Design Explanation: determining the constraints on what can be alive
Arno G. Wouters
Heyendaal Institute, Radboud University Nijmegen, The Netherlands

Final Draft
17 February 2007
To be published in Erkenntnis
The publication will be available at www.springerlink.com

Arno Wouters,
Heyendaal Institute,Radboud University Nijmegen,
P.O. Box 9103,
6500 HD Nijmegen,
The Netherlands
Arno.Wouters@hin.ru.nl+31 (0)24 36 11689

Design Explanation

Abstract
This paper is concerned with reasonings that purport to explain why certain organisms havecertain traits by showing that their actual design is better than contrasting designs. Biologists
call such reasonings ‘functional explanations’. To avoid confusion with other uses of thatphrase, I call them ‘design explanations’. This paper discusses the structure of design
explanations and how they contribute to scientific understanding. Design explanations arecontrastive and often compare real organisms to hypothetical organisms that cannot possibly
exist. They are not causal but appeal to functional dependencies between an organism’s
different traits. These explanations point out that because an organism has certain traits (e.g.,
it lives on land), it cannot be alive if the trait to be explained (e.g., having lungs) were
replaced by a specified alternative (e.g., having gills). They can be understood from amechanistic point of view as revealing the constraints on what mechanisms can be alive.

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1

Introduction

Why do fishes respire with gills rather than lungs? Why do tetrapods respire with lungs rather
than gills? Why don’t these organisms respire through their skin? These questions are
discussed in any intermediate level textbook that deals with respiratory physiology (e.g.,
Johansen 1977, Dejours 1981, Withers 1992: chs. 4 & 12, Liem et al 2001: ch. 18, Maina
2002) and they are answered by showing that in the conditions that apply to the relevantorganisms, the actual design is better than the contrasting design. Biologists call suchreasonings ‘functional explanations’. To avoid confusion with other uses of this term, I shallcall them ‘design explanations’. As I shall show, design explanations are contrastive, theyoften compare real organisms to fictitious organisms that cannot possibly exist, and they are
not causal, but appeal to a kind of relation (functional dependence) that has been ignored in
the philosophy of science. As I shall shortly discuss, the standard accounts of functional
explanation do not apply to design explanations. In this paper I aim to provide a detailed
account of the structure of design explanations and to explain how they add to scientificunderstanding.1 My main conclusion is that they reveal the constraints on being alive.
The question of how functional explanations contribute to scientific understanding is along-standing issue in the philosophy of science (see Wouters 2005 for an overview). I
discuss this question in view of the causal-mechanical approach to explanation. According tothis approach, explanations show how a certain event, state, regularity, activity or capacity
fits into the world’s causal structure. This approach is best known from Salmon (1984).
Most theories of function depict functional explanations as statements of the form ‘The
function of x is f’ brought up to answer a request for an explanation of x (x can be anything ofa living organism: a part, a behavior, a trait, the structure of an organ, and so on). This view is
nicely summarized by Andrew Woodfield in a recent encyclopedia:
Typically, a functional explanation in biology says that an organ x is present in an animal [sic!] because x
has function F (Woodfield 2000: 493)

The standard example concerns the heart. The idea is that biologists explain the presence of
this organ simply by uttering: ‘The function of the heart is to pump the blood around.’Another important assumption is the idea that functions are a special kind of effect. An
item such as the heart has many effects, but only some of them are functions.
Given this picture of functional explanations as consisting of function attributions, given
the impression that functions are effects, and assuming that explanations are causal, the mainquestion about functional explanation becomes how an appeal to an effect of a trait’s
presence could explain why that trait is present. The three main kinds of answers to thisproblem are: (1) selected effect theories, (2) systemic theories and (3) life chances theories.
These theories do not necessarily exclude each other. The dominant opinion is that
biologists employ two notions of functions associated with two kinds of explanation:systemic functions, which are used to explain how a certain capacity or activity is brought
about in a certain individual; and selected effect functions, which are used to explain why
1 It is not my goal to analyze the notion of design (I aim to analyze a kind of reasoning), to analyze a pre-existing notion of ‘design explanation’ (this notion is stipulated), or to define the phrase ‘design explanation’.
Rather, I aim to describe and account for the scientific value of a kind of reasoning that is ostensively defined.

