## 1. Life and Work



Aristotle was born in Stagira on the northern Aegean coast in 384BCE.
His father Nicomachus was physician to King Amyntas III of Macedon,
and his mother was of a wealthy family from the island of Euboea. He
was sent at the age of 17 to Athens, where he studied in Plato's
Academy for 20 years, until Plato's death in 347. By then he had
developed his own distinctive philosophical ideas, including his
passion for the study of nature. He joined a philosophical circle in
Assos on the coast of Asia Minor, but soon moved to the nearby island
of Lesbos where he met Theophrastus, a young man with similar
interests in natural science. Between the two of them they originated
the science of biology, Aristotle carrying out a systematic
investigation of animals, Theophrastus doing the same for plants.



In 343 Aristotle was asked by Philip II of Macedon to tutor his son
Alexander. By 335 he had returned to Athens, now under the control of
his former student Alexander. With Theophrastus he founded a
'school' in a public sanctuary known as the Lyceum. He
headed the Lyceum until the death of Alexander the Great in 323. With
anti-Macedonian feelings running high in Athens, Aristotle retired to
his mother's birthplace. He died there in 322BCE.



The surviving corpus of Aristotle derives from medieval manuscripts
based on a 1st century BCE edition. There were no
commentaries on the biological works written until they were
collectively translated into Arabic. The first appearance of
Aristotle's biological writings in the West are Latin
translations, by Michael Scot, of an Arabic edition, which forms the
basis of Albertus Magnus' *De animalibus*. In the
13th century, William of Moerbeke produced a Latin
translation directly from the Greek. The first printed editions and
translations date to the late 15th century, the most widely
circulated being that of Theodorus Gaza. In addition to the three
works traditionally referred to as *History of Animals*
(*HA*), *Parts of Animals* (*PA*) and
*Generation of Animals* (*GA*), there are a number of
briefer 'essays' on more specialized topics: *On animal
motion*, *On animal locomotion*, *On respiration*,
*On life and death*, *On youth and old age*, *On
length and shortness of life*, *On sleeping and waking*,
*On the senses and their objects* (the last six being included
in the so-called *Parva naturalia*). Whether one should
consider *De Anima* (*On the soul*) part of this project
or not is a difficult question. What is certainly clear, however, is
that there are important connections between the theoretical approach
to the relationship between body and soul defended in that work and
the distinctive way that Aristotle approaches the investigation of
animals.



In order to understand Aristotle's distinctive approach to the
study of living things, it is imperative to situate it within his
natural philosophy and his philosophy of science. The first book of
Aristotle's *Parts of Animals* is, in fact, largely
devoted to doing just that, and after a brief discussion of
Aristotle's general views about scientific inquiry and
explanation, we will turn to it.


## 2. Aristotle's Philosophy of Science



In sections 4, 5 and 6, we will explore the ways in which Aristotle
systematically organizes and explains an extraordinary body of
information about animal anatomy, physiology and development. But
Aristotle was able to accomplish what he did in zoology because he had
given a great deal of thought to the nature of scientific inquiry. How
does one progress from the superficial and unorganized state of
everyday experience toward organized scientific knowledge? To answer
this question, you need a concept of the goal to be achieved, and
Aristotle developed such a concept in his *Prior* and
*Posterior Analytics* (henceforth abbreviated as *APr*.
and *APo*., respectively). The goal of scientific inquiry, he
argued, was a system of concepts and propositions organized
hierarchically, ultimately resting on knowledge of the essential
natures of the objects of study and certain other necessary first
principles. These definitions and principles form the basis of causal
explanations of all the other universal truths within the domain of
study. Those other universal truths should identify attributes
belonging to a subject *per se*, in virtue of that
subject's nature. The example he uses when he introduces his
account of scientific demonstration to illustrate such propositions is
from geometry: having interior angles equal to two right angles
belongs to all and only triangles in virtue of their being triangles
(*APo*. I 4, 5). This attribute belongs to all
*equilateral* triangles as well--not, however, because
they are equilateral, but *because they are triangles*. Thus
scientific knowledge of such a proposition, knowledge that displays
the reason why any triangle has this property, must explain why this
property belongs to triangles *as such*. The explanation, of
course, will appeal to the essential character of three-sided
rectilinear plane figures, i.e. to what it is to be a triangle.



The second book of the *Posterior Analytics* discusses
*how* to achieve this goal of scientific knowledge, one central
concern being how knowledge of essences, expressed in definitions, is
related to explanations expressed in the form of causal
demonstrations. Plato had formulated a famous paradox of inquiry in
his dialogue *Meno*: either you know the object of your
inquiry, in which case inquiry is unnecessary; or you don't know
the object of your inquiry, in which case inquiry is impossible
(*Meno* 80d5-e5). Aristotle reminds us of this paradox in the
first chapter of the *Posterior Analytics*, but his full
solution only emerges in book II. There, he argues that perceptual
experience gives us a grasp of the target of inquiry that, though it
does not count as scientific knowledge, does serve to direct further
inquiry. He begins the discussion by presenting us with a claim about
how objects of inquiry are linked to objects of scientific
knowledge.



> 
> The things about which we inquire are equal in number to the things we
> know scientifically. We inquire about four things: *the fact*,
> *the reason why*, *if something is*, *what something
> is*. (*APo*. II 1, 89b23-25)
> 



Aristotle conceives of these four inquiries as *paired*, and
there is a natural *sequence* in each pair. Knowing that some
state of affairs is the case, we can inquire into the reason why it is
the case.



> 
> When we know *the fact* we inquire about *the reason
> why* (e.g., knowing *that* it is eclipsed or *that*
> the earth moves, we inquire into *the reason why* it is
> eclipsed or *why* the earth moves). (*APo*. II 1,
> 89b29-31)
> 



Similarly, if we conclude an inquiry into whether something exists, we
can go on to investigate its nature, what it is.



> 
> And having come to know *that* it is, we inquire *what*
> it is (e.g.: Then what is a god? Or what is a man?). (*APo*. II
> 1, 89b34-35)
> 



The examples reveal a distinction that structures much of the
discussion for the next ten chapters. For it looks as if
'factual' inquiries concern whether some attribute belongs
to some subject (movement to the earth, eclipse to the moon), and the
search for the reason why will be a search for the causal explanation
of the attribute belonging to that subject. By contrast, it looks like
the move from 'if' to 'what' is a move from
establishing the existence of some subject ('god',
'man') to establishing what it is.



However, the distinction is not, it turns out, so clear-cut. Having
begun to illustrate the distinction between inquiry *if* or
*whether* something is and *what* it is with the
question 'whether there is or is not a centaur or a god',
he then characterizes the knowledge achieved as 'knowing
*that* it is'. And in the second chapter, he begins to
link the two sequences of inquiry by means of his syllogistic concept
of "middle term," the term that is common to the two
premises in a syllogistic proof.



> 
> Thus it results that in *all* our research we seek either
> *if there is a middle term or what the middle term is*. For the
> middle term is the cause, and this is *in every case* what is
> sought. (*APo*. II 2, 90a7-9)
> 



That is, in any valid syllogistic inference, the middle term shared by
the premises is the warrant for the conclusion. In scientific
explanation, however, the middle term must *also* identify the
cause of the fact given in the conclusion--what that term
identifies is the causal link between the subject and attribute. To
use another of his common examples, if we seek to explain the periodic
sound of noise in the clouds, the middle term must identify the cause
of the connection between that noise and those clouds. Moreover, on
Aristotle's account of the relationship between causal
demonstration and scientific definition, knowing the *cause* of
thunder is at the same time knowing the *essence* of thunder,
what thunder really is.



