by Aristotle
explaining the works of nature, or that at any rate an attempt must be
made to include them both; and that those who fail in this tell us
in reality nothing about nature. For primary cause constitutes the
nature of an animal much more than does its matter. There are indeed
passages in which even Empedocles hits upon this, and following the
guidance of fact, finds himself constrained to speak of the ratio
(olugos) as constituting the essence and real nature of things.
Such, for instance, is the case when he explains what is a bone. For
he does not merely describe its material, and say it is this one
element, or those two or three elements, or a compound of all the
elements, but states the ratio (olugos) of their combination. As
with a bone, so manifestly is it with the flesh and all other
similar parts.
The reason why our predecessors failed in hitting upon this method
of treatment was, that they were not in possession of the notion of
essence, nor of any definition of substance. The first who came near
it was Democritus, and he was far from adopting it as a necessary
method in natural science, but was merely brought to it, spite of
himself, by constraint of facts. In the time of Socrates a nearer
approach was made to the method. But at this period men gave up
inquiring into the works of nature, and philosophers diverted their
attention to political science and to the virtues which benefit
mankind.
Of the method itself the following is an example. In dealing with
respiration we must show that it takes place for such or such a
final object; and we must also show that this and that part of the
process is necessitated by this and that other stage of it. By
necessity we shall sometimes mean hypothetical necessity, the
necessity, that is, that the requisite antecedants shall be there,
if the final end is to be reached; and sometimes absolute necessity,
such necessity as that which connects substances and their inherent
properties and characters. For the alternate discharge and re-entrance
of heat and the inflow of air are necessary if we are to live. Here we
have at once a necessity in the former of the two senses. But the
alternation of heat and refrigeration produces of necessity an
alternate admission and discharge of the outer air, and this is a
necessity of the second kind.
In the foregoing we have an example of the method which we must
adopt, and also an example of the kind of phenomena, the causes of
which we have to investigate.
2
Some writers propose to reach the definitions of the ultimate
forms of animal life by bipartite division. But this method is often
difficult, and often impracticable.
Sometimes the final differentia of the subdivision is sufficient
by itself, and the antecedent differentiae are mere surplusage. Thus
in the series Footed, Two-footed, Cleft-footed, the last term is
all-expressive by itself, and to append the higher terms is only an
idle iteration. Again it is not permissible to break up a natural
group, Birds for instance, by putting its members under different
bifurcations, as is done in the published dichotomies, where some
birds are ranked with animals of the water, and others placed in a
different class. The group Birds and the group Fishes happen to be
named, while other natural groups have no popular names; for instance,
the groups that we may call Sanguineous and Bloodless are not known
popularly by any designations. If such natural groups are not to be
broken up, the method of Dichotomy cannot be employed, for it
necessarily involves such breaking up and dislocation. The group of
the Many-footed, for instance, would, under this method, have to be
dismembered, and some of its kinds distributed among land animals,
others among water animals.
3
Again, privative terms inevitably form one branch of dichotomous
division, as we see in the proposed dichotomies. But privative terms
in their character of privatives admit of no subdivision. For there
can be no specific forms of a negation, of Featherless for instance or
of Footless, as there are of Feathered and of Footed. Yet a generic
differentia must be subdivisible; for otherwise what is there that
makes it generic rather than specific? There are to be found
generic, that is specifically subdivisible, differentiae; Feathered
for instance and Footed. For feathers are divisible into Barbed and
Unbarbed, and feet into Manycleft, and Twocleft, like those of animals
with bifid hoofs, and Uncleft or Undivided, like those of animals with
solid hoofs. Now even with differentiae capable of this specific
subdivision it is difficult enough so to make the classification, as
that each animal shall be comprehended in some one subdivision and
in not more than one; but far more difficult, nay impossible, is it to
do this, if we start with a dichotomy into two contradictories.
