the local constraints encoded in the CBN, in termsof the family relationships, to be translated and combined to generate (global) constraints involving other variables,
possibly far away in the causal diagram.
48 The duality between local and global constraints is a central theme in probabilistic reasoning, where the family
factorization dictated by the graphical model is local, while d-separation is global, allowing one to read off nontrivial constraints implied by the model [Lauritzen 1996, Pearl 1988]. The graphical model could be seen as a basis,i.e., a parsimonious encoder of exponentially many conditional independences. In causal inference, do-calculus canbe seen as a generalization of d-separation to generate global, interventional-type of constraints.
49 This criterion is trivially extendable to graphs with bidirected edges by interpreting them as common causes of thevariables, as if connected by two corresponding directed edges.
50 Interestingly, the do-calculus theorem (Thm. 5) as stated here was derived entirely within the domain of CBNs andlayer 2 constraints, which contrasts with the traditional proposition ([Pearl 1995, Thm. 3]) based on layer 3 facts.
While this is an obviously valid route, we believe the statement in its current form may allow some researchers to use
this result even when taking an alternative, non-structural route; see also footnote 40.

1.4 Pearl Hierarchy – A Graphical Perspective

Hypothetical/Mutilated Graphs

Causal Graph
Z
X

43

Z

Y

(a) Back-door graph G

Z

X

Y

(b) GXZXY(c) GXXY(d) GZ

Figure 1.10: (a) graph representing a model where the query P(y | do(x)) is identifiable. The
query can be derived using do-calculus rules licensed by the graphs (b), (c) and (d).

causal diagram is required. Next, we show how the knowledge from the diagram together
with the inference rules of do-calculus allow for the identification of the query P(y | do(x)) in
the context of the model represented in Fig. 1.10(a). First, we start with the target query andthen apply do-calculus:
P(y | do(x)) = ∑ P(y | do(x), z)P(z | do(x))

Summing over Z(1.43)

Rule 3: (Z ⊥⊥ X)GX(1.44)

Rule 2: (Y ⊥⊥ X | Z)GX .

(1.45)z= ∑ P(y | do(x), z)P(z)z= ∑ P(y | x, z)P(z)z

Each step above is accompanied by the corresponding probability axiom or rule, supportedby the licensing graphs GX and GX (Figs. 1.10(b) and (c), respectively). As desired, the righthand side of Eq. (1.45) is a function of P(V), hence, estimable from L1 -data. This meansthat no matter the functional form of the endogenous variables or the distribution over theexogenous ones, for all SCMs compatible with the graph in Fig. 1.10(a), the causal effect of
X on Y will always be equal to Eq. (1.45). This can be seen as an instance of a generalizedversion of the back-door criterion provided in the Markovian case (Corollary 2). Next, westate the criterion that validates such adjustment more generally.
Corollary 3 (Back-door Criterion). Let G be a causal diagram and X and Y be the setsof treatment and outcomes variables, respectively. A set of variables Z is said to satisfy the
back-door criterion relative to the pair (X, Y) in G if:(i) No node in Z is a descendant of X, and(ii) Z blocks every path between X and Y that contains an arrow into X.
If such Z exists, the causal effect of X on Y is identifiable and given by the expression:
P(Y | do(x)) = ∑ P(Y | x, z)P(z).
z(1.46)


Many observations follow from this result. First, it is not hard to see that the derivation
provided above for the causal diagram in Fig. 1.10(a) (i.e., Eqs. (1.43)-(1.45)) constitutes anChapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

44

Hypothetical/Mutilated Graphs

Causal GraphXZY(a) Front-door graph.

X

Z

Y

X

(b) GXZZ(c) GXZ

Y

X

Z

Y

(d) GZ

Figure 1.11: Front-door graph (a) and derived graphs used to identify P(y | do(x)).

