## 1. Framing the Debate



The discussion about biological teleology has ancient origins. It is
particularly prominent in Plato's depiction of the divine
Craftsman or 'Demiurge' in the
 *Timaeus*
 and Aristotle's discussion of final causes in the
*Physics* (see the section on teleology in the entry on
 Aristotle).
 However, Plato's and Aristotle's understanding of
teleology, as well as their arguments for teleology in the natural
world, are distinct (Lennox 1992; Ariew 2002; Johnson 2005). Whereas Plato's
teleology is anthropocentric and creationist, Aristotle's is
naturalistic and functional. On the Platonic view, the Demiurge is the
source of all motion in both the heavens and on earth, and the
universe and all living beings within it are artifacts modeled on the
Forms (see the relevant sections of the entries on
 Plato
 and
 Plato's metaphysics).
 The goal toward which all things, including living beings, are
directed is the external and eternal good of the Forms. In contrast,
on the Aristotelian view, the teleology that directs the behavior of
living beings is immanent. For instance, in organismal development,
the impetus for this goal-directed process is a principle of change
within the organism, and the *telos*, or goal, of the development is
also an inherent property. Although often conflated, the views of
Plato and Aristotle on teleology have been influential in historical
debates on biological teleology, and one can still find Platonic and
Aristotelian ideas in the current debate on biological functions. 



In addition to its role within ancient philosophy and cosmology,
teleology has long been an important topic within physiology and
medicine. Galen's *On the Use of the Parts* (*De usu
partium*) is an early example of teleological reasoning applied to
physiology (see the section on teleology in the entry on
 Galen).
 In this text, Galen presents a functional analysis of the various
parts of living organisms, in which "existence, structure, and
attributes of all the parts must be explained by reference to their
functions in promoting the activities of the whole organism"
(Schiefsky 2007: 371). For Galen, a teleological account of parts is
superior to a purely causal-mechanical one, since the function or
purpose of the part plays an ineliminable role in the explanation of
the part and its activities. This Galenic view of anatomy, with its
explicitly Aristotelian reliance on final causes, largely dominated
medical thought until the seventeenth century. William Harvey's
*On the Motion of the Heart and Blood* (*De motu
cordis*) was seen by many of his contemporaries (e.g., Hobbes,
Descartes) as a turning point away from the Galenic or Aristotelian
approaches to anatomy, with their appeal to final causes, toward the
new mechanistic and experimental science of the seventeenth century
(French 1994). His attempt to empirically establish the structure and
motion of the heart without ever claiming to have identified the final
cause of circulation, as well as his use of mechanical analogies, such
as the analogy from the expansion of the arteries to the inflation of
a glove, provide some support for this assessment. Robert Boyle,
however, offered a different interpretation of Harvey's
work. Boyle saw it as amenable to his 'compatibilist'
approach that attempted to show that "mechanical and
teleological explanations of biological phenomena are
compatible" (Lennox 1983: 38). Recent commentators have also
suggested Harvey was strongly influenced by Aristotelian teleological
thinking, and thus is a liminal figure in the transition from vitalist
to purely mechanistic explanation in physiology and medicine
(Distelzweig 2014, 2016; Lennox 2017), although Lennox notes that
"Harvey appears to be pressing the analogy between art and
nature in ways that are not Aristotelian but more Platonic in
spirit" (Lennox 2017: 191).




Immanuel Kant's analysis of teleology in the *Critique of
Judgment* (Kant 1790 [2000]) also played an influential role in
biology. According to Kant, humans inevitably understand living things
as if they are teleological systems (Zammito 2006). However, on
Kant's view, the teleology we see in the natural world is only
apparent; it is the product of our limited cognitive faculties (see
section 3 of the entry on
 Kant's aesthetics).
 But in addition, according to Kant, there is a certain
non-machine-like character of organisms, evident in their ability to
grow and reproduce, that leads to a type of mechanical
inexplicability. Hannah Ginsborg (2004) argues that for Kant this
impossibility of explaining organisms in solely mechanistic terms does
not itself distinguish them from complex artifacts; but, she argues,
Kant thought that the regenerative and reproductive aspects of
organisms lead us to attribute a kind of natural purposiveness that is
absent in artifacts, paralleling Aristotle's justification for
natural, immanent teleology. Kantian analysis of this sort shows up in
early nineteenth century research in what would come to be called
organic chemistry. Scientists at that time sought to determine whether
living systems were nothing more than complex chemical systems, fully
analyzable in terms of physical and chemical processes. Those
researchers adopting a Kantian approach advocated a teleo-mechanist
strategy to make sense of the goal-directed nature of living systems,
which sought to treat the organism as both a means and an end and thus
incorporated elements of both teleological and mechanistic explanation
(Lenoir 1982). 



