the.com/proof by contradiction

means A method of proving a statement true by assuming it's false and showing that assumption leads to a logical absurdity, which forces the original statement to be true after all.

from From logic and mathematics, where it's known by its Latin name reductio ad absurdum, 'reduction to absurdity.' The technique traces back to ancient Greek mathematics and philosophy — Aristotle described the pattern, and Euclid used it to prove, among other things, that there's no largest prime number. The word 'contradiction' itself comes from Latin contradicere, 'to speak against,' from contra ('against') plus dicere ('to speak') — fittingly, the whole method works by making an assumption speak against itself until it collapses.