outline of the proof for Corollary 3. Note that the application of the third rule of do-calculusthat led to Eq. (1.44) is satisfied whenever the set Z does not include descendants of X, asrequired by cond. (i) of the criterion. Further, the application of the second rule of do-calculusthat culminated in the adjustment given by Eq. (1.45) represents exactly cond. (ii) of thecriterion, namely, that the back-door paths (with arrows into X) are blocked. Interestingly,
the most practical way of checking the back-door admissibility of a candidate set Z is byremoving the outgoing arrows from X and confirming whether Z separates X and Y in thisgraph (Fig. 1.10(c)). This is precisely the graph associated with the second rule of do-calculus,(Y ⊥⊥ X | Z)GX , and gives the name of the criterion since it isolates specifically these paths.
Second, the importance of the back-door criterion stems from the fact that adjustment isarguably the most common technique used to identify causal effects found throughout thesciences. While the expression given in Eq. (1.46) has been used since much earlier than the
discovery of the criterion itself [Pearl 1993], this condition is the first to provide a transparent
way one could judge the plausibility of the assumptions required to map L1 -data to an L2 quantity through adjustment based on a model of the world.51
Third, for the effect of Z on X, P(X | do(z)), in the same graph (Fig. 1.10(a)), there
exists no set Z that satisfies the conditions of Corollary 3. Note that in the graph where
the arrows outgoing from Z are cut, Fig. 1.10(d), Z and X cannot be separated due to theexistence of the latent path, ZL9999KX. More strongly, P(X | do(z)) is not identifiable fromthe observational distribution by any other means. We leave as an exercise the construction ofa counter-example based on the proof provided in Example 12. Broadly speaking, the effectof a certain intervention may or may not be identifiable, depending on the particular causal
diagram and the topological relations between treatment, outcome, and latent variables. Thisdichotomy is summarized in Figs. 1.6(a) and (b).
To further ground our understanding of do-calculus, consider the causal diagram in
Fig. 1.11(a), and assume that our goal is to identify the interventional distribution P(Y | do(x))
from the observational one, P(Z, X,Y ). We start by noting that there exists no back-door admissible set with respect to (X,Y ) since X and Y are connected through a bidirected arrow
in GX . One may be tempted to surmise that this effect is not identifiable as discussed in the
51 The back-door criterion provides a formal and transparent condition to judge the validity of a condition called

conditional ignorability; see further details in [Pearl 2000, § 11.3.2].

1.4 Pearl Hierarchy – A Graphical Perspective

45

previous example. To witness that this is not the case (and that the back-door is not necessary for identification), we start by writing the target distribution and expanding it throughdo-calculus:
P(y | do(x)) = ∑ P(y | do(x), z)P(z | do(x))

Summing over Z(1.47)= ∑ P(y | do(x, z))P(z | do(x))

Rule 2: (Y ⊥⊥ Z | X)GXZ(1.48)= ∑ P(y | do(z))P(z | do(x))

Rule 3: (Y ⊥⊥ X | Z)GZX .

(1.49)zzz

The rules used in each step and licensed by the corresponding graphs are shown in
Figs. 1.11(b)-(c). At this point, Corollary 3 can be used to solve for both factors in Eq. (1.49).
In the first factor, P(y | do(z)), {X} itself is back-door admissible with respect to (Z,Y ), since(Y ⊥⊥ Z | X)GZ (see Fig. 1.11(d)). For the second factor, P(z | do(x)), the empty set is backdoor admissible with respect to (X, Z), since (Z ⊥⊥ X)GX . Putting these two results together
and replacing it back into Eq. (1.49) leads to:P(y | do(x)) = ∑ P(z | x) ∑ P(y | z, x0 )P(x0 ).
z(1.50)

x0

The right-hand side of Eq. (1.50) is expressible in terms of P(V), which means that for any
SCM compatible with the graph, the causal effect will always be the same, regardless of
its idiosyncrasies. This particular expression is known as the front-door adjustment since
it leverages the outgoing arrows from X, in contrast to the back-door which considered
the incoming arrows towards X. The conditions under which the front-door adjustment(Eq. (1.50)) can be applied are more general than the graph in Fig. 1.11(a), as shown in [Pearl
2000, § 3.3.2], and generalized in [Hünermund and Bareinboim 2019, Thm. 3.5]. The frontdoor setting can be seen as the composition of two consecutive applications of the back-door
adjustment, one with the empty set and another with the set {X}.52
Finally, there are more involved scenarios that generated surprise in the literature since they
go beyond some of the intuitions discussed in the examples above; for instance, see diagramin Fig. 1.12(a). The task is to identify the effect of X on Y , P(Y | do(x)), from P(W, Z, X,Y ).
It is obvious that the effect cannot be identified by the back-door criterion since the empty
set is not admissible, and in GX , conditioning on {Z}, {W }, {Z,W } leaves the back-door path
XL9999KW L9999KY opened. It is also clear that no front-door strategy can be used due tothe direct arrow from X to Y , X → Y . After all, one may be tempted to believe that the effect
52 The conditions under which estimands can be re-expressed in terms of a composition of back-door adjustments has

been pursued recently in [Jung et al. 2020], where the front-door constitutes just one special case. This composition
allows one to leverage statistically efficient and robust estimation methods such as the propensity score and inverse
probability weighting (IPW), going beyond the classic back-door setting.

