Theory Acquisition and the Language of ThoughtCharles Kemp

Noah D. Goodman & Joshua B. Tenenbaum

ckemp@cmu.edu
Department of Psychology
Carnegie Mellon University{ndg,jbt}@mit.edu
Department of Brain and Cognitive Sciences
Massachusetts Institute of Technology

Abstract
Everyday knowledge about living things, physical objects andthe beliefs and desires of other people appears to be organized
into sophisticated systems that are often called intuitive theories. Two long term goals for psychological research are tounderstand how these theories are mentally represented, andhow they are acquired. We argue that the language of thought
hypothesis can help to address both questions. First, compositional languages can capture the content of intuitive theories.
Second, any compositional language will generate an accountof theory learning which predicts that theories with short descriptions tend to be preferred. We describe a computationalframework that captures both ideas, and compare its predictions to behavioral data from a simple theory learning task.

Any comprehensive account of human knowledge must acknowledge two principles. First, everyday knowledge is morethan a list of isolated facts, and much of it appears to be organized into richly structured systems that are sometimes called
intuitive theories. Even young children, for instance, havesystematic beliefs about domains including folk physics, folk
biology, and folk psychology [10]. Second, some aspects ofthese theories appear to be learned. Developmental psychologists have explored how intuitive theories emerge over the
first decade of life, and at least some of these changes appear
to result from learning.
Although theory learning raises some challenging problems, two computational principles that may support this ability have been known for many years. First, a theory-learning
system must be able to represent the content of any theory
that it acquires. A learner that cannot represent a given system of concepts is clearly unable to learn this system from
data. Second, there will always be many systems of concepts
that are compatible with any given data set, and a learner must
rely on some a priori ordering of the set of possible theories
to decide which candidate is best [5, 9]. Loosely speaking,this ordering can be identified with a simplicity measure, or aprior distribution over the space of possible theories.
There is at least one natural way to connect these twocomputational principles. Suppose that intuitive theories are
represented in a “language of thought:” a language that allows complex concepts to be represented as combinations ofsimpler concepts [5]. A compositional language provides a
straightforward way to construct sophisticated theories, butalso provides a natural ordering over the resulting space oftheories: the a priori probability of a theory can be identified
with its length in this representation language [3, 7]. Combining this prior distribution with an engine for Bayesian inference immediately leads to a computational account of how
theories might be learned. There may be other ways to explain how people represent and acquire complex systems ofknowledge, but it is striking that the “language of thought”
hypothesis can help to explain both knowledge representationand learning.
This paper describes a computational framework that helps
to explain how theories are acquired, and that can be used toevaluate different proposals about the language of thought.
Our approach builds on previous discussions of concept
learning that have explored the link between compositional
representations and inductive inference. Two recent approaches propose that concepts are represented in a form ofpropositional logic, and that the a priori plausibility of an
inductive hypothesis is related to the length of its representation in this language [4, 6]. Our approach is similar in spirit,
but is motivated in part by the need for languages that arericher than propositional logic. The framework we present
is extremely general, and is compatible with virtually any
representation language, including various forms of predicate logic. Methods for learning predicate logic theories havepreviously been explored in the field of Inductive Logic Programming, and we recently proposed a theory-learning modelthat is inspired by this tradition [7]. Our current approach is
motivated by similar goals, but is better able to account forthe discovery of abstract theoretical laws.
The next section describes our computational framework,
and introduces a specific logical language that we will consider throughout. Our framework allows relatively sophisticated theories to be represented and learned, but we evaluate
it here by applying it to a simple learning problem, and comparing its predictions with human inductive inferences.

