The Anti-Dialectical Aristotle: To Be Or Not To Be

Now it is also the case that there can be nothing intermediate to an assertion and a denial. We must either assert or deny any single predicate of any single subject. The quickest way to show this is by defining truth and falsity. Well, falsity is the assertion that that which is is not or that that which is not is and truth is the assertion that that which is is and that that which is not is not. Thus anyone who asserts anything to be or not to be is either telling the truth or telling a falsehood. On the other hand, neither that which is is said either not to be or to be nor is that which is not.

And if there were an intermediate of contradictory statements, then it would either be like grey between black and white or like the non-man-non-horse between man and horse.

The Metaphysics Gamma 7 1011b