46

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

Hypothetical/Mutilated Graphs

Causal Graph
W

W

W

Z

Z

Z

X

Y

X

Y

(b) GXZ(a) Napkin graph.

W
Z

X

Y

Y

X

(c) GZX(d) GZ

Figure 1.12: Napkin graph (a) and derived graphs used to identify P(y | do(x)).

of X on Y is not identifiable in this case.53 Contrary to this intuition, consider the following
derivation in do-calculus:
P(y | do(x)) = P(y | do(x), do(z))

Rule 3: (Y ⊥⊥ Z | X)GXZ(1.51)= P(y | do(z), x)

Rule 2: (Y ⊥⊥ X)GZX(1.52)P(y, x | do(z))
P(x | do(z))

Def. of cond. probability.

(1.53)=The rules used in each step and the licensing graphs are shown in Figs. 1.12(b)-1.12(c). At
this point, Corollary 3 can be applied to solve for both factors in Eq. (1.53). To witness, notethat in the numerator, P(y, x | do(z)), {W } is back-door admissible with respect to (Z, {Y, X}),
since (Y, X ⊥⊥ Z | W )GZ , as shown in Fig. 1.12(d). The denominator follows by marginalizing
Y out. Putting these two results together and replacing it back into Eq. (1.53) lead to:P(y | do(x)) =∑w P(y, x | z, w)P(w).
∑w P(x | z, w)P(w)(1.54)The r.h.s. of Eq. (1.54) is expressible in terms of P(V), which means that for any SCM
compatible with the graph, the causal effect will always be the same, regardless of the detailsof the underlying mechanisms and distribution over the exogenous variables. The expression
shown in Eq. (1.54) is a ratio following from the application of the back-door criterion twice.
1.4.4

Summary – Background Context and Recent Developments
Broadly speaking, once one acknowledges the existence of the causal mechanisms (SCM
M ) underlying the phenomenon under investigation, all conceivable quantities from the PCH
defined over the endogenous variables obtain precise numerical values. The collection of such
quantities is called P∗ , which includes the observational (Def. 2) and interventional (Def. 5)
53 As alluded to by Pearl himself: “There was no way to block the back-door paths and no front-door condition. I tried

all my favorite shortcuts and my otherwise trustworthy intuitive arguments, both pro and con, and I couldn’t see how
to do it. I could not find a way out of the maze. But as soon as Bareinboim whispered to me, “Try the do-calculus,”
the answer came shining through like a baby’s smile. Every step was clear and meaningful. This is now the simplest
model known to us in which the causal effect needs to be estimated by a method that goes beyond the front- andback-door adjustments.” [Pearl and Mackenzie 2018, pp. 240]. The structure is known as the new Napkin graph.

1.4 Pearl Hierarchy – A Graphical Perspective

Def. 2, 5(e.g., Thm. 10)

Structural
Constraints(Def. 11, 13)

P∗

G

CBN

G

CBN = P∗
(Def. 12, 16)

Exp-CBN

Unobserved

SCM M

M = hF , P(U)i

SCM

47

L2 -connection(Thm. 2, 4)(a) Graphical Semantics

P(V)

P(Y | do(x))

Truncated Factor. (Thm. 3) /Do-calculus (Thm. 5)(b) Causal Inference

Figure 1.13: (a) Summary of the results connecting the SCM M and its corresponding
coarsening hP∗ , G i, where P∗ is the collection of distributions and G is the causal diagram.
(b) Summary of the causal inference task that entail another level of coarsening, since P∗ is
not fully observable. The goal here is to bridge distributions within P∗ by leveraging G . The
marked areas (shaded in gray) represent the unobserved components of the analysis.

distributions; see the left side of Fig. 1.13(a). Since any query is computable from the fully
specified SCM, why is there a need to pursue the graphical approach developed in this section?
As discussed earlier in the chapter, in many practical situations it is implausible forscientists to have a full specification of the SCM (i.e., hF , P(U)i). Instead, only data generated
by the underlying model is at disposal. Unfortunately, the lack of detailed knowledge ofthe causal mechanism cannot be surmounted with data alone; except, perhaps, in specificsettings where data and query match one another precisely. As stated in Corollary 1, if thequery belongs to a layer above the one where data come from, causal inference is provably
impossible. Therefore, one is compelled to investigate more relaxed inferential tasks that may
rely on some sufficient, but attainable knowledge of the underlying causal mechanisms.
For this purpose, we introduced a procedure for constructing a graphical model G encoding
the structural constraints of M , bestowed with causal interpretation (Defs. 11 and 13, forMarkovian and semi-Markovian models, respectively). In fact, G can be seen as a nonparametric coarsening of the SCM M , where the signatures (i.e., arguments) of the functions
are preserved while their specific forms are dismissed (see the right side of Fig. 1.13(a)).
To give the graphical model a precise interpretation, we introduced an object called Causal
Bayesian Network (CBN) to marry the structural constraints encoded in G and the collectionof distributions in P∗ (Defs. 12 and 16). Furthermore, and irrespective of the SCM, the CBNconstruct provides means of deciding whether a pair (G , P∗ ) is compatible in some causal
fashion. Whenever this is the case, a CBN can serve as a surrogate for the empirical content

