Descartes' axiom. This is a petitio principii. “I think” can only prove this: that “I think.” And he might just as well infer from it the existence of thought as the existence of I. He is asked to prove the latter, and immediately assumes that it exists and does something, and then infers that it exists because it does something. Suppose I were asked to prove the existence of ice, and were to say, ice is cold, therefore there is such a thing as ice. Manifestly I first assume there is such a thing as ice, then ascribe to it an attribute, and then argue back that this attribute is the outcome of ice. This is not proof, but simply arguing in a circle.