> 
> There is a difference between saying why it thunders and what thunder
> is. In the one case you will say: Because the fire is extinguished in
> the clouds. But: What is thunder?--A noise of fire being
> extinguished in the clouds. Hence the same account is given in
> different ways: in one way it is a continuous demonstration, in the
> other a definition. (*APo*. II 10, 94a4-8)
> 



In the *APo*., Aristotle returns regularly to such standard
examples of natural phenomena as thunder and eclipses--but he
also, and importantly, provides an extended biological example, the
seasonal loss of leaves in broad-leafed
 plants.[2]
 In ch. 16, Aristotle imagines an inquiry that begins with questions
such as 'Why do fig trees and grape vines lose their
leaves?'. The model answer is 'Because they are both
broad-leafed.' That is, the inquiry seeks some other feature,
*common* to both kinds, related to the target of inquiry, the
seasonal loss of leaves. The 'because' is, however,
preliminary--it is best to see being broad-leafed as a step
toward causal explanation. Chapter 17 picks up the example, in the
context of arguing that basic scientific inquiry seeks, wherever
possible, *co-extensive* predications, which those between leaf
loss and fig trees, or leaf loss and grape vines, are not. The
candidate major premise, however--'Whatever is broad-leafed
loses its leaves'--*does* identify a co-extensive
relationship between subject and predicate. It may thus serve as a
proper scientific *explanandum*, and 'broad-leafed'
can serve to identify a kind, all and only the members of which lose
their
 leaves.[3]
 Thus, the middle term of the preliminary explanation becomes the
*subject* of a more basic, co-extensive predication. The cause
of broad-leafed trees losing their leaves will, then, be something
more fundamental about broad-leafed trees, here identified as the
solidification of moisture at the leaf juncture, which can thus serve
as the middle term in a causal explanation of this fact. But it will
also serve as part of a definition of leaf loss.



> 
> The middle is the account of the first major term [i.e. the predicate
> in the conclusion], for which reason all the sciences come about
> through definition. (*APo*. II 17, 99a22-23)
> 



That is, we will have, if our research goes well, an account of what
loss of leaves is. Along the way a process of identifying the kind,
all and only the members of which will lose their leaves due to sap
coagulation, is assumed. Yet the *Analytics* provides no
systematic discussion about whether there are general criteria for
identifying these basic scientific kinds. As we will see, this is the
topic of one of the most interesting sections of *On the Parts of
Animals*, book I.



There are two quite different questions we need to ask about how these
ideas about inquiry and explanation in the *Analytics* are
related to Aristotle's investigations of animals: first, how is
the *philosophy* of biology presented in *PA* I related
to the general account of explanation, definition and inquiry in the
*Posterior
 Analytics*[4];
 and second, to what extent do the treatises reporting his actual
investigations of animals conform either to his general account of
scientific knowledge and inquiry in *APo*., or to the norms
regarding the study of animals laid down in *PA* I. The
remainder of this entry will be organized around these two
questions.


## 3. Caveat lector



First some preliminary remarks are in order about what we
are--and are not--discussing. It seems obvious, once stated,
that the actual activity of studying animals is different from the
activity of writing or teaching about animals based on that study. We
have access to a number of Aristotle's systematically organized
writings on animals; we do *not* have direct access to his
actual investigations. Some authors studied by historians of science
discuss, in their written work, the methods they used to gather the
information and work out their theoretical ideas and even provide
'diaries' describing their day-to-day
studies--Aristotle is not one of them. Nor did anyone else report
observing Aristotle carrying out his studies. There are reasonable
inferences we can make from his writings; for example that he
consulted with bee-keepers, fishermen and sponge divers, that he (or
someone under his guidance) performed a great many dissections on a
wide variety of animals, that there were at least some diagrams
produced based on these dissections, and so on. Moreover, on the
question of how he reasoned *to* specific explanations we can
make some reasonable inferences from things he says about proper
methods of biological inquiry. But it is important to keep in mind
that we are studying texts that present, in a highly structured and
theoretical manner, the *results* of an actual investigation,
the details of which we know very little.



It is also unclear what is the intent of the texts we *do* have
that report on these investigations. It is sometimes said they are
'lecture notes'. That seems pretty clearly wrong; they are
too carefully written and structured. But it does seem clear, from
cross-references, that some of them were to be studied in a certain
order, and this order *may* conform to a course of study in the
Lyceum. Interestingly, while all of the other biological treatises
refer for further information to 'the animal
inquiries',that is to something like the *History of
Animals* that has come down to us, often in conjunction with
'the dissections', our *History of Animals* has no
references to the other, explanatory studies. This suggests that the
'inquiries and dissections' had a different function in
such a course of study, perhaps something akin to 'reference
works'.



Finally, it is also worth recalling that the series of treatises we
have was likely compiled hundreds of years after Aristotle's
death from whatever he left. It seems unlikely we will ever be able to
determine the exact relationship between the so-called
'Andronican' edition, the likely source of our texts, and
what was produced during Aristotle's lifetime.



These three caveats place constraints on what I can reasonably claim
to be doing. I will be discussing the treatises that report the
results of Aristotle's investigations of animals. I will assume
that the texts that have been passed down to us reflect what he wrote
on this subject, and that the cross-references in those texts are his
and reflect his own views about how these various studies are related
to each other.



## 4. Philosophy of Biology



*On the Parts of Animals*, book I (*PA* I) begins by
outlining its purpose, which is to establish a set of standards for
judging natural investigations (639a15). Its five chapters pursue this
purpose, discussing the appropriate level of generality for such
studies, the modes of causality and of necessity to be used in
biological explanations, the relation of form to matter in living
things, the proper method of logical division for this subject matter,
the means of identifying kinds and their activities at the proper
level of abstraction, and much more. Two sorts of evidence support the
conclusion that this book is intended to deal with problems and
questions that arise in the application of Aristotle's general
philosophy of science, found in the *Posterior Analytics*, to
his theoretical investigation of living nature.



The first kind of evidence consists of passages introducing his
biological investigations that appear to make explicit reference to
the account of scientific knowledge in *APo*. The following
passage from the *History of Animals* (a better though less
familiar translation would be *Animal Inquiries*), for example,
suggests that the entire biological project is organized in accordance
with the theory of inquiry developed in *APo*. II. This passage
comes near the end of chapter six in the first book of *HA*.
After five chapters in which Aristotle lays out the kinds of
similarities and differences among animals to be studied and sketches
the ways in which they are to be investigated, he makes the following
programmatic statement about the investigation to come, and where it
fits in the entire scientific study of animals.