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certain organisms have a certain trait by specifying the effects for which that trait wasmaintained in the lineage in the past.
According to selected effect theories (for example, Williams 1966, Neander 1980,
Millikan 1984), function attributions explain the presence of a certain trait by specifying theeffects for which that trait was selected in the past. For example, according to these theories,
the statement ‘The function of tetrapods’ lungs is to respire’ explains why tetrapods havelungs by pointing out that in tetrapods’ lineage, lungs were maintained in the process ofnatural selection because they helped the organism to respire. This approach provides anelegant solution to the aforementioned problem of functional explanation: functions are pasteffects and past effects can, of course, explain current presence (see Salmon 1989, § 3.8;
Mitchell 1989; Neander 1991). However, design explanations differ from functional
explanations on the selected effect view (i.e., evolutionary selection explanations). First,
design explanations are contrastive (a feature that is remarkably absent from all existingaccounts of functional explanation), and they often contrast real organisms with merely
hypothetical alternatives that never existed (evolutionary explanations appeal to past fitness
differences between variants that actually did exist). Second, design explanations areconcerned with synchronic relations between the different traits of an organism and theenvironment in which it lives, rather than with what happened in the past. So, whatever the
merits of the selected effect approach, it does not account for design explanations (seeWouters 2003: Section 3.2).
According to systemic theories (for example, Cummins 1975, 1983; Craver 2001),
function attributions inform us about how an item contributes to the workings of a complex
system. Such attributions of systemic functions are an essential part of explanations that
explain a complex system’s behavior by describing a mechanism that produces that behavior.
For example, according to systemic theories, the attribution to the lungs of the respiratory
function helps to explain how the organism gets enough oxygen.
This notion of function as a contribution to an important capacity is, indeed, the central
notion of function in biology. I shall call a function in this sense a ‘biological role’ if its
description fits in the general scheme that biologists use to explain organisms’ maintenance,
growth and development, and the reproduction of offspring (see Wouters (2003) for an
extensive discussion of the different notions of function in biology). Explanations that explain
how a certain capacity is brought about by specifying an underlying mechanism play animportant role in functional biology. Biologists call such explanations ‘causal explanations’.
Design explanations address a different kind of question. Given a system (e.g., a gill or alung) that performs a certain biological role (e.g., gas exchange), an explanation that specifies
an underlying mechanism would relate how the capacity to perform that role is brought about
by the organized activity of that system’s parts (it would, for example, explain how gas
exchange takes place in the gills or lungs). A design explanation, on the other hand, would
point out why that biological role is performed the way it is, rather than in some otherconceivable way (for example, why it is performed by a specialized organ, and why certain
kinds of organisms perform this role with gills, whereas others do so with lungs). This design
explanation would do so by specifying the features of the relevant organisms and theirenvironment (such as the organism’s size, activity and respiratory medium) that make theactual design better than the alternatives, and by elucidating why, in these circumstances, theactual design is better than those alternatives, by appealing to invariant generalizations (suchas Fick’s law of diffusion).
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A third approach, the life chances approach (Canfield 1964, 1965; Ruse 1971, 1973;
Wimsatt 1972; Bigelow & Pargetter 1987), defines functions as hypothetical advantages: atrait’s function is an effect of that trait’s presence which makes the organism’s life chances
better than they would have been if that effect were lacking. For example, according to thiskind of theory, it is the function of the lungs to respire because the fitness of organisms thathave a lung that aids in respiration is higher than their fitness would have been if their lungs
did not do so.
This account captures an important characteristic of design explanation, namely, its appeal
to counterfactual comparison. It has been criticized for this because the truth of statements
about the life chances of hypothetical organisms lacking a certain trait would beindeterminate (e.g., Millikan 1993). However, as counterfactual comparison is ubiquitous infunctional biology,2 it should be accounted for, rather than denied. Wouters (2003: Section
3.1) argues that the problem of indeterminate truth value arises because the current life
chances theories attempt to explicate a non-comparative notion (a function of .... is to ....) incomparative terms. This works only if there is a unique reference situation. Moreover, thisway of explicating ‘function’ confuses two notions of function, namely, the non-comparative
notion of function as biological role (a role of .... is to ....) and the comparative notion offunction as biological advantage (an advantage of .... over .... is ....). The problem dissolves if(1) the comparative analysis is restricted to advantages, and (2) advantages are explicitly
defined relative to a well-specified reference situation.
Wouters’ analysis shows how counterfactual comparisons can have a definite truth value.
However, an account of the insights such comparisons provide is still lacking. Why dobiologists engage in such reasonings? What do they learn from them? Some sympathizers oflife chances views have suggested that appeals to advantageous effects can causally explain
why organisms have the fitness they have (Bigelow & Pargetter 1987, Mitchell 1989, Walsh
1996). However, biologists think of design explanations as providing insight into why
organisms have the traits they have (e.g., why certain kinds of organisms have lungs andother kinds have gills). I argue that this insight can be accounted for if design explanations
are seen as reasonings that are concerned with the constraints on what forms of organization(i.e., designs) can be alive, rather than with the causes of how these designs came into being.3
In the next section (Section 2) I give some detailed examples of the reasonings that I call‘design explanation’. In Section 3 I specify the reasoning structure that emerges from theseexamples and show that it does not fit the standard view of functional explanations asfunction attributions that answer why-questions about something biological. This description
should make it clear that the heart of a design explanation is not a function attribution, but a2 See Wouters (2003) for a half dozen examples from textbooks and research papers. A quick look into any