A Bayesian approach to theory discovery
Suppose that a learner observes some of the relationships that
hold among a fixed, finite set of entities, and wishes to discover a theory that accounts for these data. Suppose, for instance, that the entities are thirteen adults from a remote tribe(a through m), and that the data specify that the spouse relation (S(·, ·)) is true of some pairs (Figure 1). One candidate
theory states that S(·, ·) is a symmetric relation, that some of
the individuals are male (M(·)), that marriages are permitted
only between males and non-males, and that males may takemultiple spouses but non-males may have only one spouse(Figure 1b). Other theories are possible, including the theory
which states only that S(·, ·) is symmetric.
Accounts of theory learning should distinguish between atleast three kinds of entities: theories, models, and data. A
theory is a set of statements that captures constraints on thepossible configurations of the world. For instance, the theory in Figure 1b rules out configurations where the spouse

g

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0 x y S(x, y)↔ S(y, x).
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0 x y S(x, y)→ (M(x) ∧ ¬M(y)) ∨ (M(y) ∧ ¬M(x)).
A

0 x ¬M(x)→ FyS(x, y).

Model

T RUE

FALSE

M(a). M(b). M(c).

M(d). M(e). . . . M(m).

S(a, e). S(e, a).
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S(a, b). S(b, a).
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S(c, m). S(m, c).

S(m, l). S(l, m).

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Data

S(a, f). S(f, a).
S(b, i). S(c, l).

Figure 1: (a) A graph representing marital ties among a groupof 13 people. (b) A hierarchical Bayesian framework for theory discovery. The theory is a collection of statements insome logical language, the model specifies the extensions of
all predicates mentioned in the theory, and the data represent
the information observed by the learner.
relation is asymmetric. A model of a theory specifies the extension of each predicate in a way that is consistent with thetheory. Note that model is a technical term that we have inherited from standard treatments of formal logic [2]. Figure 1b
includes one model of the theory described already. The theory includes one unary predicate M(·) and one binary predicate S(·, ·), and the model specifies whether or not M(·) is truefor each individual, and whether or not S(·, ·) is true for eachpair of individuals. The actual state of the world is captured
by just one model, but typically there will be many models
consistent with a given theory. The data in Figure 1b represent the information available to the theory-learner. Oftena learner will have partial information about the state of theworld, and the data will capture only some of the information
specified by the underlying model.
A hierarchical Bayesian approach allows us to transformthe diagram in Figure 1b into a formal account of theory
learning. The theory T that best accounts for data set D is
the theory that maximizes the posterior probabilityX
P (T |D) ∝ P (D|T )P (T ) =P (D|M )P (M |T )P (T )M

where we have expanded P (D|T ) as a sum over all possiblemodels M of theory T . To complete our framework we needto specify a prior on theories P (T ), along with distributions
P (M |T ) and P (D|M ) that specify how models are generated from theories and how data are generated from models.

P (T ): A prior distribution on theories
The learning framework described in the previous section isextremely general, and can be combined with many different