In the eighteenth and nineteenth centuries, the status of teleology in biology was 
also contested as part of the vitalist-mechanist debate in physiology 
and medicine (for an overview, see the first section of the entry on
 life).
 Whereas mechanists sought to describe all living things in purely
mechanical terms, vitalists argued that physical properties alone
could not explain the goal-directed organization of living things.
They claimed that 'vital forces' were also necessary to
explain the difference between physical and living systems. Although
they fell out of favor, some vitalist accounts persisted through the
twentieth-century, for instance, in philosopher Henri Bergson's
'*elan vital*' (see the section creative
evolution in the entry on
 Bergson)
 and in biologist-cum-philosopher Hans Driesch's concept of
'entelechy' (Driesch 1908). 



Much debate over the role of teleology in biology in the twentieth
century, especially amongst the architects of the 'modern
synthesis', traces back to Charles Darwin's theory of
evolution by natural selection. Biologist and philosopher Michael
Ghiselin, expressing a common interpretation of Darwin's role in
this debate, claims in his preface to Darwin's work on orchids
that Darwin's theory succeeded in "getting rid of
teleology and replacing it with a new way of thinking about
adaptation" (Darwin 1862/1877 [1984: xiii] cited in Lennox 1993:
409). On this view, the theory of natural selection explains how
species "have been modified so as to acquire that perfection of
structure and co-adaptation" without any appeal to a benevolent
Creator (Darwin 1859: 3). Prior to Darwin, the best explanation for
biological adaptation was the argument from design, most influentially
presented in William Paley's *Natural Theology* (Paley
1802): living things have the structure and behaviors that they do
because they were designed for certain purposes by a benevolent
Creator (see the entry
on natural theology and natural religion).
Darwin's theory provides
biology with the resources to resist this argument, offering a fully
naturalized explanation for adaptation. Although most agree that
Darwin's theory does indeed purge evolutionary biology of any
illicit appeal to external, Platonic teleology, there is disagreement
as to whether or not Darwin's evolutionary explanations are
teleological (see the relevant section of the entry on
 Darwinism).
 Even Darwin's contemporaries disagreed as to whether or not the
theory of natural selection purged teleological explanations from
biology or revived them (Lennox 2010). In any case, it is clear that
Darwin used the language of 'final causes' to describe the
function of biological parts in his Species Notebooks and throughout
his life; he also reflected frequently about the relationship between
natural selection and teleology (Lennox 1993). 


## 2. Explanatory Teleonaturalism



Philosophical naturalism
 denotes a broad range of attitudes towards ontological questions. We
use "teleonaturalism" to denote a similarly broad range of
naturalistic accounts of teleology in biology that are united in
rejecting any dependence on mental or intentional notions in
explicating the use of the teleological terms in biological contexts.
Thus, those who reject teleomentalism typically seek truth conditions
for teleological claims in biology that are grounded in non-mental
facts about organisms and their traits. 



Some teleonaturalists analyze teleological language as primarily
descriptive rather than explanatory, maintaining that teleology in
biology is appropriate for biological systems which show purposive,
goal-directed patterns of behavior (for which Pittendrigh (1958)
coined the term "teleonomic"). For such views, the primary
scientific challenge is to explain teleonomy, not to use teleonomy as
explanans (see Thompson 1987). While cybernetics lost its appeal in
the latter part of the twentieth century, more recent approaches to
living systems that treat them as self-organizing or
"autopoietic" (Maturana & Varela 1980) bear certain
affinities to the descriptive attitude towards
teleonaturalism--although most proponents claim that the concepts
developed within these approaches are explanatory. 



