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In attempting to prove that the soul has three separate

faculties, Plato takes occasion to discuss what makes difference

of faculties. And the criterion which he proposes is difference

in the working of the faculties. The same faculty cannot produce

contradictory effects. But the path of early reasoners is beset

by thorny entanglements, and he will not proceed a step without

first clearing the ground. This leads him into a tiresome

digression, which is intended to explain the nature of

contradiction. First, the contradiction must be at the same time

and in the same relation. Secondly, no extraneous word must be

introduced into either of the terms in which the contradictory

proposition is expressed: for example, thirst is of drink, not of

warm drink. He implies, what he does not say, that if, by the

advice of reason, or by the impulse of anger, a man is restrained

from drinking, this proves that thirst, or desire under which

thirst is included, is distinct from anger and reason.