(Suppose for instance we start with the two contradictories, Feathered
and Unfeathered; we shall find that the ant, the glow-worm, and some
other animals fall under both divisions.) For each differentia must be
presented by some species. There must be some species, therefore,
under the privative heading. Now specifically distinct animals
cannot present in their essence a common undifferentiated element, but
any apparently common element must really be differentiated. (Bird and
Man for instance are both Two-footed, but their two-footedness is
diverse and differentiated. So any two sanguineous groups must have
some difference in their blood, if their blood is part of their
essence.) From this it follows that a privative term, being
insusceptible of differentiation, cannot be a generic differentia;
for, if it were, there would be a common undifferentiated element in
two different groups.
Again, if the species are ultimate indivisible groups, that is,
are groups with indivisible differentiae, and if no differentia be
common to several groups, the number of differentiae must be equal
to the number of species. If a differentia though not divisible
could yet be common to several groups, then it is plain that in virtue
of that common differentia specifically distinct animals would fall
into the same division. It is necessary then, if the differentiae,
under which are ranged all the ultimate and indivisible groups, are
specific characters, that none of them shall be common; for otherwise,
as already said, specifically distinct animals will come into one
and the same division. But this would violate one of the requisite
conditions, which are as follows. No ultimate group must be included
in more than a single division; different groups must not be
included in the same division; and every group must be found in some
division. It is plain then that we cannot get at the ultimate specific
forms of the animal, or any other, kingdom by bifurcate division. If
we could, the number of ultimate differentiae would equal the number
of ultimate animal forms. For assum
e an order of beings whose prime
differentiae are White and Black. Each of these branches will
bifurcate, and their branches again, and so on till we reach the
ultimate differentiae, whose number will be four or some other power
of two, and will also be the number of the ultimate species
comprehended in the order.
(A species is constituted by the combination differentia and matter.
For no part of an animal is purely material or purely immaterial;
nor can a body, independently of its condition, constitute an animal
or any of its parts, as has repeatedly been observed.)
Further, the differentiae must be elements of the essence, and not
merely essential attributes. Thus if Figure is the term to be divided,
it must not be divided into figures whose angles are equal to two
right angles, and figures whose angles are together greater than two
right angles. For it is only an attribute of a triangle and not part
of its essence that its angles are equal to two right angles.
Again, the bifurcations must be opposites, like White and Black,
Straight and Bent; and if we characterize one branch by either term,
we must characterize the other by its opposite, and not, for
example, characterize one branch by a colour, the other by a mode of
progression, swimming for instance.
Furthermore, living beings cannot be divided by the functions common
to body and soul, by Flying, for instance, and Walking, as we see them
divided in the dichotomies already referred to. For some groups,
Ants for instance, fall under both divisions, some ants flying while
others do not. Similarly as regards the division into Wild and Tame;
for it also would involve the disruption of a species into different
groups. For in almost all species in which some members are tame,
there are other members that are wild. Such, for example, is the
case with Men, Horses, Oxen, Dogs in India, Pigs, Goats, Sheep; groups
which, if double, ought to have what they have not, namely,
different appellations; and which, if single, prove that Wildness
and Tameness do not amount to specific differences. And whatever
single element we take as a basis of division the same difficulty will
occur.
The method then that we must adopt is to attempt to recognize the
natural groups, following the indications afforded by the instincts of
mankind, which led them for instance to form the class of Birds and
the class of Fishes, each of which groups combines a multitude of
differentiae, and is not defined by a single one as in dichotomy.
The method of dichotomy is either impossible (for it would put a
single group under different divisions or contrary groups under the
same division), or it only furnishes a single ultimate differentia for
each species, which either alone or with its series of antecedents has
to constitute the ultimate species.
If, again, a new differential character be introduced at any stage
into the division, the necessary result is that the continuity of
the division becomes merely a unity and continuity of agglomeration,
like the unity and continuity of a series of sentences coupled
together by conjunctive particles. For instance, suppose we have the
bifurcation Feathered and Featherless, and then divide Feathered
into Wild and Tame, or into White and Black. Tame and White are not
a differentiation of Feathered, but are the commencement of an
independent bifurcation, and are foreign to the series at the end of
which they are introduced.
As we said then, we must define at the outset by multiplicity of
differentiae. If we do so, privative terms will be available, which
are unavailable to the dichotomist.