zoology textbook or journal would reveal many more, including explanations of why bats are almost exclusively
nocturnal rather than diurnal (Thomson & Speakman 1999); of why emperor penguins produce calls by using
two independent sound generators, rather than one (see Sturdy & Mooney 2000); of why tiger moths inform
predatory bats of their unpalatability by using clicks, rather than other kinds of cues (Ratcliffe & Fullard 2005);
of why nocturnal, terrestrial vertebrates have multifocal, rather than monofocal, lenses; of why multifocal eyes
have split pupils rather then round ones (Malmström & Kröger 2006); and of why the mammalian inner ear hasa spiral shape (rather than being stretched out) (Manoussaki, Dimitriadis & Chadwick 2006).
3 I do not claim that constraints can never be causal, only that constraints on what can be alive are not.

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statement expressing a functional dependence relation: the relevant organisms’ ability to livein the conditions in which they live depends on those organisms having the traits to be
explained, rather than the contrastive ones (for example, if an organism exhibits a certain size
and activity, it cannot be alive if its specialized respiratory organs are replaced by respiration
through the skin).4 In Section 4 I discuss the nature of this functional dependence relation andthe manner in which it is explained. I argue that functional dependencies are individual level,
synchronic relations that constrain what can be alive, but do not bring about thecharacteristics satisfying this constraint (in other words, they are not causal). The existence ofsuch constraints can be explained by appealing to the relevant conditions and the invariant
relations (“laws”) that apply in those conditions. This opens the way to an account of thescientific value of design explanations in Section 5. I end up by drawing some conclusions in
Section 6.
2

ExamplesMy examples concern how vertebrates take in oxygen from, and release carbon dioxide into,
the environment. This process is called ‘respiration’. Adult tetrapod vertebrates (e.g.,
amphibians, reptiles, birds and mammals) typically respire with lungs;5 whereas fishes
typically respire with gills.6 By definition, gills are evaginated organs (i.e., outgrowths of thebody) and lungs are invaginated. Fishes’ gills typically consists of a minutely subdivided
surface area, composed of numerous ultra fine lamellae, across which water is pumped in one
direction (from the mouth over the gills to the outside). The lamellae are richly supplied withblood vessels; the distance from the water to the blood can be less than 1 µm. The tetrapodlung is essentially an ingrowth of the foregut. In its simplest form it is just a sac with ridges
and edges that increase its surface area. More advanced lungs consists of a system ofconducting and distributing tubes which divide further and further until they end in blind,thin-walled sacs (alveoli) where gas exchange occurs. Lungs are ventilated by pumping air inand out them (this is called ‘breathing’). The flow of air in the lungs has a tidal character,with both inhalation and exhalation phases.7 In contrast with gills, gases are not activelytransported over lungs’ respiratory surfaces; the alveoli contain dead air through which
oxygen and carbon dioxide are transported by diffusion alone.
My example concerns two questions about how organisms respire. The first is, why do somany organisms use specialized organs (such as gills or lungs), rather than their outer surface,
for respiration? The second question concerns these organs’ specific character: why do fishes
respire with gills, and tetrapods with lungs?
4 My account of design explanations’ structure is much closer to the accounts of Hempel (1959) and Nagel(1961) than the standard accounts are. However, my account of design explanations’ scientific value is very
different from their account of functional explanations’ scientific value. Hempel and Nagel had an inferential
view of explanation; my account is mechanistic.
5In many amphibians the skin, too, plays an important role in respiration.
6Some fish do use lungs.
7The flow in birds’ lungs is virtually unidirectional. This meets the increased demand for oxygen that their

flying lifestyle imposes on them. The flow in their upper respiratory tract is tidal.

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In order to answer such questions biologists typically refer to Fick’s law of diffusion. Thisis a law of physical chemistry which states that the rate of diffusion of a gas is proportional toboth the surface area available for diffusion and the concentration gradient over which
diffusion takes place:
J = – D A ΔC/Δx
in which:Jthe rate of diffusion (mole/s)Dthe diffusion coefficient (cm2/s)

AΔx

the surface area available for diffusion (cm2)the distance of diffusion (cm)ΔC

the concentration difference over that distance (mole/cm3).