> 
> These things, then, have now been said by way of outline to provide a
> taste of what things need to be studied, and what it is about them
> that needs to be studied, in order that we may first grasp the
> differences and the attributes belonging to all animals. After we do
> this, we must attempt to discover the causes. For it is natural to
> carry out the investigation in this way, beginning with the inquiry
> into each thing; for from these inquiries it becomes clear both about
> which things (*peri hon*) the demonstration
> (*ten apodeixin*) should be and from which things
> (*ex* *hon*) it should proceed. (*HA* I 6,
> 491a7-14)
> 



The natural way to proceed, then, is to begin with inquiry
(*historia*), with the aim of grasping the differences between,
and attributes of, all the animals; and *then* to attempt to
discover their causes. This is natural because, given that our goal is
demonstrative understanding, we want to end up with a clear
distinction between the facts to be explained (the *peri
hon*) and their explanation (the *ex*
*hon*). This statement echoes the summary, in
*APo.* I 10, of the components of demonstrative knowledge:



> 
> Nevertheless there are by nature these three [components of
> demonstrative knowledge]: that about which (*peri ho*) it
> proves, what it proves, and those things from which (*ex
> hon*) it proves. (76b21-22)
> 



The *History of Animals* characterizes itself as establishing
the attributes and the differences that belong to all animals, and
claims that by carrying out this inquiry we are prepared to go on to
search for the causes. Indeed, he appears to suggest that a successful
*historia* or factual inquiry will prepare us to grasp the
difference between those facts that need to be explained and those
that will be invoked in our explanations. In the language of the
*Posterior Analytics*: *HA* establishes the
*fact*, e.g. *that* all animals with lungs have
windpipes, or *that* all cetacea have lungs and are viviparous,
typically seeking to identify groups by means of discovering
co-extensive differentiae with the aid of the method of
 division.[5]
 The proper use of division, moreover, will give clear indications of
which predications are basic and which derivative. Works such as
*Parts of Animals* or *Generation of Animals*, on the
other hand, seek to establish the reason why--the cause--of
the fact. If Aristotle is following the method described in the
*Analytics*, these causal explanations should at the same time
point us to essential definitions of what it is to be a windpipe or to
be viviparous. It is a question currently much debated whether
definition was, in fact, an explicit goal of *HA* or simply a
consequence of the explanatory goal clearly identified in the above
passage from *HA* I 6; and if so, whether definitions of animal
*kinds* were sought, or only definitions of their attributes.
As we will see, there are a number of chapter summaries in the
explanatory treatises that make a point of claiming that *both*
an explanation of *why* a part is found in those animals that
have it, *and* an account of *what* that part is, have
been provided; but one must work very hard to reconstruct any
definitions of animal kinds in those treatises.
 [6]



A number of texts in Aristotle's causal investigations reinforce
the message of *HA* I 6, stressing that the preliminary work of
inquiries that establish and organize the facts at various levels of
generality has been accomplished; and they regularly refer to
something like what is reported in our *HA* as the place to
look for the results of this preliminary work. Two explicit statements
to that effect follow, one from the beginning of his study of the
causes of the differences in animal locomotion, one from the beginning
of his study of the causes of the differences among the parts of
animals.



> 
> Clearly there needs to be a study of all of these questions about
> animal locomotion and any others of the same kind; for *that*
> (*hoti*) these things are thus is clear from our inquiries into
> nature (t*es historias tes phusikes*);
> *the reason why* (*dioti*) must now be investigated.
> (*IA* 1, 704b7-10)
> 



> 
> From which parts and from how many parts each of the animals is
> constituted has been exhibited more clearly in the inquiries about
> them (*en tais historiais tais peri auton*); it is the
> *causes* owing to which each animal has this character that
> must now be examined, on their own and apart (*chorisantas
> kath'hauta*) from what was said in the inquiries.
> (*PA* II 1, 646a8-12)
> 



Each of these passages explicitly describes the study of animals with
which Aristotle is engaged in the language of Aristotle's theory
of research in *APo*. II 1. Indeed, the passage from
*HA* I 6 does so by insisting that the *natural*
*method* to use is to first get clear on the differences and
attributes to be demonstrated ('establish the fact
*that*...') before going on to find the causes
('the reason why, i.e. the cause') to be appealed to in
these demonstrations. Animal inquiries (*historiai*) are a kind
of *hoti* inquiry--that is, the *History of
Animals* presents the facts to be explained, organized so as to be
prepared for causal
 demonstration.[7]



The *IA* and *PA*, on the other hand, refer to
themselves as pursuing a causal inquiry into *the reason*
*why* the various kinds of animals are differentiated as they
are, and they acknowledge that they are able to do this precisely
because the factual investigation into the locomotion and parts of
animals has been accomplished. In both cases Aristotle emphasizes the
distinction on which we are focused, perhaps in order to remind his
readers of his philosophy of scientific research.



There is a second line of evidence, quite independent of these
programmatic statements, which leads to the same conclusion. The
topics covered in *PA* I take the form of specifications of the
central topics of the *Posterior Analytics*. These
specifications are required because animals are [a] complex unities of
matter (body) and form (soul); [b] arise by a complex process of
development; [c] the end--that for the sake of which the
development occurs-- is both causally and definitionally prior to
that process; [d] a distinctive kind of necessity, conditional
necessity, is operative; and [e] a special method of
multi-differentiae division is required. Such a discussion is required
by the fact that although the *Posterior Analytics* intends its
epistemic standards to be applicable to natural science - as is
clear from the many examples drawn from natural science in book II
- it provides no details as to how this application is to be
accomplished.



What, then, does *PA* I tell us about the proper way to
investigate animals? Aristotle begins by posing a problem about how to
identify the proper objects of investigation. Should we begin by
studying the features of groups close to the level of perception, such
as human beings, horses, dogs and such, or should we look for
attributes that 'belong in common according to kind, and then
later study their distinctive attributes' (639b4-5). He
notes that since there are many attributes that are common to many of
these more concrete kinds, focusing our study on them will have the
result of our repeating ourselves--much as if, to use an example
from *APo* II discussed in Section 2, we were to remain at the
level of 'olive trees lose their leaves', 'grape
vines lose their leaves'. Among these common attributes,
Aristotle distinguishes those that seem not to differ across the kinds
that have them (he cites respiration and death) from those that are
' distinctive in form ' such as locomotion. As he puts it:
"it is apparent that locomotion is not one in form, because
flying, swimming, walking, and crawling differ" (639b1-2).
Aristotle deals with this question, so reminiscent of *APo*. I
4-5, in *PA* I 4, but only after he has introduced a new
way of thinking about differentiae and division appropriate for
investigating animals. After discussing his recommendations regarding
the use of logical division in biology, we will return to look at his
answer.



Animals are complex structures organized so as to be able to perform
an integrated set of functions and activities; yet the *Posterior
Analytics* provides one with very little guidance as to how to
apply its norms to such things. Again *PA* I, and especially
chapters 1 and 5, appears designed to provide that guidance. It seeks
to establish [a] the priority of goal-causation to motive-causation,
[b] the priority of the study of an animal's form (which, he
argues, should be identified with its soul) to the study of its
material constituents (i.e. its body), and [c] the presence of a
special kind of necessity operative where goals and form take
priority, namely hypothetical or conditional necessity (cf.
*PA* I 1, 640a10-641a32, 642a1-31).



The argument is complicated in virtue of its manner of presentation.
First, it is a narrative in which Aristotle gradually develops his own
views by exposing the errors of those who investigated nature before
him; it is structured as presenting an alternative to views expressed
by Empedocles and Democritus, and as in the spirit of Socrates
(642a24-31). Second, as is so typical of him, what initially may
appear to be three separate narratives turn out to be a single,
complex case for emphasizing the study of form, teleological
explanation, and conditional necessity when studying living things.
Form, in the case of living things, turns out to be the capacity to
perform living functions such as nutrition, perception, or locomotion,
i.e. soul; and thus the form of a living thing is causally prior to
the matter, because it is the goal for the sake of which the parts of
the animal--its matter--come to be and exist. This, in turn,
provides us with the appropriate way to understand 'conditional
necessity'--parts and the processes that produce them do
not necessitate the outcome; on the contrary, the outcome necessitates
that the developmental processes bring about the parts that are
necessary for the organism to live its life, and do so in a temporally
and spatially coordinated manner (cf. *GA* II 4,
740b19-34; II 6, 743a16-36).