48

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inferenceof the underlying SCM with respect to knowledge in layers L1 and L2 (Thms. 2 and 4, forMarkovian and semi-Markovian models, respectively).
Obviously, operating at this level of coarsening does not come without a price, as challenges will appear when attempting to bridge the layers of the hierarchy. For instance, inferring an interventional distribution P(Y | do(x)) from the observational distribution P(V) andthe causal diagram G , will not always be possible. Depending on the existence of exogenous
variables and how they influence the observed system, cross-layer inferences will entail very
different answers. In Markovian CBNs, the truncated factorization product (Thm. 3) maps
any interventional distribution (L2 ) to the observational (L1 ) one, effectively bridging Layers 1 and 2 (Fig.1.6(a)). For semi-Markovian CBNs, we proved that the do-calculus holds(Thm. 5), again, irrespective of the SCM. We showed that in many, but not all, cases the target
effect is identifiable (Fig.1.6(b)). The corresponding impossibility results evidence the semantic gap between the SCM M (that generates the entire P∗ ) and its diagram G (that identifies
only a portion of it; see Fig. 1.13(b) for a graphical summary.
The problem of deciding identifiability, also known as non-parametric identification, hasbeen extensively studied in the literature. There are a number of conditions that have been
proposed to solve this problem, including [Galles and Pearl 1995, Kuroki and Miyakawa 1999,
Pearl and Robins 1995, Spirtes et al. 1993]. The do-calculus provides a general mathematicaltreatment for non-parametric identification [Pearl 1995]. It has been made systematic andshown to be complete for the task of identification from a combination of observations andexperiments [Bareinboim and Pearl 2012, Huang and Valtorta 2006, Lee et al. 2019, Shpitser
and Pearl 2006, Tian and Pearl 2002a]. In words, given a causal diagram G and a collectionof observational and experimental distributions, the target effect of X on Y given a set ofcovariates Z, P(y|do(x), z), is identifiable if and only if there exists a sequence of applicationof the rules of do-calculus that reaches an estimand in terms of the available distributions.

Recent Developments
The clarity gained from theoretical understanding of the do-calculus machinery has inspired
numerous developments in the field. We summarize next a few important threads in theliterature that seem especially significant for the data sciences, given their generality.
Stochastic, conditional, and non-atomic interventions. There are complex interventions
that cannot be described in terms of atomic ones, as delineated by the do-operator, including
conditional and stochastic [Pearl 2000, Ch. 4]. It may be challenging in many settings to asses
the effect of new soft-interventions from non-experimental data, including when designing
adaptive strategies to treat patients in medicine, developing new incentives structures in an
economy, or inferring a new dynamic policy in reinforcement learning. In response to thatchallenge, a novel treatment building on the do-calculus has recently been introduced [Correa

1.5 Reflections and Conclusions 49

and Bareinboim 2019], and developed in great generality under the rubric of σ-calculus(shortcut for soft-do-calculus) [Correa and Bareinboim 2020].
Data-driven identification. On another line of investigation, one can use the structural
constraints (Fig. 1.4(d)) learned from L1 -data to induce an equivalence class of causal
diagrams (i.e., all the diagrams that respect the constraints imposed by the observed data)[Peters et al. 2017, Spirtes et al. 2001, Zhang 2008]. The problem of identification in thesestructures is considerably more challenging due to the inherent uncertainty about the causal
structure itself. An initial solution has been proposed in [Jaber et al. 2018, Perkovic et al.
2017], and further developed with greater generality in [Jaber et al. 2019a,b,c].
Estimation beyond the back-door adjustment. Most of the results found in the literature
are about identification but, ultimately, one needs to estimate the effect from a finite number ofsamples. One line of investigation led to efficient and robust estimation techniques [Bang andRobins 2005, Horvitz and Thompson 1952, Imbens and Rubin 2015, Rosenbaum and Rubin
1983, Van Der Laan and Rubin 2006], which was specific to adjustment-like estimands. A
recent treatment leveraging insights following form the do-calculus generalized some of thesetechnique based on weighting operators for the non-backdoor cases [Jung et al. 2020].
Data-Fusion – Integrating Heterogeneous Data Collections. The do-calculus was developed for solving the problem of confounding bias, which had been the primary concern inthe modern literature of causation at the time [Pearl 1995], and going back at least to [Rubin1974]. In practice, however, there exist other issues that plague most data collection, including selection bias, external validity, transportability, and, broadly speaking, due to issues ofgeneralizability. Building on the insights learned by handling the problem of confounding,a general framework has been introduced and developed to solve the data-fusion problem,i.e., cohesively combining multiple datasets coming from various, heterogeneous sources, andplagued with different types of biases [Bareinboim and Pearl 2016]. For some recent developments on the fusion framework, refer to [Correa et al. 2019a,b] and [Lee et al. 2020], and forsome applications in the context of econometrics, see [Hünermund and Bareinboim 2019].

