Many a
contrary is transformed into its other and vice versa, where neither
is even a quantum and so cannot increase in bulk, e.g. an invalid into
a healthy subject. It is clear that not even those contraries which
satisfy both the conditions mentioned above are food to one another in
precisely the same sense; water may be said to feed fire, but not fire
water. Where the members of the pair are elementary bodies only one of
the contraries, it would appear, can be said to feed the other. But
there is a difficulty here. One set of thinkers assert that like
fed, as well as increased in amount, by like. Another set, as we
have said, maintain the very reverse, viz. that what feeds and what is
fed are contrary to one another; like, they argue, is incapable of
being affected by like; but food is changed in the process of
digestion, and change is always to what is opposite or to what is
intermediate. Further, food is acted upon by what is nourished by
it, not the other way round, as timber is worked by a carpenter and
not conversely; there is a change in the carpenter but it is merely
a change from not-working to working. In answering this problem it
makes all the difference whether we mean by 'the food' the
'finished' or the 'raw' product. If we use the word food of both, viz.
of the completely undigested and the completely digested matter, we
can justify both the rival accounts of it; taking food in the sense of
undigested matter, it is the contrary of what is fed by it, taking
it as digested it is like what is fed by it.
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