The sign indicates the direction of diffusion; it is negative here because diffusion goes fromhigher to lower concentrations. The diffusion coefficient depends on the nature of the
diffusing gas, the medium in which diffusion takes place and the temperature. The quantity
ΔC/Δx is the concentration gradient (mole/cm4).
The answer to the first question (why is respiration concentrated in a specialized organ?)points to the many disadvantages of using the body’s entire surface for respiration. In order toreach a certain level of activity the organism must generate a certain amount of energy, andthis, in turn, requires a certain amount of oxygen and generates a certain amount of carbon
dioxide. From Fick’s law it follows that if the organism’s need for oxygen exceeds a certain
level, the respiratory surface must meet certain requirements in order to assure that the
diffusion rate (J) is large enough to fulfill the organism’s needs: (i) the distance across which
the gas must diffuse (x) must be small enough, (ii) the surface available for diffusion (A) mustbe large enough, and (iii) the material across which the gas diffuses must be readily
permeable to that gas (D must be large). In other words, the respiratory surface must be thin,large and permeable. This precludes the animal from using its body’s entire surface forrespiration: a thin skin is easily damaged, enlargements of the outer surface would disturb its
streamline, and a skin that is easily permeable to oxygen and carbon dioxide is also easily
permeable to water (which is a severe disadvantage both on land, and in aquatic environments
whose osmotic pressure differs from the organism’s). Specialized respiratory organs provide
the means to solve this problem.
In order to answer the second question (why do fishes respire with gills and tetrapods withlungs?), biologists compare the physical qualities of water and air. The concentration ofoxygen in air is about 30 times higher than in air-saturated water, the diffusion coefficient ofoxygen is about 30,000 times higher in air than in water, water is more than 800 times as
dense as air, and water is about 60 times more viscous than air. Animals that obtain oxygenfrom water have to solve three problems. First, due to water’s low oxygen content, they needto ventilate a much larger volume than air breathers to extract the same amount of oxygen.
Second, uptake of oxygen from water is very inefficient, due to both its lower oxygen content(ΔC/Δx is lower) and lower diffusion coefficient (D). Third, because of water’s higher densityand viscosity, it costs much more energy to move a certain volume of water over a certain
distance than to move the same volume of air over that distance. Respiration in air faces other-6-

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Design Explanationdifficulties, including the constant risk of desiccation and the problems caused by gravitation.
These problems do not occur in water.
The differences in design between gills and lungs are explained by these physicaldifferences and the different problems that “result” from them. The respiratory surface of gills(A) is immensely larger than that of lungs, and gills’ respiratory membranes are much thinner(Δx is lower). This increases oxygen uptake efficiency. The unidirectional character of thewater’s flow across the gills saves the costs of reversing the flow. In addition, the continuous
stream of water over the respiratory surface increases the oxygen uptake because it increases
the oxygen concentration at the outside of the respiratory surface. Due to the problems ofgravitation, the vast increase in gills’ surface area is not possible on land; the finely divided
and thin lamellae would collapse against each other. The thin membranes and unidirectional
flow also would increase the risk of desiccation.
Lungs are internal, which reduces evaporation and provides structural support tocounteract gravity. The tidal flow of air further reduces the loss of water. Lungs would notwork in an aquatic environment for several reasons.8 One is that the lungs’ pumping
mechanism, breathing, has much less power than gills’ pumping mechanism (this is no
problem for air breathers because moving air requires much less energy than moving water).
Another reason is that the tidal character of air flow in the lungs greatly decreases ventilation
efficiency (recall that respiration in water imposes much more demands on ventilation thandoes aerial respiration).
3

The basic structure of design explanations

As I indicated in Section 1 above, functional explanations are routinely viewed as answering
a question of the form ‘Why do certain organisms have / perform x?’ with a function
attribution of the form ‘In those organisms the function of x is to do f’. It will be clear frommy examples that design explanations’ structure is much more complex than this.
First, the questions design explanations address are explicitly or implicitly comparative.
The first example asked why so many animals respire by means of a specialized organ rather
than using the entire skin. The second example asked why aquatic vertebrates use gills rather
than lungs for respiration, and why land vertebrates use lungs rather than gills. This question
is a shorthand for more specific questions, such as, ‘Why is the surface of fishes respiratoryorgan more minutely divided than that of land vertebrates?’; ‘Why do land vertebrates use aninternal, rather than an external, organ to respire?’; and ‘Why do fishes have a unidirectional
flow over their respiratory surface, whereas land vertebrates employ a tidal flow system?’.9
Second, the explanations employ two kinds of function attributions, namely, functions as
biological roles (the kind of function envisaged by the systemic view) and functions as
biological advantages (the kind of function envisaged by the life chances view). Attributing
the biological role of respiration to the lungs and the gills plays a key role in my examples’
explanations. Those explanations use this attribution to explore the advantages and
8Fishes with lungs are no exception, as they use their lungs to breathe air.
9 Note that design explanations always compare the real design with a limited range of alternative designs. Theynever address the question of why a certain design is better than all other conceivable ones. We can always
imagine better solutions (whose lack of existence has no design explanation – see Section 5).