Chapters 2 and 3 have a similar narrative structure. Gradually, a
positive theory of biological division emerges from the ashes of a
detailed critical analysis of those (Plato and certain of his
students) who "attempt to grasp the particular by dividing the
kind into two differences" (642b5), a practice he finds in
"the written divisions" (642b11), perhaps a reference to
divisions we find in some of Plato's later dialogues, or perhaps
to lost works by other members of Plato's Academy. The method he
is criticizing combines two basic techniques that lead to things being
grouped together that are fundamentally different and things being
grouped separately that are fundamentally similar: dichotomy and
division by non-essentials. Dichotomy is the method of dividing the
more general differentia into two exhaustive alternatives, which often
entails that one of the alternatives is simply the privation of the
other (winged, wingless). Division by non-essentials involves dividing
a prior differentia-class by means of an unrelated difference. For
example, you might first divide animals into winged and wingless, and
then winged animals into wild and tame.



> 
> I mean the sort of thing that results by dividing animals into the
> wingless and the winged, and winged into tame and wild, or pale and
> dark. Neither tame nor pale is a difference of winged; rather each is
> the origin of another difference, while here it is incidental.
> (643b19-22)
> 



To avoid the problems created by this dichotomous and arbitrary
method, Aristotle recommends a method that divides each wider
differentia by more and more determinate forms of that differentia
(wings into forms of wing, beaks into forms of beak), and does so
simultaneously on as many differentiae as are correlated at a given
level of universality.



> 
> Rather, one should try to take animals by kinds, following the lead of
> the many in demarcating a bird kind and a fish kind. Each of these has
> been defined by many differences, not according to dichotomy.
> (643b9-12)
> 



Thus, if all birds have beaks, feathers, wings of a sort, are bipedal,
etc. , the biologist needs to perform the proper sort of division on
all of these in order to grasp the various sub-kinds of bird. This
will not, of course, give us demonstrative knowledge; but it will help
us to grasp the co-extensively correlated features, at each level of
specificity, that are the proper things about which to seek
demonstrative knowledge.



An interesting feature of this extended criticism of dichotomous,
arbitrary division and the development of a systematic,
multi-differentiae alternative is that it takes for granted the
'naturalness' of certain kinds that 'the many'
have identified. As he says above, 'these [birds and fish] have
been defined by many differences'; earlier he had said that the
terms 'bird' and 'fish' name a
'similarity', a group of animals that are 'alike in
kind' that dichotomous division inappropriately tears apart
(642b13-19). During the discussion of division there is very
little clarification of what underlies this notion of similarity or
likeness in kind. However, armed with a method of division suited to
the study of animals, the discussion quickly turns to the question of
what it is that makes it appropriate to treat all birds or all fish as
a single kind, and to the specification of rules for identifying other
such kinds. What 'bird' and 'fish' name are,
he claims, kinds with "common natures, and forms not very far
apart" (644b2-3). Prior to understanding these kinds
through a scientific account of this common nature, we can--as
the *Posterior Analytics* argues--grasp these kinds, i.e.
grasp that all birds *have* a common
 nature.[8]
 We do this by noting that they have many general differences that
vary in measurable ways--by the more and less, as he puts it.



> 
> ...[these kinds] are correctly defined in this way. For those
> animals that differ by degree and the more and the less have been
> brought together under one kind, while those that are analogous have
> been kept apart. I mean, for example, that bird differs from bird by
> the more or by degree (for one has long feathers, another short
> feathers), while fish differs from bird by analogy (for what is
> feather in the one is scale in the other). (644a16-22)
> 



It is with this very topic of different degrees of similarity and
difference of parts that the *Historia Animalium* begins,
extending this way of characterizing similarity and difference
immediately to differences in activities, ways of life and traits of
character. (*HA* I 1, 486a5-487a13) This is, then,
clearly a discussion of how one achieves 'knowledge of the
fact', rather than 'knowledge of the reason
why'.



The closing section of *PA* I (treated since the Renaissance as
its fifth chapter) is often thought of as having two, essentially
unrelated parts. And there is some justification for this; if one were
to remove the first half of the chapter, one would be left with an
argument the flows quite naturally out of the conclusion of chapter 4.
At that point, Aristotle states that once one has made an initial
division of the attributes that belong to all the animals *per
se*, the next step is to attempt "to divide their
causes," after which he applies the analysis of sameness and
difference from the previous discussion of parts to the activities
performed by these parts. He then closes the discussion by arguing
that the differences we find in animal parts are to be explained by
reference to differences found among their activities. Thus a proper
division of activities is a division of the causes of the parts of
animals.



Nevertheless, a good case can be made that this 'chapter'
forms a coherent whole, a sort of culmination of the book. It begins
with perhaps the best known passage in Aristotle's biological
works, a stirring and beautifully crafted encomium to the joys of
studying animals, even the most lowly. An elegant introduction divides
natural beings into those that are eternal and those that partake of
generation and perishing, noting that there are attractions to
studying both: though access to information about the former is
limited, he likens it to "...a chance, brief glimpse of the
ones we love"; on the other hand the latter, perishable things
"take the prize in respect of understanding because we know more
of them and know them more fully" and they are "more of
our own nature" (644b23-645a3). He then elaborates on the
value of the study of the living things around us. It is a passage
worth quoting in full.



> 
> 
> Since we have completed stating the way things appear to us about the
> divine things, it remains to speak about animal nature, omitting
> nothing in our power, whether of less or greater esteem. For even in
> the study of animals disagreeable to perception, the nature that
> crafted them likewise provides extraordinary pleasures to those able
> to know their causes and who are by nature philosophers. Surely it
> would be unreasonable, even absurd, for us to enjoy studying
> likenesses of animals--on the ground that we are at the same time
> studying the art, such as painting and sculpture, that made
> them--while not prizing even more the study of things constituted
> by nature, at least when we can behold their causes.
> 
> 
> 
> For this reason we should not be childishly disgusted at the
> examination of the less valuable animals. For in all natural things
> there is something marvelous. Even as Heraclitus is said to have
> spoken to those strangers who wished to meet him but stopped as they
> were approaching when they saw him warming himself at the
> oven--he bade them to enter without fear, 'For there are
> gods here too'--so too one should approach research about
> each of the animals without disgust, since in every one there is
> something natural and good. For what is not haphazard but rather for
> the sake of something is in fact present most of all in the works of
> nature; the end for the sake of which takes the place of the good. If
> someone considered the study of the other animals to lack value, he
> ought to think the same thing about himself as well; for it is
> impossible to look at that from which mankind has been
> constituted--blood, flesh, bones, blood vessels, and other such
> parts--without considerable disgust. Just as one who discusses
> the parts or equipment of anything should not be thought of as doing
> so in order to draw attention to the matter, nor for the sake of the
> matter, but rather in order to draw attention to the overall shape
> (e.g. to a house rather than bricks, mortar, and timbers); likewise
> one should consider the discussion of nature to be referring to the
> composite and the overall substantial being rather than to those
> things which do not exist when separated from their substantial being.
> (*PA* I 5, 645a4-36)
> 
> 
> 



Readers, often carried away by the rhetoric of this passage, might
fail to notice that there is a compelling argument here, with two key
conclusions. The first is that, while studying the Sun, Moon, starts
and planets may be a noble pursuit, if scientific understanding is
your goal, you are much more likely to achieve it in the study of
animals and plants: "...anyone wishing to labor
sufficiently can grasp many things about each kind...we know more
of them and we know them more fully...they are nearer to us and
more of our own nature..." (excerpted from
644b29-645a3). The second is that if one has the proper
philosophical focus, then one realizes that the somewhat unpleasant
task of dissecting blood vessels and the like, is a means to
understanding *that for the sake of which* and *soul*,
those objects of inquiry argued to be primary in chapter 1. For the
lesson of chapter 1 is that understanding living matter is achieved
primarily by means of an understanding of the functional goals served
by an animal's parts, discovering what each of its parts is for,
what it contributes to the life of the animal. This is the
philosophical way to investigate animals, and by doing so you learn
the value of an animal's parts and activities to its life.