Hence, most teleonaturalists favor accounts of biological function
which make the explanatory role of this notion a desideratum.
Naturalistic accounts typically aim to satisfy two additional
desiderata. They should distinguish genuine biological functions from
accidental utility (such as noses supporting glasses), and they should
capture the normative dimension of function in order to preserve a
function-malfunction distinction. Although these three desiderata are
neither universally accepted nor are they adequacy conditions in the
strict sense, they have nevertheless achieved canonical status within
the contemporary debate over biological functions. 



In the subsequent sections we divide various ways in which different
teleonaturalist accounts of function can be distinguished. Our first
distinction is between (a) views which assimilate functional
explanations in biology to patterns of explanation in the
non-biological sciences, and (b) views treat functional explanation as
distinctively biological. 


## 3. Assimilation to Non-Biological Explanations



Ernest Nagel (1961) and Carl Hempel (1965) provide early attempts by
philosophers of science to directly assimilate functional explanation
in biology to more general patterns of explanation. In particular,
they both consider functional explanation within the framework of the
 Deductive-Nomological account of scientific explanation.
 They consider the functional claims to be related to explanations of
the presence of a trait in an organism. Their accounts differ
primarily on whether to say that a trait *T* has function
*F* in organism *O* when *T* is sufficient to produce
*F* in *O* (Hempel's version) or when *T* is
necessary to produce *F* in *O* (Nagel's version).




Larry Wright (1973, 1976) also offers an explanatory account targeted
on the presence of the trait, but he criticizes the prior accounts for
failing to capture the apparent goal-directedness of functional
traits. His so-called "etiological" analysis holds that
the function of *X* is *Z* means (a) *X* is there
because it does *Z*, and (b) *Z* is a consequence (or
result) of *X*'s being there. Because of the intended
breadth of Wright's analysis, it has been attacked on conceptual
grounds (e.g., Boorse 1976), but the general thrust of the etiological
account survives in accounts of function based on natural selection,
discussed in section 4. 



Cummins (1975) criticized both Hempel and Nagel on the grounds that
the proper target of explanation of biological function claims is not
the presence of a trait, but the capacities of biological organs and
organisms. Sophisticated capacities can be analyzed in terms of the
contributions that their components make to those capacities. For
example, the heart of a bilaterian animal pumps blood, which in this
way contributes to the capacity of the organism to deliver oxygen and
nutrients to its tissues. The heart itself can be further decomposed
into parts (chambers, valves, etc.) which each play different
functional roles in contributing to that organ's capacity to
pump blood. Among philosophers, this approach to functional analysis
is most associated with Cummins (1975), although biologists have
advanced similar ideas, sometimes independently (Hinde 1975; Lauder
1982), and indeed the approach can be traced much further back, as the
quotation from William Harvey in the introduction suggests.
Cummins' ideas about functional analysis have been incorporated
in recent discussion on mechanisms in the biological sciences (see the
entry on
 mechanisms in science).
 For instance, Craver (2007) explicitly draws on Cummins (1975) in his
account and is important for moving teleological descriptions down to
the molecular level (see also, Craver 2001, 2013). 



According to Cummins, although biological systems certainly have
capacities that are uniquely biological, there is nothing specifically
biological about the pattern of explanation offered by functional
analysis; it applies equally well, for example to the contributions
made by the components of artifacts (Lewens 2004), for example the
contributions of the engine's pistons in the capacity of an
automobile to transport people. Because of the generality of the
framework, it is also possible to give a functional analysis of how
some part of a biological system contributes to outcomes usually
treated as negative, such as disease or death. Some commentators
regard this as a virtue of the approach, while others regard it as too
detached from standard biological practice. Relatedly, the functional
analysis approach fails to live up to the commonly-held desideratum that
an adequate account should provide an analysis of malfunction--a
desideratum that Cummins explicitly rejects (see also Wouters 1999 and
Davies 2001). A heart with a hole in its septum may not circulate
blood at a level sufficient to sustain life, but it thereby simply
lacks that function. 


