Aristotle, Metaphysics

[1062b] [1] (I mean "that the same thing can at one and the same time be and not be") will be true;because just as, when they are separated, the affirmation is no more true than the negation, so in the same way, if the complex statement is taken as a single affirmation, the negation will be just as true as the whole statement regarded as an affirmation.And further, if nothing can be truly affirmed, then this very statement—that there is no such thing as a true affirmation—will be false. But if there is such a thing, the contentions of those who raise objections of this kind and utterly destroy rational discourse may be considered to be refuted.¹

Very similar to the views which we have just mentioned is the dictum of Protagoras²; for he said that man is the measure of all things, by which he meant simply that each individual's impressions are positively true.But if this is so, it follows that the same thing is and is not, and is bad and good, and that all the other implications of opposite statements are true; because often a given thing seems beautiful to one set of people and ugly to another, and that which seems to each individual is the measure. [20] This difficulty will be solved if we consider the origin of the assumption. It seems probable that it arose in some cases from the doctrine of the natural philosophers, and in others from the fact that everyone does not form the same opinion about the same things, but to some a given thing seems sweet and to others the contrary.For that nothing comes from what is not, but everything from what is, is a doctrine common to nearly all natural philosophers.³ Since, then, a thing does not become white which was before completely white and in no respect not-white, that which becomes white must come from what was not-white. Hence according to this theory there would be generation from what is not, unless the same thing were originally white and not-white.However, it is not hard to solve this difficulty. We have explained in the Physics⁴ in what sense things which are generated are generated from what is not, and in what sense from what is.

But to attach equal importance to the opinions and impressions of opposing parties is foolish, because clearly one side or the other must be wrong.⁵ This is evident from what happens in the sphere of sensation;