The noun antithesis comes from a Greek root meaning "opposition" and "set against." It's often used today when describing two ideas or terms that are placed in strong contrast to each other. We might come across antithesis in school if we learn about the "Hegelian dialectic." There, the thesis, or main idea put forward in an argument, is countered with its opposite idea — the antithesis — and the two are finally reconciled in a third proposition, the synthesis. An antithesis wouldn't exist without a thesis because it works as a comparison.