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Works by Aristotle
Pages of Metaphysics

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And if the things of this earth continuously flow and move in
respect of quantity-if one were to suppose this, although it is not
true-why should they not endure in respect of quality? For the
assertion of contradictory statements about the same thing seems to
have arisen largely from the belief that the quantity of bodies does
not endure, which, our opponents hold, justifies them in saying that
the same thing both is and is not four cubits long. But essence
depends on quality, and this is of determinate nature, though quantity
is of indeterminate.
Further, when the doctor orders people to take some particular
food, why do they take it? In what respect is 'this is bread' truer
than 'this is not bread'? And so it would make no difference whether
one ate or not. But as a matter of fact they take the food which is
ordered, assuming that they know the truth about it and that it is
bread. Yet they should not, if there were no fixed constant nature
in sensible things, but all natures moved and flowed for ever.
Again, if we are always changing and never remain the same, what
wonder is it if to us, as to the sick, things never appear the same?
(For to them also, because they are not in the same condition as
when they were well, sensible qualities do not appear alike; yet,
for all that, the sensible things themselves need not share in any
change, though they produce different, and not identical, sensations
in the sick. And the same must surely happen to the healthy if the
afore-said change takes place.) But if we do not change but remain the
same, there will be something that endures.
As for those to whom the difficulties mentioned are suggested by
reasoning, it is not easy to solve the difficulties to their
satisfaction, unless they will posit something and no longer demand
a reason for it; for it is only thus that all reasoning and all
proof is accomplished; if they posit nothing, they destroy
discussion and all reasoning. Therefore with such men there is no
reasoning. But as for those who are perplexed by the traditional
difficulties, it is easy to meet them and to dissipate the causes of
their perplexity. This is evident from what has been said.
It is manifest, therefore, from these arguments that contradictory
statements cannot be truly made about the same subject at one time,
nor can contrary statements, because every contrariety depends on
privation. This is evident if we reduce the definitions of
contraries to their principle.
Similarly, no intermediate between contraries can be predicated of
one and the same subject, of which one of the contraries is
predicated. If the subject is white we shall be wrong in saying it
is neither black nor white, for then it follows that it is and is
not white; for the second of the two terms we have put together is
true of it, and this is the contradictory of white.
We could not be right, then, in accepting the views either of
Heraclitus or of Anaxagoras. If we were, it would follow that
contraries would be predicated of the same subject; for when
Anaxagoras says that in everything there is a part of everything, he
says nothing is sweet any more than it is bitter, and so with any
other pair of contraries, since in everything everything is present
not potentially only, but actually and separately. And similarly all
statements cannot be false nor all true, both because of many other
difficulties which might be adduced as arising from this position, and
because if all are false it will not be true to say even this, and
if all are true it will not be false to say all are false.

Every science seeks certain principles and causes for each of
its objects-e.g. medicine and gymnastics and each of the other
sciences, whether productive or mathematical. For each of these
marks off a certain class of things for itself and busies itself about
this as about something existing and real,-not however qua real; the
science that does this is another distinct from these. Of the sciences

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