The remainder of the chapter, therefore, recommends approaching the
*activities* associated with animals and their parts in a
manner exactly analogous to the approach to the study of parts
recommended in chapter 4 (645b1-14); and it concludes with a
schematic discussion of the variety of teleological relationships that
hold *among* parts, *between* parts and their
activities, and *among* activities. Aristotle encourages us to
conceive of these relationships in a yet more *unified* way, as
a single complex relationship between a single complex activity
(living, I suppose) and a single complex instrument, the
animal's body. (645b15-36)



In the first book of *On the Parts of Animals*, then, we have a
systematic philosophy of biology, which in many respects seems to
provide what is missing in the *Analytics* regarding the
application of its program to the study of living nature. But does
Aristotle's biological practice carry through on this program?
It is worthwhile keeping that question in mind as we survey those
books that report the results of Aristotle's animal studies. To
those books we now turn. 


## 5. Aristotle's Biological Practice


### 5.1 The *History of Animals* as a Report of the Results of a *Hoti*-Investigation



As we saw earlier, Aristotle introduces his systematic study of the
differences and attributes of animals as a
*hoti*-investigation, a factual investigation preliminary to
the search for causal demonstrations. By studying it, then, we ought
to be able to develop a rich picture of what this theoretically
motivated, preliminary stage of investigation aims to achieve, and
perhaps a glimpse of what such an investigation entails.



As many studies, following up on the pioneering work of David Balme
(1961, 1987b) have now established, the *History of Animals*
(*HA*) is a work that from first to last displays and builds
upon multi-differentiae division of animal differences (see Gotthelf
1988, 1997b; Lennox 2001b, chs. 1, 2; Pellegrin 1986). It is organized
as a study of four kinds of animal differences first mentioned in
*HA*'s first chapter as the principal objects of study
(at 486b22-487a14)--differences in parts (the topic of
Books I-IV), in modes of activity and ways of life (those
related to generation in Books V-VI--and human generation in
IX--others in Book VII) and in characters (Book VIII). These in turn
are sub-divided; for example, discussion of the *non-uniform*
parts (organs, as we say) of animals with blood (I 7-III 1) is
followed by that of their *uniform* parts (tissues, III
2-22). A discussion of the parts of animals without blood then
concludes the discussion of differences in parts (IV 1-8). Book
IV concludes with a discussion of differences in sensory faculties,
voice, and differences related to
 sex.[9]



It is only once Aristotle begins to distinguish *differences*
among, say, uniform parts that various groupings of animals play an
important role in organizing the discussion. And in so far as there
are relatively stable general groupings of animals, these are
identified by noting their possession of stable correlations among
differentiae--for example, there are a large number of different
animals all of which have wings, feathers, beaks, and two fleshless
legs, and this group of animals has been given the designation
 'bird'.[10]
 In other cases, such groups have not been generally recognized, and
Aristotle simply refers to them by means of some of their most
important correlated differences--the four-legged and
live-bearing animals, for example, corresponding to our land mammals
(with one important exception).



What is clear from the practice of the *History of Animals* is
both the value of division and its limitations. Division by itself
does not provide you with the *axes* of division; rather they
are presupposed. Division does not give you animal kinds; as we saw in
the previous section, one needs to turn to *PA* I 4 and
*HA* I 6 for Aristotle's thoughts on how those kinds are
established. Something besides division is needed in order for a
researcher to recognize *theoretically* *significant*
kinds, such as cephalopods or cetaceans. Why group animals together
based on their possession of four legs and the ability to give birth
to living offspring (rather than eggs)? Certainly each of these traits
is the product of a division, one of modes of locomotion and one of
modes of reproduction. But those divisions do not tell you that
animals with four legs that bear living young constitute a
scientifically significant group.



A second limitation of division is its indifference to the distinction
between causally fundamental characteristics and proper attributes, to
use the language of the *Analytics*. Yet, being able to
distinguish these is absolutely fundamental to Aristotelian science. A
careful comparative study of the *History of Animals*, on the
one hand, and works such as *On the Parts* or *On the
Generation of Animals*, on the other, provides insight into how
Aristotle understands and deploys this distinction in his actual
scientific practice. And as we have seen above, Aristotle draws
explicit attention to its importance for his biological investigations
in a number of key texts within those investigations themselves. To
study in detail the interplay between definition, causal demonstration
and division in the biology is to see Aristotle working through just
those problems which form the central question of *Posterior
Analytics* II--how precisely are definition, causal
demonstration and division related to one another in the quest for,
and achievement of, scientific understanding?



It is impossible to present even a hint of the richness of the
empirical content of the *Historia Animalium* here. Before
going on to a discussion of the works that report the results of
Aristotle's causal investigations of animal parts and animal
generation, however, two examples of reports from *HA* are
provided for comparison with corresponding discussions in *PA*
and *GA*. The first example discusses the organs of respiration
in blooded animals; these are the first *internal* organs
discussed after Aristotle has concluded his review of their
*external* parts. Here is how he introduces that
discussion.



> 
> 
> So, then, the number and character of the external parts of the
> blooded animals and how they differ from one another has been stated.
> We must now discuss how the internal parts are situated, first in the
> blooded animals.
> 
> 
> 
> As many as (*hosa*) are four legged and live-bearing all
> (*panta*) have an esophagus and windpipe, arranged just as in
> human beings; it is also arranged similarly in as many of the four
> legged animals as lay eggs, and in the birds; but they differ in the
> forms (*tois eidesi*) of these parts. Generally, all and only
> (*panta hosa*) those receiving air by inhaling and exhaling in
> every case (*panta*) have a lung, windpipe and esophagus; and
> the *position* of the esophagus and windpipe is alike in all,
> but the organs are not, while the lung is neither alike in all nor is
> it positioned in a like manner. And again as many as (*hosa*)
> have blood all (*panta*) have a heart... . But not all
> [blooded animals] have a lung, for instance fish do not, nor any other
> animal there might be that has gills. (505b23-506a12,
> excerpts)
> 
> 
> 



There are a number of features in this passage that are common
throughout the *Historia Animalium* and provide insight into
its methods and aims. First is the syntactically redundant linguistic
pattern *hosa...panta* ("as many as...all"),
variants of which are common in this treatise. Aristotle appears to
use the sentence form "As many as are X, all have Y" for a
quite specific reason. It is not *just* to note a universal
correlation; it is to do so while leaving the extension of the
correlation open--a brilliant methodological innovation. New
animals with the correlation can be discovered, but the generalization
will not change, since it is about the universal correlation among
differentiae--in this case internal organs--not about the
kinds that exemplify it. It appears that when Aristotle intensifies
the expression by writing *panta hosa*, as he does once here,
he means to signal a *convertible* universal, an
'entailment' as we would say. Aristotle first lists a
number of distinct groups, all the members of which have the three
organs being discussed, windpipe, lung and esophagus. Then, using the
term 'generally' to signal what he is about to do, he
identifies a differentia that all these groups have in common, and
which is *coextensive* with these three organs--inhaling
and exhaling air. The inductive pattern has the following form:



> 
> 
> 'As many as are *P* all have *Y*; As many as are
> *Q* all have *Y*... .
> 
> 
> 
> Generally, all and only those that are *X* (a feature common to
> *P*, *Q*...) have *Y*'.
> 
> 
> 



By going on to note that as many as have blood have a heart, he
implicitly is warning the reader about a potential false inference one
might draw from the examples so far: that all blooded animals have a
lung. He mentions fish as an example of blooded animals without a
lung, but immediately identifies the *relevant* differentia:
'those blooded animals *that have gills* do not have
these organs [the lung and its associated parts]'. He is careful
not to restrict the universal to a kind--the relevant correlation
is with *gills*, in whichever animals they might be found.



