It is the rock on which all thought rests, and most people have never heard its name: the principle of non-contradiction.
The statement
Formulated by Aristotle in the Metaphysics, the principle says: one and the same thing cannot be and not be at the same time and under the same aspect. A door cannot be open and closed at the same time, in the same sense. It seems obvious, and it is: precisely because it is the most evident of all, it grounds all the others.
Why no one escapes
No one can actually deny it. Even the most convinced relativist walks through the door, not the wall: he acts as one who knows that being-a-wall and not-being-a-wall are not the same. And whoever tries to deny the principle already uses it in the very denial, because he wants "it is false" and "it is true" not to mean the same thing.
The principle holds in ethics too: good is good and evil is evil, and calling them the same thing is a contradiction. That is why it is the silent guardian of truth: without it, no statement would have content, because it could be and not be what it says at the same time.
In-depth study
Church History Course, with Prof. Dr. Rodrigo Bitencourt
In-depth reading of the work of Daniel Rops, where Thomas Aquinas and scholasticism, which ground truth in God, appear in full.
Discover the courseFrequently asked questions
What is the principle of non-contradiction?
The principle, formulated by Aristotle, that one and the same thing cannot be and not be at the same time and under the same aspect.
Can the principle be proved?
Not by deduction, because every proof already presupposes it. But Aristotle shows that whoever denies it already uses it: to say it is false, 'false' and 'true' must not be the same thing.
Where does it apply?
Everywhere: in logic, science and ethics. It is the basis for distinguishing the true from the false and good from evil.
Continue: What is truth · What is relativism · Truth, goodness and beauty
Source class (NousCast Community): The Idea of Truth (F&T 1.7)