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it is impossible for any one to believe the same thing to be and not
to be, as some think Heraclitus says. For what a man says, he does not
necessarily believe; and if it is impossible that contrary
attributes should belong at the same time to the same subject (the
usual qualifications must be presupposed in this premiss too), and
if an opinion which contradicts another is contrary to it, obviously
it is impossible for the same man at the same time to believe the same
thing to be and not to be; for if a man were mistaken on this point he
would have contrary opinions at the same time. It is for this reason
that all who are carrying out a demonstration reduce it to this as
an ultimate belief; for this is naturally the starting-point even
for all the other axioms.

There are some who, as we said, both themselves assert that it
is possible for the same thing to be and not to be, and say that
people can judge this to be the case. And among others many writers
about nature use this language. But we have now posited that it is
impossible for anything at the same time to be and not to be, and by
this means have shown that this is the most indisputable of all
principles.-Some indeed demand that even this shall be demonstrated,
but this they do through want of education, for not to know of what
things one should demand demonstration, and of what one should not,
argues want of education. For it is impossible that there should be
demonstration of absolutely everything (there would be an infinite
regress, so that there would still be no demonstration); but if
there are things of which one should not demand demonstration, these
persons could not say what principle they maintain to be more
self-evident than the present one.
We can, however, demonstrate negatively even that this view is
impossible, if our opponent will only say something; and if he says
nothing, it is absurd to seek to give an account of our views to one
who cannot give an account of anything, in so far as he cannot do
so. For such a man, as such, is from the start no better than a
vegetable. Now negative demonstration I distinguish from demonstration
proper, because in a demonstration one might be thought to be
begging the question, but if another person is responsible for the
assumption we shall have negative proof, not demonstration. The
starting-point for all such arguments is not the demand that our
opponent shall say that something either is or is not (for this one
might perhaps take to be a begging of the question), but that he shall
say something which is significant both for himself and for another;
for this is necessary, if he really is to say anything. For, if he
means nothing, such a man will not be capable of reasoning, either
with himself or with another. But if any one grants this,
demonstration will be possible; for we shall already have something
definite. The person responsible for the proof, however, is not he who
demonstrates but he who listens; for while disowning reason he listens
to reason. And again he who admits this has admitted that something is
true apart from demonstration (so that not everything will be 'so
and not so').
First then this at least is obviously true, that the word 'be'
or 'not be' has a definite meaning, so that not everything will be 'so
and not so'. Again, if 'man' has one meaning, let this be
'two-footed animal'; by having one meaning I understand this:-if 'man'
means 'X', then if A is a man 'X' will be what 'being a man' means for
him. (It makes no difference even if one were to say a word has
several meanings, if only they are limited in number; for to each
definition there might be assigned a different word. For instance,
we might say that 'man' has not one meaning but several, one of
which would have one definition, viz. 'two-footed animal', while there
might be also several other definitions if only they were limited in
number; for a peculiar name might be assigned to each of the
definitions. If, however, they were not limited but one were to say

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