proposals about how theories are mentally represented. We
work with the idea that theories are mentally represented insome language, and that the prior probability of any theory
is inversely related to the length of its description in this language. As a starting point, we work with a language that isclosely related to function-free first-order logic. The theory
in Figure 1 is expressed in this language, and many additional
examples are shown in Figure 2.
Our language includes symbols representing predicates ofdifferent arities (e.g. M(·) and S(·, ·)) variable symbols (e.g.
x, y and z), and Boolean connectives which capture negation (¬), conjunction (∧), disjunction(∨), material implication (→), and biconditional implication (↔). Four quantifiers are included: for all ( ), there exists at least one (E),there exists one or fewer (F), and there exists precisely one(P). The language includes the identity symbol (=), and we
also include an operator T (·, ·), where T (R, C) indicates thatthe transitive closure of R(·, ·) is C(·, ·) (in other words, thatC(x1 , xn ) is true if and only if there is a set {x1 , x2 , . . . , xn }such that R(x1 , x2 ) is true, R(x2 , x3 ) is true, and so on).
Many aspects of this language are inherited from standard
treatments of first-order logic, but several representational
choices deserve some attention. First, note that the language
includes no symbols for constants. We are especially interested in how abstract theories might be learned, and it will be
convenient to restrict our attention to laws that do not referdirectly to individual objects. Second, the language includes
two quantifiers (F and P) that are missing from most logical languages, in part because statements that rely on thesequantifiers can be rewritten as statements that use the familiar
existential quantifier and the identity symbol instead. These
conversions, however, often produce long and unwieldy statements, and our language is based on the hypothesis that F
and P are no more complex psychologically than the familiar
quantifier E.
The transitive closure operator T (·, ·) represents our greatest departure from familiar first-order logic. Unlike the quantifiers F and P, the operator T (·, ·) cannot be defined using afirst-order language. The concept of transitive closure, however, seems psychologically natural, and is probably one of
the most important concepts that cannot be formulated in a
first-order language. First-order logic provides a simple starting point for investigations of the language of thought, butthere is no reason to think that mental representations are limited to a first-order language. We believe that attempts to formalize the language of thought will eventually need to draw
on the expressive resources of higher order logics, and theoperator T is a preliminary step in this direction.
Given any theory T expressed in our language, the priorprobability P (T ) is determined by the number of symbols in
T : P (T ) ∝ λ|T | , where λ is a parameter between 0 and 1.
For all applications in this paper we set λ = 0.9.
Once we have committed to a representation language anda setting of λ, the prior distribution P (T ) is unambiguously
specified. This prior, however, depends critically on the lanA

(a)

A
A
A
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1. x Fy R(x, y).
At most one outgoing edge per node
2. y Fx R(x, y).
At most one incoming edge per node
At least one outgoing edge per node
3. x Ey R(x, y).
4. y Ex R(x, y).
At least one incoming edge per node
5. x Py R(x, y).
Exactly one outgoing edge per node
6. y Px R(x, y).
Exactly one incoming edge per node
7. Ex Py R(x, y).
At least one node with exactly one outgoing edge
8. Ey Px R(x, y).
At least one node with exactly one incoming edge
9. Px y ¬R(x, y).
Exactly one node with no outgoing edge
Exactly one node with no incoming edge
10. Py x ¬R(x, y).
11. x y R(x, y) → Pz (¬y = z ∧ R(x, z)).
Zero or two outgoing edges per node
No cycles in R(·, ·)12. x ¬C(x, x).
13. Ex C(x, x).
At least one cycle in R(·, ·)R(·, ·) has a path between any pair of nodes
14. x C(x, x).
15. Ex y ¬x = y → C(x, y).
At least one node is an ancestor of every other node
16. Px y ¬x = y → C(x, y).
Exactly one node is an ancestor of every other node
17. Ey x ¬x = y → C(x, y).
At least one node is a descendant of every other node
18. Py x ¬x = y → C(x, y).
Exactly one node is a descendant of every other nodeNo paths of length three
19. w x y z ¬R(w, x) ∨ ¬R(x, y) ∨ ¬R(y, z).
R(·, ·) is transitive
20. x y z R(x, y) ∧ R(y, z) → R(x, z).
R(·, ·) is transitive with all self-edges removed.
21. x y z R(x, y) ∧ R(y, z) ∧ ¬x = z → R(x, z).
22. x y R(x, y) → R(y, x).
R(·, ·) is symmetric
23. Ex Ey R(x, y) ∧ R(y, x).
At least one symmetric edge
R(·, ·) is connected.
24. x y S(x, y) ↔ R(x, y) ∨ R(y, x). T (S, T). x y T(x, y).
25. Ew Ex Ey Ez ¬w = x ∧ ¬w = y ∧ ¬w = z ∧ ¬x = y ∧ ¬x = z ∧ ¬y = z ∧ Exhaustive description of the chainu v R(u, v) ↔ (p = w ∧ q = x) ∨ (p = x ∧ q = y) ∨ (p = y ∧ q = z).
26. (law not shown)
Exhaustive description of the tree
27. (law not shown)
Exhaustive description of the transitive relation
28. (law not shown)
Exhaustive description of the pair of cliques
29. (law not shown)
Exhaustive description of the ring
30. (law not shown)
Exhaustive description of the random relation
A
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Figure 2: Thirty laws used to approximate the predictions of our model. Relation C(·, ·) is the transitive closure of R(·, ·).
guage chosen, and our goal is to work towards a languagethat captures the language of thought as closely as possible. Our current language does not achieve that goal: note,for instance, that all of the Boolean connectives are assumed
to produce the same cognitive burden, but it is well known
that conjunctions are easier to learn than disjunctions in somecontexts [1]. Although our current language is little morethan a starting point, it allows us to demonstrate our theorylearning framework in action, and to show how proposals
about knowledge representation can guide and be guided byproposals about theory learning.