While the inductive strategy of this work, directing us to high-level,
convertible universals is, I hope, now clear, it is also important to
note what is *absent* from this passage. Aristotle rarely in
this work claims any of these correlations is *necessary*, or
that others are *impossible*. He does not distinguish features
that are part of the *essence* of a kind from others that are
mere attributes; and he typically avoids any hint of *causal
explanation*. All of this is to be expected, given the
methodological guidelines for *historia* that we discussed
earlier--*historia* is preliminary to causal investigation
and prepares us for it. In the passage we are currently considering,
for example, it is striking that he notes the convertible correlation
between lung, windpipe and esophagus, on the one hand, and inhaling
and exhaling of air, on the other, but does *not* claim that
two of the organs are present for the sake of breathing. (In fact, the
word for 'breathing' is never used!) Though he uses the
term 'nature' regularly in *HA*, he does
*not*, as he does so often in his causal investigations, claim
that nature does nothing in vain, or that it always does what is best
for each organism. Nor does he talk about the parts of an animal as
its material nature, nor about its living capacities as aspects of its
formal nature. Indeed there is virtually no mention of the matter-form
distinction in *HA*. It is only when we turn to the various
biological treatises devoted to causal investigation that this
language associated with the search for definitions and demonstrations
comes to the fore.



The aim of this brief analysis of one small passage is to convey a
taste of the methodological depth and complexity of this great
scientific treatise, so often misunderstood as a poorly organized
compendium of nature lore. I'll conclude this section with one
more example, from the books dealing with generation, so that we may
compare it with a corresponding discussion in *De*
*Generatione Animalium*.



*HA* V-VI discuss all aspects of animal generation, moving from
consideration of differences in modes and timing of copulation and
reproduction to the actual *process* of generation (or
'development', as we more often say). As in his discussion
of parts, he moves through the various blooded kinds and then
discusses generation in bloodless kinds and from identifying the
universals common to all members of a group to identifying the
peculiarities of more specific kinds. At *HA* VI 3, having
previously described the common and peculiar characteristics of
copulation in birds, he turns to recording his observations of the
development of the chick at the critical fourth day after the egg has
been laid.



> 
> Generation from the egg occurs in an identical manner in all birds,
> though the time taken to termination varies, as we have said. In the
> case of the hen, the first signs of the embryo are seen after three
> days and nights; in larger birds it takes more time, in smaller birds
> less. During this time the yolk travels upwards to the point of the
> egg--that is where the starting point of the egg is and where it
> opens up, and the heart is no bigger than just a small blood-spot in
> the white. This spot beats and moves as though it were alive; and from
> it, as it grows, two vein-like vessels with blood in them lead on a
> twisted course to each of the two surrounding membranes. A membrane
> with bloody fibers already surrounds the white of the egg, at this
> time coming from the vessel-like channels. A bit later the body can
> also be distinguished, at first very small and pale. The head is
> apparent, and its eyes, very swollen; and this continues for a long
> time, for it is later that they contract and become smaller.
> (561a4-21)
> 



He begins with a wide generalization about all birds, followed by a
'more and less' correlation between the *size* of
the bird and the *rate* of development. This device allows
Aristotle to use the careful description of the chick egg opened on
day four as a universal description of development; the reader is
licensed to infer that the same stage of development will occur in all
birds, but may occur either earlier or later, depending on whether
they are larger or smaller than the domestic chicken.
Aristotle's description of this classic experiment led to its
being repeated many times in the
 Renaissance.[11]
 Anyone who has seen modern films of the chick at this stage of
differentiation can appreciate the accuracy of Aristotle's
description, especially when one takes into account the size of the
chick embryo at this stage of growth.


### 5.2 From Inquiry to Understanding; from *hoti* to *dioti*.



For Aristotle, '*historia*', the scientific
organization of information about animals--serves the goal of
causal explanation; and, as I discussed in section 4., explanation by
reference to goals and functions is the primary form of causal
explanation in biology.



*On the Parts of Animals* II-IV presents Aristotle's
attempt to provide causal explanations for the sort of information one
finds organized in *HA* I-IV, information about the uniform and
non-uniform parts of animals. (*PA* I, it will be recalled, is
a discussion of the norms that are needed to guide the scientific
investigation of animals; see section 4, above.) Here, I will be
narrowly focused on just a couple of passages in *PA* II-IV,
but will begin with a very brief sketch of its overall structure. It
begins (*PA* II 1-2) with a careful and complex
discussion of the causal relationships between uniform and non-uniform
parts (roughly, this distinction corresponds to our distinction
between bodily fluids and tissues and organs), stressing the
teleological priority of the whole organism to the parts, and among
parts the priority of the non-uniform to the uniform. *PA* II
3-10 then discusses the uniform parts, beginning with that out
of which all the others are nourished, blood, and ending with flesh,
bone, and analogous materials (nail, horn, hoof). From II 10 to III 3,
Aristotle discusses the external parts of blooded animals associated
with the head such as eyes, ears, nose and mouth. But when he gets to
the neck, he turns to the internal parts, and from III 3 to IV 4 he
discusses the 'innards' of the blooded animals, parts such
as heart, lung, liver, kidneys, and so on. From IV 5 to 9 he discusses
the parts, both internal and external, of the 'bloodless'
animals, i.e. the crustaceans, testaceous mollusks, cephalopods and
insects. Finally, at IV 10, he returns to the external parts
associated with the torso and limbs of the live bearing and egg laying
quadrupeds (10-11), birds (12), and fish (13); and ends by
discussing a number of animals that 'dualize' between
being land-dwellers and water dwellers or flyers.



In order to compare the methods of this treatise with the discussion
of the parts in *HA*, it will be useful to look briefly at
*PA* III 3, a discussion of the lung, windpipe and esophagus,
the part discussed in the passage from HA II quoted in section 5.1,
above.