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disadvantages of the item that performs this role having a certain character rather than another(e.g., having the character of a lung, rather than a gill). The first example details the many
disadvantages of using the skin, rather than a specialized organ, for respiration. The second
example details the advantages of using lungs, rather than gills, on land and gills, rather thanlungs, in water.
Third, the answer consists of much more than function attributions. In addition to
specifying the biological role of the item or activity in question, and what advantages it has inperforming that role the way it is performed, design explanations point out what internal andexternal conditions make this character advantageous, and elucidate how those conditions are
related to the advantages in terms of the laws of nature. In the first example, the relevant
conditions include such things as the organisms’ activity, their size and how they obtainenergy. In the second example, the respiratory medium is an important condition. Fick’s lawof diffusion plays an important role in all these examples.
In my view, a design explanation addresses the question of why certain organisms have
one trait rather than another. The design explanations in my examples compare different ways
in which the biological role of respiration is performed. In my first example (why aspecialized organ?) the question concerns those animals that use a specialized organ to
respire, and asks why they do so rather than using the entire outer surface. My second
example (why gills in water and lungs in air?) is concerned with two such questions. One isabout fishes that perform aquatic gas exchange, and it asks why they respire with an organthat has the specific characters of gills (such as their minutely divided lamellae andunidirectional flow), rather than with an organ that has the specific characters of lungs (suchas the fact that they are internal, the existence of dead space, and the tidal character ofventilation). The other question concerns tetrapods that respire with lungs, and it asks why
these organisms respire with an organ with the specific characters of lungs, rather than withan organ with the specific characters of gills.
The explanation that answers this type of question:(1) specifies conditions that apply to the relevant organisms;(2) states that, due to those conditions, the trait those organisms have is more useful tothem than the alternative (the nature of this relation is discussed in Section 4 below);and(3) explains (2) (this part of the explanation, too, is discussed in Section 4 below).
In the examples above:
Animals that use a specialized organ to respire do so (rather than using their body’s entire outer surface)because:(1a) Those organisms’ activity and size exceed a certain level.
(1b) They use oxygen as an electron acceptor in the metabolic pathway with which they obtain energy.
(2)

If an organism’s activity and size exceed the level mentioned in (1), the biological role of respiration isbetter performed with a specialized organ than by using its entire outer surface.

(3a) In order to reach the level of activity mentioned in (1a), an organism of the size mentioned in (1a) needsa certain amount of energy (law of physics).
(3b) To get this amount of energy the organism needs a certain supply of oxygen (the way that aerobic
organisms get their energy, see 1b).
(3c) In order to satisfy this need the respiratory surface must be thin, large and permeable (follows from
Fick’s law).

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Design Explanation(3d) A skin that satisfies these conditions would have the disadvantages of being fragile (material physics),
lacking a streamline (hydrodynamics) and suffering from osmotic problems (physical chemistry) and/or
desiccation (physics). These problems are avoided if a specialized organ is used.
Fishes that employ aquatic gas exchange use gills rather than lungs for respiration because:(1a) They take their oxygen from water.
(1b) They are moderately active and their size is within certain limits.
(1c) They use oxygen as an electron acceptor to generate organic energy.
(2)

If the respiratory medium is water and the fishes’ level of activity and size are as mentioned in 1b, thebiological role of respiration is better performed with gills than with lungs.