P (M |T ) and P (D|M ): Generating models and data
Since we are working within a finite domain, the number ofmodels for any given theory is finite. For any theory T , weuse a distribution P (M |T ) which assigns uniform probability
to any model M that is consistent with T , and zero probability
to all remaining theories.
A model M unambiguously specifies which statements aretrue, but often only some of these statements will be available to a theory-learner. Figure 1b, for instance, shows acase where a learner must reason about the social structure of a tribe given only a limited sample of positive examples. The distribution P (D|M ) should capture the assumptions that were used to generate the data D. In somecases, the data will include both positive (S(a, e)) and negative examples(¬S(a, b)), but in other cases a learner may observe only positive examples. In some cases, the data D will
be known to be uncontaminated by noise, but in other cases

the distribution P (D|M ) should acknowledge that some partsof the data D may not accurately represent the underlying
model M . Our framework can handle all of these cases, but
we will focus on a simple setting where P (D|M ) is 1 if eachobservation in D is true according to M , and 0 otherwise.
Now that all components of our model have been specified,
we can combine them to discover the theory T that best accounts for a given data set D. Our framework makes two psychological contributions: our language for representing theories is a proposal about the language of thought, and ourmodel predicts which theories people will infer when presented with a given data set. Note, however, that our approach
does not attempt to capture the psychological mechanisms orprocesses that allow humans to identify the best-scoring theory according to our model.

Learning abstract relational laws
In principle, the formal framework we described can be used
to study the acquisition of rich and complex theories. Here,however, we explore one of the simplest settings where abstract theoretical laws must be discovered. We consider problems where a learner observes a single binary relation R(·, ·)and must discover a theory that explains the structure of thisrelation. Working in this relatively simple setting allows us toprovide a transparent demonstration of our model in action,and to test some behavioral predictions of this model.
Any binary relation R(·, ·) can be represented as a graphwhere there is an edge from node i to node j if and only
if R(i, j) is true. The six binary relations that we will con-

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Figure 3: (a) Six binary relations provided as input to our theory learning framework. (b) Laws used to construct candidate
theories for each relation (numbers correspond to laws in Figure 2). (c) Posterior distribution, likelihood function, and priordistribution for each relation. Each bar represents a theory, and the theories along the x-axis of each plot are sorted in order ofdecreasing posterior probability. (d) Top-scoring theories and their posterior probabilities. For instance, the two best theories
for the ring had posterior probabilities of around 0.2, and the first of these theories included laws 5 and 14. Theories marked
with C are cases where the observed relation is defined as the transitive closure C of a latent relation R.
sider are shown in Figure 3a. Note that each relation is consistent with many abstract regularities: for instance, the firstgraph in Figure 3a is a graph where each node has at mostone child, a graph where each node has at most one parent,a graph without cycles, and a connected graph. These andmany other regularities can be formulated as abstract theoretical laws, and deciding which laws account best for a given
relation is a challenging problem.
When considering potential explanations of each data set,
our model uses a hypothesis space that includes all theories
that can be formulated in the language we have chosen. Implementing this approach raises some formidable challenges,
but we can approximate the predictions of our model. Figure 2 shows 30 theoretical laws that are consistent with one
or more of the relations in Figure 3a, and we can approximate our model by restricting ourselves to a hypothesis spacethat includes any theory which corresponds to a set of laws
from this list. In practice, even this approximation is challenging to compute, and we make one further simplification.
For each relation in Figure 3a, we choose some subset of thelaws in Figure 2, and consider all theories that can be createdby combining laws from this subset. Future work can attempt
to develop better approximations of our model, including approximations that do not rely on a hypothesis space of hand-