## 4. Natural Selection Accounts



Many philosophers of biology believe that functional explanation is
uniquely appropriate to biology, turning to Darwin's theory of
descent with modification to ground the practice of attributing
functions. Like Wright, Hempel, and Nagel, natural-selection
teleonaturalists take the primary target of explanation to be the
presence of various traits in organisms. 



Here we distinguish between two ways of using natural selection to
ground biological teleology. 


* Indirect approaches treat the adaptive, self-organizing nature of
living cells and organisms as the natural basis for teleological
properties of their traits, but give background credit to the power of
natural selection to produce such self-organizational complexity as is
found in living systems.
* Direct approaches invoke natural selection explicitly when
explicating functional claims, either in an etiological sense based on
the history of selection or in a dispositional sense based on the
fitness of organisms possessing the traits.


### 4.1 Indirect



The primary motivation for the earliest indirect, cybernetic accounts
of biological teleology were to explain the apparent purposiveness of
biological organisms, for instance, the maintenance of constant body
temperature in endotherms. These accounts aimed to provide a
naturalized explanation for the goal-directed behavior of biological
systems through reference to their organization. In an influential
early paper, Norbert Wiener and colleagues sought to explain the
goal-directed behavior of biological organisms and machines as arising
from their utilization of negative feedback mechanisms (Rosenblueth et
al. 1943; for further development see also Braithwaite (1953),
Sommerhoff (1950) and Nagel (1953)). Attributions of teleological, or
goal-directed, behavior to animals or machines, they argued, meant
nothing more than "purpose controlled by feed-back" (Rosenblueth et al. 1943, 23). 



This cybernetic account of teleology inspired biologist Colin
Pittendrigh to introduce the term 'teleonomy' into the
literature (Pittendrigh 1958). With this neologism, Pittendrigh hoped
to purge biology of any vestiges of Aristotelian final causes whilst
providing biology with an acceptable term to describe adapted,
goal-directed systems. This term was taken up in the 1960s by
evolutionary biologists such as Ernst Mayr (1974) and George Williams
(1966), as well as by scientists studying cell metabolism and
regulation, who were just beginning to elucidate the structural and
molecular basis for cellular feedback mechanisms (Monod & Jacob
1961; Davis 1961). According to proponents, adopting a cybernetic
account of goal-directed behavior in biological systems splits the
explanatory problem in two. On the one hand, teleological activity in
the biological world could be explained by the presence of teleonomic
systems with negative feedback mechanisms, whereas the very presence
of those teleonomic systems in living organisms, on the other hand,
could be explained by the action of natural selection (Monod 1970
[1971]). 



Although explicit cybernetic accounts of biological teleology have
fallen out of favor, other organizational approaches to biological
function have had a recent resurgence in the function literature.
These organizational, or systems-theoretic, approaches often build
upon early cybernetic accounts or aim to extend Maturana and
Varela's (1980) influential notion of autopoiesis, which refers
to the self-organizing, self-maintaining characteristic of living
systems (see the entry on
 Embodied Cognition
 for further description). These accounts identify the function of a
biological trait through an analysis of the role the trait plays
within an organized system in contributing to both its own persistence
and the persistence of the system as a whole (Schlosser 1998;
McLaughlin 2001; Mossio et al. 2009; Saborido et al. 2011; Moreno
& Mossio 2015). Although they differ in their details,
organizational approaches to biological function generally agree that
a trait token *T* has a function *F* when the performance of
*F* by *T* contributes to the maintenance of the complex
organization of the system, which in turn results in *T*'s
continued existence. For example, the heart has the function to pump
blood, according to these accounts, because it contributes to the
maintenance of the entire organism by causing the blood to circulate,
which facilitates the circulation of oxygen and nutrients. At the
same time, this circulation is also responsible in part for the
persistence of the heart itself, since the heart also benefits
directly from this function (i.e., the cardiac cells receive the
oxygen and nutrients necessary for their survival). 