(3a) Given the activity and size mentioned in 1b the organism needs a certain supply of oxygen (laws ofphysics and way that aerobic organisms get their energy).
(3b) Due to water’s low oxygen content and very small diffusion coefficient, the organism must ventilate
huge amounts of water in order to get enough oxygen (arithmetic + Fick’s law).
(3c) Due to water’s density and viscosity, much power is needed to move this amount of water (physics).
(3d) Because of their smaller respiratory area, their larger distance between the medium and the blood, thedead air in their alveoli and the tidal character of their air flow, lungs are less efficient than gills (Fick’slaw). This means that if fishes were to use lungs to take up oxygen from water, they would need evenmore power than they do now. The lungs’ ventilation mechanism is less powerful than that of gills (theway lungs and gills are built) and does not have enough power to move the required amount of water.
(3e) Due to their large respiratory area, their small distance between the medium and the blood, and theircontinuous, unidirectional flow over the respiratory surface, gills are more efficient than lungs (Fick’slaw). In addition they have much more power (the way lungs and gills are built). As a result they areable to provide the required amount of oxygen at the required rate.
Tetrapods that respire with lungs do so rather than with gills because:(1)

They live on land and perform a certain level of activity, their size is within certain limits, they use
oxygen as an electron acceptor to generate organic energy and they take their oxygen from the air.

(2)

If an animal lives on land and its respiratory medium is air, the biological role of respiration is better
performed with lungs than with gills.

(3a) Animals that live on land and/or breathe air are subject to gravitation (physics).
(3b) Animals that breathe air lose water via their respiratory surface (physics).
(3c) Due to gravitation, gills would collapse on land (physics); the resulting decrease in respiratory surface
would kill the animal (aerobic metabolism, certain activity).
(3d) Furthermore, air breathing animals with a unidirectional stream over a thin respiratory surface would dryout (physics).
(3e) Lungs are internal. This reduces the loss of water via the respiratory surface (physics) and allows forstructural support that prevents the respiratory surface’s collapse (geometry). This solves the problemsof gravitation and desiccation.

4

Functional dependencies and their explanation

The heart of a design explanation is a statement (2) that says that due to certain conditions, a
trait that an organism actually has is more useful to that organism than a conceivable
alternative. The notion of utility or biological value refers to an organism’s ability to stayalive (i.e., to maintain itself, to grow, to develop and to produce offspring). Roughly
speaking, a certain trait (e.g., using a specialized organ for respiration) is more useful to a-9-

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certain organism than is another trait (e.g., using its entire outer surface for respiration), if thereal organism’s ability to stay alive is higher than that of a hypothetical organism in which theactual trait is replaced by the alternative. In many cases, the hypothetical organism cannot
live. In such cases it is said that the actual trait ‘is needed’ by the real organism (in therelevant circumstances).
Statement (2) above says that the difference between the real and the hypotheticalorganisms’ ability to stay alive ‘is due to’ certain conditions. I call the kind of relation thatexists between the conditions and the trait to be explained ‘functional dependence relations’.
Biologists often express this kind of relation by saying that the conditions ‘demand for’ theorganism to have the actual trait rather than the alternative. If the condition is another of theorganism’s traits it is often said that this latter trait ‘depends on’ the trait to be explained (e.g.,
living on land depends on using lungs (rather than gills) to respire). Phrases such as ‘is dueto’, ‘depend on’, and ‘demand for’ indicate that the conditions are causally relevant to theutility of the trait to be explained. Roughly speaking ‘causally relevant’ means that if theconditions did not apply, the hypothetical organism’s ability to stay alive would not be lessthan that of the actual one.
Functional dependence relations are individual level relations in the sense that they relatedifferent traits of the same individual (e.g., an animal that lives on land needs lungs). They
are synchronic in the sense that the need must be satisfied at the time that the demand arises(an organism that has the need but cannot satisfy it is not viable). This is an importantdifference between design explanations and evolutionary explanations. The latter appeal tohistorical relations between different individuals (that is, at the level of the lineage); in an
evolutionary explanation it is the past performance of ancestral individuals with a certain trait
that explains the presence of that trait in current individuals.
The nature of functional dependence relations is different from the nature of relations thatfigure in causal explanations. Woodward (1997) argues that the kinds of generalizations thatfigure in causal explanations are generalizations that are invariant under at least some(hypothetical) interventions that change the conditions described in the antecedent. Following
Glennan (2000: S344), I call such generalizations ‘invariant, change-relating generalizations’.
Invariant, change-relating generalizations are weaker than the kind of generalizations thatphilosophers call laws, and stronger than accidental generalizations (such as thegeneralization that all the coins in my pocket are made of copper) and common cause
generalizations (such as the relation between the barometer’s reading and the weather).
Statements expressing functional dependencies are not invariant, change-relating
generalizations; the presence of the needed trait is not invariant under interventions on thedependent trait. For example, although (given a certain size and activity) living on land
depends on the presence of lungs and living in water depends on the presence of gills, wecannot change a dog’s lungs into gills by forcing that dog to breathe underwater. Yet, unlike
accidental generalities and like causal relations, functional dependencies are in a sense
physically necessary: an organism that has the dependent trait cannot be alive (or will be lessviable) if it has the alternative trait instead of the needed one. In other words, a functional
dependency is a constraint on what can be alive.
Unlike accidental generalizations and like causal regularities, functional dependencies canbe explained on the basis of invariant relations. This is what part (3) of a design explanation
does. Part (3) (I) specifies a structure of functional dependencies that relate the conditions
under (1) to the utility of having the trait to be explained, and (II) relates this structure of- 10 -