1.5

Reflections and Conclusions
We investigated a mathematical structure called the Pearl Causal Hierarchy (PCH), which was
discovered by Judea Pearl when studying the conditions under which some types of causal
explanations can be inferred from data [Pearl 2000, Pearl and Mackenzie 2018]. The PCH
is certainly one of the most productive conceptual breakthroughs in the science of causal
inference over the last decades. It highlights and formalizes the distinct roles of some basic
human capabilities – seeing, doing, and imagining – spanning cognition, AI, and scientificdiscovery. The structure is pervasive in the empirical world: as long as a complex system canbe described as a collection of causal mechanisms – that is, a structural causal model (Def. 1)– the hierarchy relative to the modeled phenomena emerges (Def. 9).

50

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

The main contribution of this chapter is a detailed analysis of the PCH through three lenses:
one semantical (Sec. 1.2), another logical-probabilistic (Sec. 1.3), and another inferentialgraphical (Sec. 1.4). These complementary approaches elucidate the PCH from different
angles, ranging from when one knows everything about a specific SCM (semantical), totalking about classes of SCMs in general (probabilistic), and ending with one SCM thatis particular to the environment of interest but which is not fully observed (graphical).
We believe these distinct interpretive angles provide a powerful and appealing tool forstudying causation across different research communities, with far reaching implications forscientific practice in a wide range of data-driven fields. This work also promises to shape thedevelopment of the next generation of AI systems.
Semantically speaking (Sec. 1.2), we start from the building block of any causal analysis,
a fully specified SCM M , which represents the dynamics of a specific system or phenomenon
under investigation. Following Pearl’s own presentation, we then develop basic intuitions for
how the PCH naturally emerges from an SCM, conveying how an agent could (1) perceive acertain part of the world unfolding in time and predict its next stage, (2) deliberately intervenein this world and evaluate the effects of such an action, and (3) imagine alternative ways theworld could be and speculate what would have happened had something different occurred.
Provided the SCM M is fully known, all these and other conceivable quantities of the threelayers are well-defined and immediately computable from the model (through Defs. 2, 5, 7).
From this starting point, we move on to the original contributions of the chapter. First,
in Sec. 1.3, we define (symbolic) logical languages that were implicit in our discussions
in the previous discussion. These languages encode the types of statements and questions
associated with the three layers of the hierarchy. We then formally state Thm. 1, the Causal
Hierarchy Theorem (CHT), which tells us that the ability to answer questions at one layer,
by itself, almost never guarantees the ability to answer all the relevant questions at higher
layers. Depending on the specific dynamics of the underlying SCM, it is conceivable that theeffects of actions (Layer 2) can be fully determined merely by passively observing the world
unfolding in time (Layer 1); or, all the facts about which one could imagine or speculate(Layer 3) are fully determined by the experimental evidence (Layer 2). The CHT, on the other
hand, says that this is generically impossible (i.e., it is measure zero): each layer makes claims
about the world not entailed by lower layers. The theorem can be seen as formal grounding forthe mantra “no causes in, no causes out” [Cartwright 1989], commonly taught in introductory
causal inference courses.
As a practical matter, one might wonder under what conditions a quantity at one layer of
the PCH could be learned from data collected at another. For instance, how could the effect of
a new intervention of X on an outcome Y , P(y | do(x)), a layer-2 quantity, be predicted fromdata collected observationally, P(x, y), i.e., from Layer 1?
In Sec. 1.4, we explore a solution to this inferential task through the lens of graphicalmodels. The departing point of most of the causal inference literature is the acknowledgment

