It
may, however, be asked, of what mode of necessity are we speaking when
we say this. For it can be of neither of those two modes which are set
forth in the philosophical treatises. There is, however, the third
mode, in such things at any rate as are generated. For instance, we
say that food is necessary; because an animal cannot possibly do
without it. This third mode is what may be called hypothetical
necessity. Here is another example of it. If a piece of wood is to
be split with an axe, the axe must of necessity be hard; and, if hard,
must of necessity be made of bronze or iron. Now exactly in the same
way the body, which like the axe is an instrument-for both the body as
a whole and its several parts individually have definite operations
for which they are made-just in the same way, I say, the body, if it
is to do its work, must of necessity be of such and such a
character, and made of such and such materials.
It is plain then that there are two modes of causation, and that
both of these must, so far as possible, be taken into account in
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.
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