swer Yes and No cannot be given to one and the same question
understood in the same sense.

But why did Aristotle consider it necessary to lay down a principle
so obvious? Simply because among the subtle dialecticians who
preceded him the principle had been challenged. The Platonic dialogue
Euthydemus shows the farcical lengths to which such quibbling was
carried. The two brothers vanquish all opponents, but it is by
claiming that the answer No does not preclude the answer Yes. "Is not
the honourable honourable, and the base base?" asks Socrates. "That is
as I please," replies Dionysodorus. Socrates concludes that there
is no arguing with such men: they repudiate the first principles of
dialectic.

There were, however, more respectable practitioners who canvassed on
more plausible grounds any form into which ultimate doctrines about
contraries and contradictions, truth and falsehood, could be put, and
therefore Aristotle considered it necessary to put forth and defend
at elaborate length a statement of a first principle of demonstration.
"Contradictions cannot both be true of the same subject at the same
time and in the same sense." This is the original form of the Law of
Contradiction.

The words "of the same subject," "at the same time," and "in the
same sense," are carefully chosen to guard against possible quibbles.
"_Socrates knows grammar._" By Socrates we must mean the same
individual man. And even of the same man the assertion may be true at
one time and not at another. There was a time when Socrates did not
know grammar, though he knows it now. And the assertion may be true
in one sense and not in another. It may be true that Socrates knows
grammar, yet not that he knows everything that is to be known about
grammar, or that he knows as much as Aristarchus.

Aristotle acknowledges that this first principle cannot itself be
demonstrated, that is, deduced from any other. If it is denied, you
can only reduce the denier to an absurdity. And in showing how to
proceed in so doing, he says you must begin by coming to an agreement
about the words used, that they signify the same for one and the other
disputant.[3] No dialectic is possible without this understanding.
This first principle of Dialectic is the original of the Law of
Identity. While any question as to the truth or falsehood of a
question is pending, from the beginning to the end of any logical
process, the words must continue to be accepted in the same