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functional dependencies to invariant relations that result from the way the organism is wired,and from more general invariant relations that scientists call ‘physical laws’ (in the schemesof reasoning at the end of Section 3 above I have indicated part (II) between brackets).
For example, part I of the explanation of why so many organisms use a special organ to
respire specifies that the organisms’ level of activity and size depend on their ability togenerate a certain amount of energy (3a). This, in turn, depends on a certain supply of oxygen(3b). This, in turn, demands that the respiratory surface be thin, large and permeable (3c);which, in turn, demands a specialized organ (3d). Part II relates each of these dependencies toinvariant, change-relating relations that apply to the relevant organisms in the conditions
mentioned in (1). The explanation of the functional dependence of activity and size on acertain level of energy production (3a) points out that (i) there is an invariant relation between
body mass and energy consumption (energy consumption increases with activity and bodymass), and (ii) energy consumption can never be larger than energy production. The
explanation of energy production’s functional dependence on oxygen supply (3b) points outthat (i) there is an invariant relation between energy production and oxygen consumption (theproduction of a certain amount of energy uses a certain amount of oxygen), and (ii) the
oxygen supply must meet the oxygen consumption. The explanation of the oxygen supply’s
dependence on the respiratory surface being thin, large and permeable (3c) employs Fick’s
law: the only combination that satisfies the oxygen need of a large and active organism is one
in which Δx is small, A is large and D is small. The final step (3d) aims to show that the
hypothetical organism with the alternative trait would not satisfy its needs as well as the realorganism does. This can be done directly, but the example does it by means of an advantage
articulation. The explanation states at this point that an organism that satisfies the need for alarge, thin and permeable respiratory surface and uses its skin to respire will suffer somedisadvantages that the solution in the real organism does not present; e.g., its skin will beeasily damaged, the organism will lack streamline and may have problems withosmoregulation and desiccation. The disadvantages of this implementation are calculated withthe help of invariant generalizations.
The invariant, change-relating generalizations used in (3II) to explain the functional
dependencies of (3I) need explaining. This is done by describing the mechanism underlying
this relation (see Glennan 2000). The process of providing deeper explanations ends if the
explanation of a certain generalization is considered to be part of another discipline or if the
explanation is registered as a yet unsolved problem. For example, the explanation of the
precise relation between body mass and energy needs (3ai) is a major unsolved problem inanimal physiology. It is known from physics that the energy need increases with activity andbody mass. However, the relation between body mass and energy need is not linear as might
be expected at first. Measurements show that energy needs are proportional to body mass by afactor of (about) 0.75 (this factor is called the mass exponent), but it is unclear why this is thecase. The explanation of the relation between the needs for energy and oxygen (3bii),
however, is well understood. This relation is determined by the metabolic pathways involvedin the production of organic energy. Knowledge of the relevant pathways allows us tocalculate the oxygen need. The explanation of Fick’s law (3c) is a matter of physicalchemistry, which biologists simply take as fundamental.