Similar to direct natural selection accounts, organizational accounts
can be forward or backward-looking: the function of a trait may
identify its dispositional contribution to the complex organization of
the system which results in its own persistence or reproduction in the
future (forward-looking; Schlosser 1998), or a functional attribution
may identify a trait's past contribution (etiological, or
backward-looking; McLaughlin 2001). Alvaro Moreno's group adopts
a third position. They claim their organizational account of function
unifies these two perspectives (Mossio et al 2009; cf. Artiga &
Martinez 2016). All these organizational accounts differ from direct
natural selection accounts, however, in that they make no appeal to
the selection history of the trait. Instead, the function of a trait
can be inferred from the present or past role of the trait in
maintaining itself within the complex, organized system without
further holding that the trait was selected for that role. On this
view, functional attributions in biology are explanatory not because
of selection, but rather because of the causal role traits play in
contributing to the maintenance of the organization of a system, which
in turn enables the traits themselves to persist. 


### 4.2 Direct Natural Selection Approaches



Accounts of biological function which refer to natural selection
typically have the form that a trait's functions causally explain
the existence or maintenance of that trait in a given population via
the mechanism of natural selection. William Wimsatt (1972), Ruth
Millikan (1984), and Karen Neander (1991a), all treat the past history
of natural selection as the selection process that legitimizes the
notion of a biological function. Within such approaches there is a
dispute about the exact role of natural selection, whether as a source
of variation (sometimes referred to as the "creative" role
of natural selection, e.g., Neander 1988; see also Ayala 1970, 1977),
or only as a filter on variations that arise independently (Sober
1984). 



Positions which ground functional claims in natural selection have
much in common with Wright's etiological account. However,
because the grounding is specific to biology, they may avoid the kinds
of counterexamples to Wright's account introduced by critics
such as Christopher Boorse, predicated on the idea that Wright's
account is intended to provide a more general conceptual analysis. A
related challenge stems from the claim that pre-Darwinian thinkers
such as Harvey correctly identified functional properties of
biological organs, and that natural selection cannot therefore be a
requirement for the proper conceptual analysis of function. Defenders
of direct natural selection accounts of function have responded in
different ways. One way, exemplified by Millikan (1989), is to argue
that conceptual analysis has no role to play in articulating what is
essentially a theoretical term within modern evolutionary biology.
Another way, exemplified by Neander (1991b), is to say that the task
of conceptual analysis is appropriate but restricted to the concepts
of the relevant scientific community. 



Paul Davies (2001) and Arno Wouters (2005) argue that both Millikan
and Neander are incorrect to treat malfunction as an important
theoretical or conceptual aspect of the practice of attributing
functions by biologists. Wouters declares the wish that the study of
biological function should be liberated "from the yoke of the
philosophy of mind" (2005: 148). However, Ema Sullivan-Bissett (2017) argues
that while the task of explicating biological practice by philosophers
of biology is usefully distinguished from the broader goals of
philosophers pursuing naturalistic accounts of mind and language (see
the entry on
 teleological theories of mental content),
 the latter serves legitimate goals. She regards an account of
malfunction to be integral to the latter project even if not to the
former. Davies (2001) argues that the natural selection accounts are
unable to provide an account of malfunction insofar as they
individuate traits functionally, entailing that a putatively
malfunctioning trait is not an instance of the functionally-defined
kind. Sullivan-Bissett addresses Davies' objection by
incorporating a structural condition on the individuation of traits.
(See also Garson 2016: 48-49, for additional discussion and
critique of Davies' view.) 



Returning to the kinds of traits studied by biologists, some theorists
make a distinction between the initial spread of a new phenotypic
trait in a population and the more recent maintenance of traits in
populations. Take a trait such as feathers, arising in a population by
whatever means. Initially this trait may have spread because of a role
in mating displays. Later, feathers may have contributed to improved
thermoregulation. And still later, the trait may have become more
widely distributed because feathers make good flight control surfaces.
If display or thermoregulatory functions of feathers become less
important in some niches, the trait may nonetheless be maintained in a
population due to selection for its flight-control function. The
shifting functional profile may also be correlated with
differentiation in form, such as between downy feathers and flight
feathers.



