COGITO, ERGO SUM ( I THINK, THEREFORE I EXIST )

You can challenge the existence of every thing, but your own existence cannot be doubted. Even when you deny your existence, you, the denier, exists. Descartes is said to have brought in this fundamental truth by saying, cogito, ergo sum- I think therefore I exist. But this truth existed even in ancient times. Asti Atma ( Self exists ), nastikya sadhana ( establishing denial ) asambhavat ( being impossible ) is the text of this in Sanskrit.
Adi Sankara finding the Self as the undeniable epistemological reality,made it also the basic ontological reality and claimed that the Individual Self (Atman ) is the same as the Universal Self(Brahman ).This is perhaps the greatest glorification of the human self found in the history of thought.