1.5 Reflections and Conclusions 51

that the SCM M as well as the corresponding PCH are there even though M may be
hard to obtain in practice.54 Even when M is not fully known, one may still have someunderstanding of its structure at a coarse level. Accordingly, we study the structural constraints– different types of invariances – implied by the SCM at each layer of the PCH. For instance,a projection of the SCM downwards to the first layer, the world of associations, displays
conditional independences. There exists a rich literature on probabilistic reasoning and how
to parsimoniously organize knowledge in this fashion, including the use of Bayesian networks[Koller and Friedman 2009, Pearl 1988]. We observe that layer-1 constraints and models are
insufficient to answer questions of interventional nature (see Example 10).
Going up the hierarchy, we study layer 2 type constraints, which leads to a family ofgraphical models called Causal Bayesian Networks (CBNs). There are two types of CBNs
depending on whether there are unobserved confounders (i.e., latent variables that affectmore than one observable) in the system. We study Markovian CBNs [Bareinboim et al.
2012, Pearl 2000] and show that it is always possible to answer interventional questions fromobservational data via the truncated factorization product (Thm. 3). Given that unobserved
confounders exist in most practical settings, we introduce a general version of CBNs forsemi-Markovian systems (Def. 16). We show that in the presence of unobserved confounders
the empirical gap between the CBN and the true SCM widens, and hence the knowledge
encoded in the CBN is not always sufficient to support cross-layer inferences (Example 12).
We subsequently discuss an inferential system that allows one to perform inferences acrosslayers of the PCH, known as do-calculus [Pearl 1995]. We prove that do-calculus can beused to facilitate cross-layer inferences from a CBN perspective (Thm. 5), agnostic to theunderlying generating model. In other words, one could take an axiomatic view and satisfy
the constraints that CBNs require through means other than a structural model (for example,
from a set of experimental distributions [Kocaoglu et al. 2017]), which will still lead tocausally-valid inferences. By the end of the section, we study instances of identifiable andnon-identifiable effects, and summarize new challenges and relaxations of the identification
problem leading to more robust and general cross-layer inferential settings.
The work presented so far on the foundations of causal inference has been somewhat
technical, and we believe it is worth closing the chapter with some reflections on how
these ideas and results relate to some practical concerns arising in modern AI and machinelearning. As a discipline largely concerned with the replacement of explicit “hand-coded”
rules and algorithms with rules and behaviors learned directly from “patterns in data” [Bishop
2006], ideally with minimal human supervision, machine learning faces formidable inferential
challenges that can be understood through the lenses of the PCH. In fact, a central question
for machine learning concerns what kinds of inferences one can possibly hope to make from

54 Given that this is a very dense and technical section, a detailed summary of the results is provided in Sec. 1.4.4.

52

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

what kinds of data (which may include, e.g., data produced via interactions in the world). The
PCH addresses this question in a direct and transparent way.
As a suggestive final example to illustrate the subtleties as well as the significance of the
PCH in practically important tasks, consider a problem that has been increasingly prominent
in discussions around the role of AI in contemporary society, namely whether we ought toprefer an AI system or a human medical doctor in a decision-making setting.55
Example 14 (MD versus AI physicians). Imagine a case of a patient who needs to decide
between two styles of treatment: an AI physician, who is a black-box and cannot explain
itself, or a human physician, who is a medical doctor (MD) and can, for instance, offer clear
explanations for their decisions. Let X represent the treatment received, X = 0 meaning the
MD, X = 1 meaning treatment by the AI system – and let Y = 0 mean that the patient has
died, while Y = 1 means that the patient is alive after one year of treatment. How should one
choose between these two completely different styles of treatment?
One way of proceeding might be to collect data on previous patients in the population who
received these two treatments, computing statistics, for example, about the recovery rate foreach treatment. This could be implemented with standard methods (including some of themost modern neural methods). The corresponding statistics can be computed after careful andextensive data collection and analysis, and in this case they come back as follows:
P(Y = 1 | X = 1) = 0.9, andP(Y = 1 | X = 0) = 0.8.