 The first twenty lines of that discussion read as follows:


> 
> 
> Not all animals have a neck, but only those with the parts *for the
> sake of which* the neck is *naturally* present--these
> are the windpipe and the part known as the esophagus. Now the larynx
> is present *by nature for the sake of* breathing; for it is
> through this part that animals draw in and expel air when they inhale
> and exhale. *This is why* those without a lung have no neck,
> e.g. the kind consisting of the fish. The esophagus is the part
> through which nourishment proceeds to the gut; so that animals without
> necks manifestly do not have an esophagus. But *it is not
> necessary* to have the esophagus *for the sake of*
> nutrition; for it concocts nothing. And further, it is
> *possible* for the gut to be placed right next to the position
> of the mouth, while for the lung this is *impossible*. For
> there *needs* first to be something common like a conduit,
> which then divides in two and through which the air is separated into
> passages--in this way the lung *may best* accomplish
> inhalation and exhalation.
> 
> 
> 
> So, then, the organ connected with breathing *from necessity*
> has length; therefore it is *necessary* for there to be an
> esophagus between the mouth and the stomach. The esophagus is fleshy,
> with a sinuous elasticity--*sinuous* *so that* it
> may dilate when food is ingested, yet *fleshy* *so that*
> it is soft and yielding and is not damaged when it is scraped by the
> food going down. (664a14-34)
> 
> 
> 



I have highlighted the language of *nature, necessity, possibility
(and impossibility), and being for the sake of* in this passage in
order to highlight the contrast with the discussion of the same
organic correlations in the corresponding *HA* passage. Here
the goal is *explanation*--parts are present and have the
character they do primarily due to the conditional necessity imposed
by the organism's functional requirements. One sees here the
Aristotle that so impressed the great French naturalist Georges
Cuvier: Aristotle is not only systematically discussing the adaptive
functions of each of these organs; he is also displaying the complex
way in which the internal parts of animals constitute an organic
*system*.



The lack of names for animal kinds is also noteworthy; as in the
corresponding passage from *HA* II, fish are mentioned once as
an instance of blooded animals that lack the organs in question; but
the stress throughout is on this particular system of organs in
whichever blooded animals have them. Aristotle's goal is to
explain why it is that certain animals have these organs, and why it
is that, *if* they have one of these organs, they *must*
have the others. Some, but not all, of the explanations specify the
function for the sake of which the parts are present; and even the
material nature of the tissues, whether fleshy or sinuous, may be
explained in functional terms. This discussion also displays a
*hierarchy* of teleological connections of the sort described
at the close of *PA* I 5, 645b28-33--one activity is
present for the sake of another more basic one; a part that performs a
subordinate activity is present for the sake of a part that performs a
more important activity. Inhaling and exhaling are for the sake of
respiration (which is for the sake of cooling the blood); a windpipe
is present for the sake of transporting air to and from the lung; a
neck is present to protect the windpipe.



For reasons that will become clear, the relationship between the books
of the *Historia Animalium* dealing with generation and *On
the Generation of Animals* (*GA*) is somewhat more complex.
*GA* has a reasonably clear organization of its own; but its
central (though not *only*) project of explaining the causal
roles of the male and female in sexual generation and the causality of
embryological development raises a host of topics and problems well
beyond the scope of *HA*. But a study of how the two works are
related is nonetheless illuminating. This discussion of
Aristotle's biological practice will thus close by considering
the causal explanation of embryological development and in particular
the explanation for the early appearance of the heart, with a final
look at the application of this explanation to the case of embryonic
birds.



As I noted earlier, taken on its own terms, *GA* has a
reasonably clear structure. Here again, as with PA II-IV, I will begin
with a brief sketch of its overall structure, before focusing narrowly
on its method as applied to one problem. It begins (715a1-18) by
noting that it will explain two sets of facts left unexplained in
*PA*: the differences among the parts related to generation;
and the *generation* of the parts generally. Though he
acknowledges that some animals do not have males and females, he
treats sexual generation as his central topic. Initially he provides a
theoretically 'neutral' account of the distinction: the
male is what generates in another, the female what generates in itself
(716a14-15). It is not until late in book I that he begins to
develop his own detailed theory of their distinctive contributions to
generation.



The first 13 chapters characterize the reproductive organs in the
males and females of blooded animals, while 14-16 do the same
for the bloodless animals that have males and females. Beginning in
chapter 17, the discussion turns to the relevant *uniform*
parts related to generation, *sperma* (roughly
'seed') and milk, but the discussion of milk is postponed
until book IV. This immediately raises the contentious issue of
whether *both* male and female contribute *sperma*, and
if both, then what the nature of their respective contributions is.
This is a large part of the discussion from I 17-II 5, one of the most
difficult stretches of text in the Aristotelian corpus, and the one on
which our attention will be focused. But to continue our overview: the
remainder of book II (from chapter 5 on) discusses the causes of the
embryological development of live-bearing animals, while the
egg-laying animals are the primary focus in book III, which closes
with a discussion of animals that are *not* sexually generated,
including those that arise 'spontaneously.' *GA* IV
turns to the development of inherited differences within kinds,
beginning with the most important intra-specific difference for his
theory of generation, the difference between male and female. From
there it moves on to family resemblance, differences in the number of
offspring produced and to the causes of deformities. *GA* V
aims to explain 'more and less' variations within kinds
that appear late in development and that may be fully accounted for by
identifying material level causal interactions--differences
within a kind in the color, texture or density of a part, for
example.



The remainder of this discussion will focus on the general causal
theory of animal generation developed in the first half of *GA*
 II.[12]
 In the closing chapters of book I, Aristotle argues that both the
male and female contributions are formed from a 'useful
residue' of blood by a concocting process, the male's
being more fully concocted than the female's. From this point on
the male's contribution is typically termed *gone*
(semen), and the female's *katamenia* (menses). Aristotle
is not entirely consistent, however: occasionally he uses the generic
term *sperma* for the male contribution and explicitly
contrasts it with *katamenia* (727a27-30); while at other
times he refers to *katamenia* as a sort of *sperma*
(728a26-29).



The two contributions are variously described. The male contributes a
source of movement or *dunamis* ('power' or
'potential'), which, as the argument unfolds, turns out to
be a special sort of capacity to heat present in the semen's
*pneuma* or air, which is part of its nature. (This gets fully
worked out in *GA* II 2-3) The semen itself is mere a
vehicle for delivering this heating capacity; the male makes no
material contribution to the offspring. The female contributes what
might be called 'prepared matter'; all it needs is the
presence within it of the heating capacity from the male and it begins
a more or less lengthy and complicated developmental process, which
Aristotle analogizes to a sort of automaton performing a complex set
of coordinated movements once it is set in motion.



He relies at several points in the argument on an analogy with crafts,
such as house building:



> 
> In animals that emit semen the nature in the male uses the semen as an
> instrument possessing active movement, just as the tools are moved in
> things that come to be by craft; for the movement of the craft is
> somehow in these tools. (730b20-23)
> 



This obscure analogy gradually gets unpacked in the first four
chapters of *GA* II. The general principle common to art and
nature is this:



> 
> As many things as come to be by nature or by art come to be by means
> of a being in actuality from that which is potentially such as that
> being. (734b22-23)
> 



It is the prepared portion of the female's *katamenia*
that is potentially such as the contributor of semen is actually. And
the semen conveys to it a movement or power, such that "when the
movement ceases each of the parts comes to be and is ensouled
(734b24-25)." That is, in embryological development, each
part emerges within a system of parts that is organized to provide
nutrition for its growth, and that is guided by a complex
'program' for its development. Notice that as each part
comes to be it already has the functional capacity associated with
it--it comes to be *and is ensouled*.

 Aristotle
characterizes the process as follows:


> 
> As for hardness, softness, toughness and fragility and all the other
> such affections present in these ensouled parts, these affections
> might be produced by heat and cold, but not the defining character
> (*logos*) in virtue of which the one part is flesh and the
> other bone; that is the result of the movement derived from the
> generator, being in actuality what that out of which it comes to be is
> potentially, just as in things that come to be by craftsmanship.
> (734b31-35)
> 



This passage comes immediately after Aristotle has stressed two
related points: that the uniform parts and the instrumental parts
arise simultaneously; and that even flesh and bone have a function to
perform. These points are tightly connected. The defining features of
tissues are not merely their affective qualities such as being hard or
soft, but their biological functions as well; and their biological
functions are intimately connected to their roles in
constituting--*and being able to play functional roles
in*--instrumental parts; many, if not all, of a part's
affective qualities are a consequence of its biological role.



