On The Parts Of Animals   

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.
Part 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.
Part 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