(1.55)Naturally, one may be tempted to conclude based on these results that the AI physician is tobe preferred to the MD. If we are concerned solely with maximizing the chances of survival,
not availability of explanation (nor, e.g., the human-human contact that might be lacking inan artificial system), then why would we opt for a physician with a lower success rate?
Digging a bit deeper in the Pearl Hierarchy, suppose that, unbeknownst to both the MD
and the AI physician (not to mention the patient), the following SCM M ∗ encodes thetrue causal mechanisms of how patients decide their treatment and respond to it – formally,M ∗ = hV = {X,Y }, U, F = { fx , fy }, P(U)i, where:
U = 0, 4
0,




0,
X,
U = 0, 1, 2, 3
U = 1, 5
fX (U) =,fY (X,U) =,(1.56)
1,

U = 4, 5, 6, 7
1 − X,
U = 2, 6




1,
U = 3, 7
55 To cite just one recent discussion appearing in the public sphere, see the spirited discussion following thisprovocative tweet by Geoffrey Hinton (2/20/2020, 3:37pm): “Suppose you have cancer and you have to choose
between a black box AI surgeon that cannot explain how it works but has a 90% cure rate and a human surgeon withan 80% cure rate. Do you want the AI surgeon to be illegal?”1.5 Reflections and Conclusions 53

and the distribution over the exogenous variable is P(U = u) = (1/40, 1/40, 1/80, 3/16,
1/160, 1/160, 11/160, 107/160), where u = 0, 1, ..., 7. The endogenous variables X and Y
are both binary and the exogenous variable U is 8-dimensional, so as to represent the different
types of preferences patients could have over treatments, and how they respond to them.
As expected, the underlying SCM M ∗ produces exactly the same conditional probability
statements as the statistics supported by the data and shown in Eq. 1.55. Suppose that,
impressed by these numbers, a new regulation is put in place, whereby AI physicians are
deployed across the country, implying that patients are uniformly submitted to treatment
do(X = 1). What is the effect of this new policy in the general population? Again, following
the discussion in Section 1.2 (and Def. 5), M ∗ gives meaning to this statement as:P(Y = 1 | do(X = 1)) ≈ 0.89.

(1.57)

Likewise, we can compute what would happen in the hypothetical scenario in which no AI
system replaces any human MDs, which would lead to the following average treatment effect:
P(Y = 1 | do(X = 0)) ≈ 0.94.

(1.58)As Eqs. 1.57 and 1.58 reveal, the result of this policy is quite disastrous – given the true causal
mechanism M ∗ , it is clear that significantly more lives would be saved by relying on human
doctors; on average AI is hurting more people than helping.
Tension between Layers 1 and 2. The trend reversal revealed in Eq. 1.55 (Layer 1) andEqs. 1.57-1.58 (Layer 2) is no surprise from the standpoint of the PCH (for example, see[Pearl 2000, Ch. 6]). The purpose of the example is to clarify how the whole analysis
is contingent on the type of data collected with respect to PCH’s layers. For instance, if
the samples are drawn from a layer 1 distribution, the results are likewise constrained tolayer 1 claims, viz. predictions. To be sure, the best learning algorithm (e.g., a deep neuralnetwork) will be able to approximate with great precision the probability of recovery for
those individuals deciding by themselves whether they opt for the AI or the human-type oftreatment. Still, no claim can be made about the effect of the intervention and what would be
the medical recommendation; only a prediction can be offered.
Similarly, if the data is collected from experiments with respect to a layer 2 distribution,
the results are again constrained to the corresponding layer 2 claims (viz. interventions). Afterall, we learned from the Causal Hierarchy Theorem (Thm. 1) that claims from a certain layer
need to be supported by knowledge at the same layer or above, regardless of the amount ofdata or computational resources available.
Climbing further up the PCH, we could imagine another group of more enlightened policymakers who have internalized the lesson that correlation is not causation. Since they do nothave access to the real collection of causal mechanisms underlying the system (as shown in
Eqs. 1.56), they decide to conduct a careful randomized clinical trial (RCT) leveraging modern

54

Chapter 1 On Pearl’s Hierarchy and the Foundations of Causal Inference

reinforcement learning techniques. Lo and behold, the statistics come as describe in Eqs. 1.57
and 1.58. Naturally, they may be tempted to conclude based on these results that the humanphysician is to be preferred to the AI, flipping the decision of the first group of policy-makers.
Again, if we are concerned with maximizing the chances of survival, then why would we opt
for an AI with a lower success rate?
Further, digging even deeper in the Pearl Hierarchy, it makes sense to ask about theprobability of survival under a certain treatment, given that the patient is inclined to opt for
one or the other treatment. For instance, the patient may be tempted to go with the AI system(X = 1), given their age group, gender, socioeconomic status, personality, and other factors,
and may wonder whether they would survive (Y = 1) had they been treated by the human(X = 0). This query has meaning in counterfactual language (Layer 3), and can be formally
written as P(YX=1 = 1 | X = 0). In particular, they can be evaluated by M ∗ (Eq. (1.56)), as
discussed in Section 1.2 (and following Def. 7),
P(YX=1 = 1 | X = 0) = 0.85

P(YX=1 = 1 | X = 1) = 0.90

P(YX=0 = 1 | X = 0) = 0.80

P(YX=0 = 1 | X = 1) ≈ 0.98.

