Ch. 8 De Interpretatione - Consistent Use of Terms is Required for Opposition - by Aristotle

Use Words Homonymously

A statement is single or one, when it either affirms or denies some one thing and no more of another, be the subject universal or not and the statement universal or not. We may take for examples the following, provided that 'white' has one meaning : 

Every man is white. Not every man is white,

Man is white. Man is not white. 

No man is white. Some men are white.

If, however, one word has two meanings, which do not combine to make one, the affirmation itself is not one. If, for instance, you gave the name 'garment' alike to a horse and a man, then it follows that 'garment is white' would be not one but two affirmations, as also would 'garment is not white' be not one denial but two. For the statement that 'garment is white' really means 'horse and man both are white.' And this statement, in turn, is the same as to say 'horse is white,' 'man is white.' And if these have more meanings than one and do not, in effect, make one statement, it follows that 'garment is white' must itself have more meanings than one or, if not, it means nothing at all. For no particular man is a horse. And accordingly not even here is one necessarily true and one false of two statements opposed contradictorily.

Aristotle is reaffirming something he taught in the very first chapter of The Categories, namely, that if we do not define our terms so that we are using them homonymously (same symbol, same definition), then we will fall into error.