5
There is a principle in things, about which we cannot be deceived,
but must always, on the contrary recognize the truth,-viz. that the
same thing cannot at one and the same time be and not be, or admit any
other similar pair of opposites. About such matters there is no
proof in the full sense, though there is proof ad hominem. For it is
not possible to infer this truth itself from a more certain principle,
yet this is necessary if there is to be completed proof of it in the
full sense. But he who wants to prove to the asserter of opposites
that he is wrong must get from him an admission which shall be
identical with the principle that the same thing cannot be and not
be at one and the same time, but shall not seem to be identical; for
thus alone can his thesis be demonstrated to the man who asserts
that opposite statements can be truly made about the same subject.
Those, then, who are to join in argument with one another must to some
extent understand one another; for if this does not happen how are
they to join in argument with one another? Therefore every word must
be intelligible and indicate something, and not many things but only
one; and if it signifies more than one thing, it must be made plain to
which of these the word is being applied. He, then, who says 'this
is and is not' denies what he affirms, so that what the word
signifies, he says it does not signify; and this is impossible.
Therefore if 'this is' signifies something, one cannot truly assert
its contradictory.
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