(1.59)

In words, among those patients inclined to opt for the MD (X = 0 after the conditioning bar),treatment by the AI physician (X = 1) would actually be better (since P(YX=1 = 1 | X = 0) >P(YX=0 = 1 | X = 0)), while the opposite pattern can be seen for those inclined to prefer the
AI physician (since P(YX=1 = 1 | X = 1) < P(YX=0 = 1 | X = 1)). Knowing these additional,
quintessentially layer 3 facts, the best policy would actually be to give patients whatevertreatment they would be inclined not to choose. (We leave as a challenge to the readers tointerpret how this population is structured, in terms of the generating M ∗ (Eq. (1.56), andhow this type of pattern could arise in the real world. One hint would be to dissect the Uspace and understand the endogenous and exogenous components of the SCM.)
Tension between Layers 2 and 3. The reversal shown of the statistics obtained in Eqs. (1.57)(1.58) (Layer 2) and Eqs. (1.59) (Layer 3) is again not surprising from the standpoint of thePCH (for example, see [Bareinboim et al. 2015, Forney et al. 2017]). Still, it does make clearthat the output of the analysis is contingent on the type of data collected, even when it comesfrom direct experimentation (Layer 2). To be sure, the best learning algorithm will be able toapproximate with great precision the treatment effect when we randomize across units in the
population, but this will miss an opportunity to leverage information revealed by the human’s
decision, imperfect as this may be. This turns out to be a great opportunity for collaboration– human decision-makers may perform poorly (Eq. 1.55); purely automated decision-making
procedures may also perform poorly (Eqs. 1.57-1.58). From the perspective of Layer 3, as
evident from Eqs. (1.59), optimal decision-making emerges from a combination of humans
intuitions, even when wrong, with machine capabilities. The lesson here is, of course, more

1.5 Reflections and Conclusions 55

general, as the example does not depend on what exactly X and Y are. For a detailed discussion
on the implications of causality to decision-making, see [Bareinboim et al. 2020b].

To summarize, whenever a domain of interest can be construed as a collection of causal
mechanisms (i.e., as an SCM), the PCH emerges at once. To the extent that the SCM isalready known, all the relevant quantities at all layers can in principle be computed. For almost
any SCM, however, higher layers in the hierarchy remain independent of those lower in thehierarchy; it is a generic property of SCMs that information at lower layers underdetermines
information at higher layers. Because in practice the SCM is rarely known, we thus face
the formidable challenge of cross-layer inference, a challenge that has been illustrated in
Example 14 and the other examples in this chapter. Any sensible approach to this problem
must involve a subtle interplay between (1) prior knowledge we may have in the form ofa partial specification of the underlying SCM, (2) the type of data that we can collect, and(3) the inferential target of the analysis (Fig. 1.4). Pearl’s original presentation of the PCH
and the results presented here that build on this conceptual and formal foundation allow us
to understand from first principles when a question about some aspect of the world (whether
associational, interventional, or counterfactual) can – at least in principle – be answered froma combination of prior knowledge about the domain and the available data.
We note that the Pearl Causal Hierarchy has a certain inevitability to it. Many of themost fundamental problems that we need to solve – “What will happen if I do this?,” “Why
did that happen?,” etc. – compel us upward in the hierarchy. Even when we lack data orexperience at the relevant layer, there are still powerful methods we can use to answer
such questions. If our goal is to build the next generation of AI systems that are humancompatible, capable of explaining themselves, aligned with the social good, safe and robust
to changing conditions, we submit that connecting with the fundamental dimensions of thehuman experience delineated by the PCH is a critical step towards this goal.
Acknowledgements
This work has benefited immensely from conversations with David Blei, Carlos Cinelli, Philip
Dawid, Sanghack Lee, Judea Pearl, Jin Tian, Yuhao Wang, and Junzhe Zhang. We are also
grateful to the Editors, Professors Rina Dechter, Hector Geffner and Joe Halpern, for theopportunity to contribute to this special volume. Elias Bareinboim and Juan D. Correa werepartially supported by grants from NSF IIS-1704352 and IIS-1750807 (CAREER). Duligur
Ibeling was supported by the NSF Graduate Research Fellowship Program under Grant No.
DGE-1656518. Thomas Icard was partially supported by the Center for the Study of Language
and Information.

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