coded laws. Even the rough approximation considered here,
however, can provide some insight into theory learning.
The laws considered for each relation are shown in Figure 3b, and in each case we have tried to include the lawsthat seem most likely to produce theories with high posterior probability. Some theories include laws (e.g. law 12) thatrefer to relation C, and to any such theory we add the statement T (R, C) which indicates that C(·, ·) is the transitive closure of R(·, ·). For the fifth relation (the order) we include
two copies of any theory that refers to relation C: one where
the observed relation is assumed to be R and any statement
involving C merely places constraints on the extension of R,
and one where the observed relation is defined as the transitive closure C of some latent relation R(·, ·). Finally, we add
add two extra theories to each hypothesis space: the empty
theory which is consistent with any possible model, and atheory which simply enumerates the structure of the observed
relation (see laws 25 through 30 in Figure 2).
When computing the posterior probability of a given theory
T,
P the primary challenge is to compute the sum P (D|T ) =M P (D|M )P (M |T ). Since the examples in Figure 3a all
use relatively small domains, we compute this sum using thenauty program [8] to enumerate all models up to isomorphism, and to count the number of models consistent with

(b)Transfer predictions(c)Transfer results

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Figure 4: (a) Six training relations. (b) Transfer predictions generated by our hierarchical framework. Each prediction is labeledwith its posterior probability. (c) Experimental results. The number of participants who made each prediction is shown.
each isomorphism class. Note that this approach is not intended as a model of psychological processing, and is merely
a convenient way to compute the predictions of our model.
Figure 3c shows the prior distribution P (T ), the likelihood
function P (D|T ) and the posterior distribution P (T |D) for
the six problems we consider. The theories considered foreach relation are arranged along the x-axis of each plot inorder of decreasing posterior probability. The plots confirm
that the prior and the likelihood both play important roles in
our framework. Among all theories consistent with the data,
the best are those that are consistent with few other possible
data sets (i.e. theories with high likelihood) and that can beconcisely described (i.e. theories with high prior probability).
Figure 3d identifies the top four theories for each problem.
For instance, the equal top theory for the chain relation indicates that each node has at most one incoming edge (law 2),
that there is exactly one node with no outgoing edges (law9), and that there are no cycles (law 12). Some reflection
should confirm that the only relations consistent with these
constraints are chains. For each of the first five relations, thebest theory according to our framework captures some of theregularities that are apparent in the data, but the best theory
for the random relation is the empty theory.

Inductive predictions
Theory acquisition supports at least two kinds of inductive
predictions. In previous work we have explored inferences

about new or sparsely observed elements from a known domain [7]. After observing marriage ties within tribe T, for instance, a learner who has concluded that “Bob tends to havelots of spouses” can predict that a new member of tribe T is
fairly likely to end up married to Bob. Here we focus on inductive transfer, or predictions about domains that are entirely
novel. After learning about tribe T, for instance, a learner
may form expectations about the likely structure of a novel
tribe S. For instance, a learner who concludes that “exactly
one person has lots of spouses” should expect that exactly
one member of S will have many spouses.
Each relation R(·, ·) in Figure 3a is defined over a domain that includes four or five entities. Given experience with one of these domains, we explored whether ourmodel could transfer its knowledge to a novel domain with
six entities. The posterior distributions P (T |D) in Figure
P 3c provide the basis for these predictions: P (Rnew |D) =T P (Rnew |T )P (T |D) where D represents the data observed for the original domain, and Rnew is the version ofrelation R defined over the novel domain. For instance, if
the posterior distribution P (T |D) assigns high probability totheories which state that R is symmetric, then our framework
predicts will predict that Rnew is very likely to be symmetric.
Figure 4 shows the top transfer predictions for each of thesix relations. Note that each problem is highly underconstrained: we ask our framework to make predictions aboutnovel relations after observing only a single training rela-tions. Despite the challenges involved, our framework makes
predictions that seem relatively intuitive, and we tested thesepredictions in a behavioral experiment.

