"I think, therefore I am"
René Descartes sought an unshakeable foundation for knowledge. He employed Methodological Doubt, deciding to treat anything that could possibly be doubted as false. He doubted his senses (optical illusions), his waking state (dreams), and even math (an Evil Demon could be deceiving him).
Suppose a powerful, malicious demon is tricking me about everything. Is there anything he cannot deceive me about? Yes. Even if I am being deceived, I must exist to be deceived. Even if I am doubting, I am thinking.
Thus, he arrived at the famous conclusion: "Cogito, ergo sum" (I think, therefore I am). The existence of the thinking self is the one absolute truth that cannot be doubted. This established the subject/ego as the center of modern philosophy.