” a thing cannot be and not be at the same time”— St. Thomas (Summa Contra Gentiles, Book 1, chap. 84)

“…reason’s first principle is the principle of contradiction. He who denies this principle affirms a self-destructive sentence.

To deny this principle is to annihilate language, is to destroy all substance, all distinction between things, all truths, thoughts, and even opinions, all desires and acts.

We could no longer distinguish even the degrees of error. We would destroy even the facts of motion and becoming, since there would be no distinction between the point of departure and the point of arrival … without purpose or nature. It would be attraction and repulsion, freezing and melting, both simultaneously” … More

–Reality A Thomistic Synthesis, ch 1, Reginald Garrigou-Lagrange, O. P. 

A thing can only be what it is. It can never be what it is not. Any assertion to the contrary would be an erroneous construction or hallucination.