Descartes' Cogito.

Descartes' Cogito, often summarized as "I think, therefore I am," is a foundational statement in philosophy. It suggests that the very act of thinking indicates one's existence. In exploring self and self-consciousness, Descartes argues that even in doubt or deception, the thinker must exist to doubt or be deceived. Thus, the essence of self is linked to thought and awareness. This realization forms the basis for understanding consciousness, identity, and the nature of existence, emphasizing that our ability to reflect on our thoughts is key to defining who we are.