Descartes was the father of "foundational epistemology" . This is a theory of knowledge that tries to find a basis for saying one knows by starting from a few "unassailable" beliefs. So, he used a method of radical doubt, looking closely at his beliefs and finding out which ones could be doubted without contradiction, and found that most could. He stumbled upon the fact that even to doubt one exists requires that one exists to have the doubt. Thus, his words are best translated "I am conscious, therefore I exist".