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5

How design explanations contribute to scientific understanding

According to the causal-mechanical view of explanation, explanations reveal the mechanismsthat produce the event or regularity to be explained. This means that the main insightproduced by a causal-mechanical explanation is the insight into how (i.e., by means of whatcauses and mechanisms) an event or regularity is brought about. However, as I haveemphasized in the preceding section, design explanations are not causal. The functional
dependence relation that is central to design explanations (for example, the relation between
an organism’s activity and size and its use of a special organ for respiration, or the relation
between the environment (water/land) and the character of the respiratory organ (gills /lungs)) does not vary under interventions on the related variables, and it provides no insight
into how the respiratory organs are brought about (neither in the course of individual
development, nor in the course of evolution).
To understand what design explanations add to scientific understanding, it should be notedthat the state of being alive critically depends on the individual’s organization, that is, on thespatial arrangements of its parts and the timing of their activities (see Wimsatt (1997) andCraver (2001) for this notion of organization). As the diversity of living organisms testifies,
there are many ways (‘forms of organization’ or ‘designs’) to construct mechanisms that arealive. However, not all arbitrary combinations of parts result in a living mechanism. Certain
ways of life (e.g., living an active life on land) are possible only if certain constraints are
satisfied (e.g., the problems of desiccation and gravitation must be solved). This makes it
possible to explain the structure and activity of a living mechanism’s parts on the basis of theconstraints imposed on it by other characteristics of this mechanism and the conditions underwhich it works. This is what design explanations do.
Design explanations explain the structure and activity of a certain part of a living
mechanism by (i) showing that this mechanism would not work as well as it does if this partor behavior were replaced by one with other characteristics (design explanations show that
this part or behavior is useful or needed), (ii) specifying other traits of the mechanism thatgive rise to this constraint on being alive (design explanations specify which traits
functionally depend on the part or behavior to be explained), and (iii) describing how thisfunctional dependency results from the conditions and invariant, change-relating relations that
apply to this mechanism (design explanations tell us why this dependency holds). As a result
it becomes clear why the mechanism consists of the parts of which it consists and why thoseparts are organized the way they are in the context of the overall design of the organism.
Some readers might object that, precisely because the reasonings that I have called ‘design
explanations’ do not give insight into the causes of the phenomenon to be explained, they do
not deserve to be called ‘explanations’. For example, in the view of Mahner and Bunge(2001), explanation is by definition causal. To show that a certain structure or activity isuseful is “a description or perhaps even a subsumption, but not a mechanismic explanation”(p. 83). I have no opinion about the answer to the question of whether or not design
explanations really are explanations. In fact, I don’t see that this question makes any sense atall. However, it should be noted that design explanations do not merely show that a certain
structure is useful, but also why it is so. The latter is done not merely by subsuming it under afunctional dependence relation, but also by explaining why this relation holds. In this respect,design explanations do not differ from mechanistic explanations of causal regularities such as
the explanation of the general gas law by statistical mechanics.

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Design Explanation

Like the explanation of the periodic table of elements in chemistry and the explanation of
Kepler’s laws by Newtonian mechanics, design explanations are concerned with what canpossibly exist and what cannot. Design explanations seek to explain why an organism withthe trait to be explained can be alive whereas a similar organism in which that trait is replacedby another one cannot. No principles restrict the kinds of contrasts for which we might want
to seek design explanations. Note, however, that not all contrastive why-questions have adesign answer. If the alternative trait is more useful than or functionally equivalent to the real
trait (why are our bones white rather than purple?), a design explanation of why certain
organisms have that trait rather than the alternative cannot be given. This is simply not thekind of question a design explanation can address. It is an empirical issue (rather than aphilosophical one) which particular contrasts and how many contrasts can be explained by adesign explanation.
There are also many questions for which a design answer would be irrelevant. There arethree types of limitations on living beings’ design: (1) it must be possible to generate
individuals with that design, (2) once generated, a design must be viable (the organisms
having that design must be able to stay alive), and (3) there must be an evolutionary path tothat design. Design explanations are concerned with only the second kind of limitation.
Clearly, if there is reason to assume that a certain design cannot be generated (how to supply
a freely rotating wheel with blood vessels?), consideration of its ability to stay alive would be
a waste of energy, time and money. Similarly, if we have reason to assume that no viable
evolutionary path can connect an alternative design to (past or current) living forms, there
would be no need to seek a design explanation.
6

Conclusion

Functional biologists devote a large part of their time to the construction of design
explanations. Up to now there was no satisfactory philosophical account of what those
explanations add to science. This paper sketches how such an account can be given within amechanistic view of explanation. Design explanations address the question of why it is more
useful to certain organisms to have a trait they have rather than some conceivable alternative.
They answer this question by appealing to the constraints on being alive set by other traits of
the organism and its environment. The explanation details those traits and shows that due tothose other traits, the organism would be worse off if the trait to be explained were replacedby the alternative. The existence of this functional dependency in its turn is explained by
specifying a structure of intermediate functional dependencies that relate the explaining traits
to the traits to be explained. Each dependency in this structure is subsequently explained interms of invariant, change-relating relations (some of them result from how the organism isbuilt; others are more general). In doing so, design explanations provide both anunderstanding of why a certain trait is needed in the context of the overall design of a living
mechanism, and an insight into the constraints on the construction of living mechanisms.
Acknowledgements
Earlier versions of this paper were presented at the ISHPSSB meeting in Quinnipiac (June
2001) and at the Séminaire de la Philosophie de la Biologie of the IHPST in Paris (December- 13 -

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2005). The author thanks Bob Richardson, Fred Boogerd, Stuart Glennan, Françoise Longy,
Michel Morange, Jean Gayon, Philippe Huneman and several anonymous referees for theiruseful comments.
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