Some biologists used the term pre-adaptation to capture the idea that
a trait selected for one function may turn out to be very useful for
something else. However, Gould and Vrba (1982) introduced the term
'exaptation' to capture such transitions, and avoid what
they saw as the overly teleological implications of pre-adaptation, as
well as to recognize that non-selected traits of organisms could also
be co-opted to serve a function, increasing fitness without having any
further modification by natural selection (Lloyd & Gould 2017).
Critics of the etiological natural selection approaches sometimes
argue that backward-looking approaches are too vague with respect to
questions about the point at which traits acquire or lose functions,
and that they are consequently untestable empirically (Amundson &
Lauder 1994). Godfrey-Smith (1994) independently proposed a
"modern history" theory of functions to address these
problems. Similarly, Griffiths (1993: 417) invokes a notion of the
"last evolutionarily significant time period" to handle
these issues, but many critics remain unconvinced (e.g., Wouters 1999;
Davies 2001). 



Another issue confronting direct natural selection accounts is the
evident utility of attributing functions to novel traits of organisms
developed within a single lifetime such as the capacity of brains to
acquire new concepts of kinds of things not previously experienced in
the evolutionary lineage, or of the immune system to develop
antibodies to new infectious agents. Previously, Millikan (1984) had
suggested a notion of "derived proper function" to capture
this kind of example. More recently, Bouchard (2013) and Garson (2017)
have developed more detailed accounts of derived function,
respectively using "differential persistence" and
"differential retention" within an organism's
lifespan to play the role that differential reproduction plays in
direct natural selection accounts. 



Some biologists and philosophers of biology have been motivated by
problems with the backward-looking etiological approach, or by seeing
examples from biology that seek to identify the present functions of a
trait. To deal with these issues they propose a dispositional or
forward-looking approach that analyzes function in terms of those
effects it is disposed to produce that tend to contribute to the
present or future maintenance of the trait in a population of
organisms. Various ways of spelling this out include Hinde's
(1975) account of strong function, Boorse's (1976, 2002)
biostatistical theory, Bigelow and Pargetter's (1987) propensity
theory, and Walsh's (1996) relational theory (see also and Walsh
& Ariew 1996). 


## 5. Unification and Pluralism



Some theorists have argued for the pluralistic idea that biology may
incorporate (at least) two notions of function, one to explain the
presence of traits and the other to explain how those traits
contribute to the complex capacities of organisms (Millikan 1989;
Sober 1993; Godfrey-Smith 1994). Ron Amundson and George Lauder
(Amundson & Lauder 1994) argue that paleontology is a part of
biology which cannot make use of the etiological account because the
selection regime for extinct organisms is generally inaccessible, and
it must therefore depend on Cummins-style functional analysis of
fossilized remains. However, defenders of pluralism may respond that
even if it is correct to say that not all parts of biology can use
both notions of function, this is consistent with both accounts having
a role within biology. 



Some theorists have argued that these two apparently distinct notions
of function can be unified by regarding the target of explanation as
the biological fitness of a whole organism (e.g., Griffiths 1993;
Kitcher 1994; and perhaps Tinbergen 1963 according to Peter
Godfrey-Smith 1994). Moreno and colleagues (Mossio et al. 2009; Moreno
& Mossio 2015) have also claimed that their organizational
approach unifies across backward-looking and forward-looking accounts
by describing activities that atemporally account for the continuing
persistence of traits. The viability of this account as one that is
distinct from etiological accounts has been challenged by Marc Artiga
and Manolo Martinez (Artiga & Martinez 2016), who
argue that the necessarily multi-generational characterization of
organizational closure needed to accommodate biological functions for
parental endowments to offspring (whether transmitted via gametes or
behavior) entails the standard etiological structure found in
Wright's account. 



Attention to the actual explanatory practices of contemporary
biologists is central to these discussions. This focus on scientific
practice reflects broader trends within the philosophy of science,
expanding beyond the general and abstract questions about scientific
epistemology and metaphysics that dominated the twentieth century. The
diversity of life itself is reflected in the variety of scientific
attempts to understand it, and as philosophers of biology pursue
further engagement with this variety, novel perspectives on the role
and appropriateness of functional and teleological notions seem likely
to result.