Experiment
We trained participants on the six relations shown in Figure 4a and asked them to describe similar relations over novelsets of entities.
Participants. 12 members of the MIT community were paid
for participating in this experiment.
Materials and Methods. The experiment included six
within-participant conditions that correspond to the six relations shown in Figure 3a. In each condition, participantslearned a single relation (the training relation) then generated
similar relations for two novel domains, one with six entities
and the other with seven entities.
The cover story informed participants that they were learning about the organization of several small companies. Each
relation in Figure 3a captures information flow within one of
the companies. The experiment was carried out on a computer, and during the training phase the interface had a single
button labeled “Observe.” Upon clicking this button, participants were told about an event corresponding to one of theedges in the current training relation: they might be told, forexample, that “John sends an envelope to Bill” (employee
names were randomized across participants). After somenumber of observations, participants were given a test which
included a yes/no question about each pair of employees (e.g.
“Does John send envelopes to Bill?”). Participants continued
to observe edges in the training relation until they were able
to answer all of the test questions correctly.
After participants had passed this test, they were told aboutanother company in the same industry with six employees,
and were asked to “indicate one way in which the company
might be organized.” Responses were provided by checking
boxes on a screen which included one box for each (directed)
pair of employees.
Results. The relations most commonly generated for the sixemployee companies are shown in Figure 4. These results
confirm that people are able to discover abstract relational
regularities given a single training relation. Our model provides a good account of these findings: for all conditions except the tree condition, the top (or equal top) choice is identical to the top (or equal top) choice according to our framework. Some aspects of the data, however, are not captured by
our approach. For instance, humans often choose a transfer
relation that can be generated by connecting new nodes to a
copy of the training relation, and this preference might explain why humans do not generate a set of three pairs in thecliques condition, even though our model rates this configuration as its equal top choice.
Although our formal framework does not account for all
aspects of our results, we know of no other computationalframework that is likely to perform better. In particular, approaches (e.g. [7]) that simply search for the shortest description of the data in a first-order language will not be adequate.

Note, for instance, that the shortest description of the ring is
an exhaustive list of the edges in this relation, and contains noabstract laws that provide a basis for inductive transfer. The
hierarchical aspect of our approach is essential for discovering abstract regularities in cases like this, and may capture
one of the principles that allow humans to discover abstract
theoretical laws.

Conclusion
Proposals about the language of thought and about theory acquisition should be tightly coupled. Knowing how theories
are represented should allow us to predict which theories will
be readily learned by people, and identifying the theories thatpeople learn readily should provide important clues about the
representations that support this ability. We described a computational framework that supports both kinds of investigation. We suggested that theories that are represented in a
logical language, and explored the inductive consequencesof one such language. Our language is proposed as a very
rough approximation to the language of thought, but can certainly be improved in many respects. More important than
the specific language we considered is the general framework
we described: a framework for simultaneously exploring how
theories are represented and how they are acquired.
Many knowledge representation schemes have been proposed by psychologists, computer scientists, and philosophers, including the lambda calculus, many flavors of logic,
and several languages for representing semantic networks.
Previous work, however, has not always focused on the inductive consequences of these languages. An important direction
for future work is to combine some of these proposals with
our framework, and to discover which languages lead to inductive predictions that best match human inferences. Probably no existing language will turn out to be entirely satisfactory, but understanding the strengths and weaknesses of existing representation languages should lead to future languages
that correspond more closely to the language of thought.

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