The impossibility of reaching the definition of any of the
ultimate forms by dichotomy of the larger group, as some propose, is
manifest also from the following considerations. It is impossible that
a single differentia, either by itself or with its antecedents,
shall express the whole essence of a species. (In saying a single
differentia by itself I mean such an isolated differentia as
Cleft-footed; in saying a single differentia with antecedent I mean,
to give an instance, Manycleft-footed preceded by Cleft-footed. The
very continuity of a series of successive differentiae in a division
is intended to show that it is their combination that expresses the
character of the resulting unit, or ultimate group. But one is
misled by the usages of language into imagining that it is merely
the final term of the series, Manycleft-footed for instance, that
constitutes the whole differentia, and that the antecedent terms,
Footed, Cleft-footed, are superfluous. Now it is evident that such a
series cannot consist of many terms. For if one divides and
subdivides, one soon reaches the final differential term, but for
all that will not have got to the ultimate division, that is, to the
species.) No single differentia, I repeat, either by itself or with
its antecedents, can possibly express the essence of a species.
Suppose, for example, Man to be the animal to be defined; the single
differentia will be Cleft-footed, either by itself or with its
antecedents, Footed and Two-footed. Now if man was nothing more than a
Cleft-footed animal, this single differentia would duly represent
his essence. But seeing that this is not the case, more differentiae
than this one will necessarily be required to define him; and these
cannot come under one division; for each single branch of a
dichotomy ends in a single differentia, and cannot possibly include
several differentiae belonging to one and the same animal.
It is impossible then to reach any of the ultimate animal forms by
dichotomous division.
4
It deserves inquiry why a single name denoting a higher group was
not invented by mankind, as an appellation to comprehend the two
groups of Water animals and Winged animals. For even these have
certain attributes in common. However, the present nomenclature is
just. Groups that only differ in degree, and in the more or less of an
identical element that they possess, are aggregated under a single
class; groups whose attributes are not identical but analogous are
separated. For instance, bird differs from bird by gradation, or by
excess and defect; some birds have long feathers, others short ones,
but all are feathered. Bird and Fish are more remote and only agree in
having analogous organs; for what in the bird is feather, in the
fish is scale. Such analogies can scarcely, however, serve universally
as indications for the formation of groups, for almost all animals
present analogies in their corresponding parts.
The individuals comprised within a species, such as Socrates and
Coriscus, are the real existences; but inasmuch as these individuals
possess one common specific form, it will suffice to state the
universal attributes of the species, that is, the attributes common to
all its individuals, once for all, as otherwise there will be
endless reiteration, as has already been pointed out.
/> But as regards the larger groups-such as Birds-which comprehend many
species, there may be a question. For on the one hand it may be
urged that as the ultimate species represent the real existences, it
will be well, if practicable, to examine these ultimate species
separately, just as we examine the species Man separately; to examine,
that is, not the whole class Birds collectively, but the Ostrich,
the Crane, and the other indivisible groups or species belonging to
the class.
On the other hand, however, this course would involve repeated
mention of the same attribute, as the same attribute is common to many
species, and so far would be somewhat irrational and tedious. Perhaps,
then, it will be best to treat generically the universal attributes of
the groups that have a common nature and contain closely allied
subordinate forms, whether they are groups recognized by a true
instinct of mankind, such as Birds and Fishes, or groups not popularly
known by a common appellation, but withal composed of closely allied
subordinate groups; and only to deal individually with the
attributes of a single species, when such species, man, for
instance, and any other such, if such there be-stands apart from
others, and does not constitute with them a larger natural group.
It is generally similarity in the shape of particular organs, or
of the whole body, that has determined the formation of the larger
groups. It is in virtue of such a similarity that Birds, Fishes,
Cephalopoda, and Testacea have been made to form each a separate
class. For within the limits of each such class, the parts do not
differ in that they have no nearer resemblance than that of
analogy-such as exists between the bone of man and the spine of
fish-but differ merely in respect of such corporeal conditions as
largeness smallness, softness hardness, smoothness roughness, and
other similar oppositions, or, in one word, in respect of degree.
We have now touched upon the canons for criticizing the method of
natural science, and have considered what is the most systematic and