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cannot, then, be a definition of anything; or in a sense there can be,
and in a sense there cannot. And what we are saying will be plainer
from what follows.

It is clear also from these very facts what consequence
confronts those who say the Ideas are substances capable of separate
existence, and at the same time make the Form consist of the genus and
the differentiae. For if the Forms exist and 'animal' is present in
'man' and 'horse', it is either one and the same in number, or
different. (In formula it is clearly one; for he who states the
formula will go through the formula in either case.) If then there
is a 'man-in-himself' who is a 'this' and exists apart, the parts also
of which he consists, e.g. 'animal' and 'two-footed', must indicate
'thises', and be capable of separate existence, and substances;
therefore 'animal', as well as 'man', must be of this sort.
Now (1) if the 'animal' in 'the horse' and in 'man' is one and the
same, as you are with yourself, (a) how will the one in things that
exist apart be one, and how will this 'animal' escape being divided
even from itself?
Further, (b) if it is to share in 'two-footed' and
'many-footed', an impossible conclusion follows; for contrary
attributes will belong at the same time to it although it is one and a
'this'. If it is not to share in them, what is the relation implied
when one says the animal is two-footed or possessed of feet? But
perhaps the two things are 'put together' and are 'in contact', or are
'mixed'. Yet all these expressions are absurd.
But (2) suppose the Form to be different in each species. Then
there will be practically an infinite number of things whose substance
is animal'; for it is not by accident that 'man' has 'animal' for
one of its elements. Further, many things will be 'animal-itself'. For
(i) the 'animal' in each species will be the substance of the species;
for it is after nothing else that the species is called; if it were,
that other would be an element in 'man', i.e. would be the genus of
man. And further, (ii) all the elements of which 'man' is composed
will be Ideas. None of them, then, will be the Idea of one thing and
the substance of another; this is impossible. The 'animal', then,
present in each species of animals will be animal-itself. Further,
from what is this 'animal' in each species derived, and how will it be
derived from animal-itself? Or how can this 'animal', whose essence is
simply animality, exist apart from animal-itself?
Further, (3)in the case of sensible things both these
consequences and others still more absurd follow. If, then, these
consequences are impossible, clearly there are not Forms of sensible
things in the sense in which some maintain their existence.

Since substance is of two kinds, the concrete thing and the
formula (I mean that one kind of substance is the formula taken with
the matter, while another kind is the formula in its generality),
substances in the former sense are capable of destruction (for they
are capable also of generation), but there is no destruction of the
formula in the sense that it is ever in course of being destroyed (for
there is no generation of it either; the being of house is not
generated, but only the being of this house), but without generation
and destruction formulae are and are not; for it has been shown that
no one begets nor makes these. For this reason, also, there is neither
definition of nor demonstration about sensible individual
substances, because they have matter whose nature is such that they
are capable both of being and of not being; for which reason all the
individual instances of them are destructible. If then demonstration
is of necessary truths and definition is a scientific process, and if,
just as knowledge cannot be sometimes knowledge and sometimes
ignorance, but the state which varies thus is opinion, so too

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