The catuṣkoṭi, or tetralemma, lists four possible answers to a question. The Buddha set aside certain questions in all four forms, such as whether the Tathāgata exists after death. Nāgārjuna used the four corners to show that none of them applies to the ultimate nature of things.
Tetralemma
The four-cornered logic: a thing is, is not, both is and is not, or neither.
Also: catuskoti