In the process of making this argument, Aristotle also stresses a
crucial and problematic *dis*-analogy: Natural things, once
their generation begins, 'grow by means of themselves'
(735a13-14). This is the most fundamental difference between art
and nature for Aristotle; a nature is a source of change within the
thing itself (*Physics* II. 1). Natural development, then,
though initiated by the warmth contributed by the male parent,
continues by means of a source of change within the developing embryo.
The carpenter may not mix any of his own material in his buildings;
but he must be continuously, physically involved in the movements that
create those buildings. Not so with the male parent in natural
generation. So how does a complex process that has its origins in an
external generator become a natural process of development?



For Aristotle, the heart (or its analogue in bloodless animals) is
crucial. It is the first part created by the heat derived from the
semen of the male parent, and henceforth it is the primary internal
source of that heat and thus of further development (735a16-26).
The following explanation begins to bring us into contact with our
passage on generation from the *History of Animals*.



> 
> ...in the embryo (*kuema*), while in a way all the parts
> are present potentially, the source [of development] is most along the
> path to actuality. This is why the heart is first to be differentiated
> in actuality. And this is not only clear to perception (as it
> happens), but is also clear to reason. When the developing thing is
> distinct from both the male and female contributions, it must manage
> itself, as when a son sets up a home away from his father. So in
> animals there must be a source of generation from which the later
> organizing of the body comes about. ... Wherefore the heart
> appears differentiated first in all blooded animals; for it is the
> source of both the uniform and the non-uniform parts. ... And the
> prepared blood or its analogue is nutrient for the animal, and the
> blood vessels are its repository; for this reason the heart is their
> source. This is clear from a study of 'the histories' and
> 'the dissections'. (740a2-23, excerpted; compare
> 735a12-26, 738b17-18 740a27-36, 742b34-743a1,
> 743b26)
> 



Having developed this general explanation during his discussion of his
central case, viviparous development, in *GA* II, Aristotle
moves on to egg-laying animals in book III. *GA* III 1 carries
over to them whatever is common to the two cases, with discussion of
the development of birds in their eggs beginning in *GA* III 2.
A recurring aspect of his argument, here as in *PA*, is to
refer us for descriptive details to the *histories* (presumably
the basis of our *HA*) and to a collection of representations
of dissections (tragically not preserved, but cited repeatedly in
Aristotle's biological writings and in ancient catalogues of his
writings).



The most general differences among egg-layers relate to the
'completeness' of the egg when laid. Because birds lay
eggs that become hard as soon as they are exposed to the air
(752a30-b2), they are taken to be better prepared when laid than the
eggs of other egg-laying animals. Ignoring much interesting and
important detail, we will take up the discussion of development within
the eggs of birds as they incubate. Aristotle distinguishes the
'white' and the 'yolk', not descriptively as
he did in *HA*, but functionally (752b18-28).



> 
> ...it is the white out of which the animal is constituted, while
> the yolk becomes nutrient, and growth for the constitution of the
> parts is from it. This is also why the yolk and the white, having
> distinct natures, have been kept distinct by membranes. For precise
> details concerning how these parts are situated in relation to each
> other in virtue of the sources of generation and the constitution of
> animals, as well as the membranes and umbilical cords, one should
> study what has been outlined in the histories. The following appears
> to be sufficient for the present investigation: the heart is the first
> thing constituted; after this the great blood vessel is marked off,
> and two umbilical cords extend from this blood vessel, one to the
> membrane around the yolk, the other to the membrane surrounding the
> animal itself. This latter is thus next to the membrane of the shell.
> (753b11-23)
> 



In this case (though not always) one does get far more detail in
*HA*. These references to histories and dissections can be
misleading unless one takes into account the differences in the aims,
and thus in the organization, of these treatises. In this case, for
example, nothing is said in *GA* about the heart 'beating
and moving as though it were alive'. As we saw, however,
*GA* II stresses that in the development of live-bearing
animals, the heart is first to be formed, is ensouled from the start,
and is the source of development for all the other parts--a much
more 'theory-laden' characterization that stresses the
heart's causal role in development. And because of the
organizational structure of *GA*, much of what is said in his
discussion of the live-bearing animals (those that are 'more
complete' when they are born) is assumed to apply to animals
that lay eggs (that are 'less complete' when born).



Recall the stunningly accurate discussion of the head and eyes of the
four day old chick embryo in *HA* (in section 5.1, above). This
is not discussed in *GA* III 2; however in *GA* II 6, in
a long discussion of early organ formation in *live-bearing*
animals, Aristotle has a good deal to say on the subject:



> 
> What happens concerning the eyes of animals presents a problem. For
> from the beginning, whether in land animals, swimmers or flyers, they
> appear very large and yet they are among the last of the parts to be
> completed and in the mean time contract in size. *This is
> because* the sense organs of the eyes are, as with the other sense
> organs, on channels; but while those for touch and taste are simply
> bodily, or some part of the body of animals, and those for smell and
> hearing are channels connected to the air outside and are full of
> connate pneuma...the eyes alone have a proper sensory body.
> (743b33-744a6)
> 



The full explanation goes on much longer, but we need not follow it
out for the purposes of making two important points about the
relationship of *GA* to *HA*. The first is that the
organization of *GA* dictates that for the explanation of facts
of wide generality such as this one, we must look in book
II--notice that he claims that this explanation for the large
size of eyes early in development and for their subsequent contraction
applies to all land, air and water animals. The second point to be
stressed is the relentlessly causal/explanatory focus of the
*GA* discussion, a feature (properly) completely absent in the
corresponding discussions in *HA*. As impressive as the
descriptive accuracy of the embryological passage cited earlier from
*HA* is, we again need to remember what is *absent* from
that discussion. For the equally impressive embryological
*theory*, the reader of Aristotle's biology must turn to
the *Generation of Animals*.


## 6. A Concluding Puzzle



This entry on Aristotle's biology and its philosophy closes with
a puzzle about the organization of his biological works when compared
with the remainder of the Aristotelian corpus. I began by noting that
it is a fundamental aspect of Aristotle's theory of science that
investigation proceeds from a stage where one establishes the
existence of the things being studied and the facts about them to a
stage where the focus is on causal explanations by reference to the
natures and essences of those things. It seems clear that the
biological works honor this distinction, and do so self-consciously.
Yet Aristotle never suggests, in the *Posterior Analytics* or
anywhere else, that these stages should be represented by *distinct
treatises* devoted to the same subject. Indeed, in no other domain
does Aristotle do this: whether we look to meteorology, cosmology,
psychology, ethics, drama or rhetoric, we find single treatises
presenting both the facts and their explanation. Nor can this puzzle
be easily dismissed as an artifact of later editing--as I have
pointed out a number of times, these treatises have a most interesting
and consistent pattern of cross-referencing. The treatises that report
the results of causal investigations regularly refer to *HA*
(and the dissections) for more detail regarding the facts being
explained; the *HA*, in contrast, *never* refers to the
causal treatises. Moreover, the striking avoidance in the *Historia
Animalium* of *all* of the concepts associated with
definition and explanation cannot reasonably be laid at the feet of
some imagined later editor. We are left, then, to ponder what it was
about the investigation of animals that led Aristotle to take a
*methodological* distinction regarding stages of investigation
and reify it in methodologically distinct treatises devoted to these
different stages.