On The Parts Of Animals   

many different senses, and that no one substance can be hotter than
others in all these senses, we must, when we attribute this character
to an object, add such further statements as that this substance is
hotter per se, though that other is often hotter per accidens; or
again, that this substance is potentially hot, that other actually so;
or again, that this substance is hotter in the sense of causing a
greater feeling of heat when touched, while that other is hotter in
the sense of producing flame and burning. The term hot being used in
all these various senses, it plainly follows that the term cold will
also be used with like ambiguity.
So much then as to the signification of the terms hot and cold, hotter
and colder.
Part 3
In natural sequence we have next to treat of solid and fluid. These
terms are used in various senses. Sometimes, for instance, they denote
things that are potentially, at other times things that are actually,
solid or fluid. Ice for example, or any other solidified fluid, is
spoken of as being actually and accidentally solid, while potentially
and essentially it is fluid. Similarly earth and ashes and the like,
when mixed with water, are actually and accidentally fluid, but
potentially and essentially are solid. Now separate the constituents
in such a mixture and you have on the one hand the watery components
to which its fluidity was due, and these are both actually and
potentially fluid, and on the other hand the earthy components, and
these are in every way solid; and it is to bodies that are solid in
this complete manner that the term 'solid' is most properly and
absolutely applicable. So also the opposite term 'fluld' is strictly
and absolutely applicable to that only which is both potentially and
actually fluid. The same remark applies also to hot bodies and to
cold.
These distinctions, then, being laid down, it is plain that blood is
essentially hot in so far as that heat is connoted in its name; just
as if boiling water were denoted by a single term, boiling would be
connoted in that term. But the substratum of blood, that which it is
in substance while it is blood in form, is not hot. Blood then in a
certain sense is essentially hot, and in another sense is not so. For
heat is included in the definition of blood, just as whiteness is
included in the definition of a white man, and so far therefore blood
is essentially hot. But so far as blood becomes hot from some external
influence, it is not hot essentially.
As with hot and cold, so also is it with solid and fluid. We can
therefore understand how some substances are hot and fluid so long as
they remain in the living body, but become perceptibly cold and
coagulate so soon as they are separated from it; while others are hot
and consistent while in the body, but when withdrawn under a change to
the opposite condition, and become cold and fluid. Of the former blood
is an example, of the latter bile; for while blood solidifies when
thus separated, yellow bile under the same circumstances becomes more
fluid. We must attribute to such substances the possession of opposite
properties in a greater or less degree.
In what sense, then, the blood is hot and in what sense fluid, and how
far it partakes of the opposite properties, has now been fairly
explained. Now since everything that grows must take nourishment, and
nutriment in all cases consists of fluid and solid substances, and
since it is by the force of heat that these are concocted and changed,
it follows that all living things, animals and plants alike, must on
this account, if on no other, have a natural source of heat. This
natural heat, moreover, must belong to many parts, seeing that the
organs by which the various elaborations of the food are effected are
many in number. For first of all there is the mouth and the parts
inside the mouth, on which the first share in the duty clearly
devolves, in such animals at least as live on food which requires
disintegration. The mouth, however, does not actually concoct the
food, but merely facilitates concoction; for the subdivision of the