Descartes' statement "I think therefore I exist" raises questions about the meaning of thought, the meaning of existence but most fundamentally, in what sense he can be certain. The difficulty in establishing the certainty of "I think" and "I exist" is that the two concepts are interrelated. Thus, for example, differing interpretations of what it is to think will have a profound impact on the question of whether Descartes can achieve the certainty of his existence. The success of his attempt to achieve certainty can be analysed in relation to how far he overcomes the universal doubt he has introduced in the first meditation. It is in this sphere of meaning that "I" becomes very relevant as it is possible to argue that his use of "I" demonstrates that his own interpretation of what is certain is partially based on intrinsically held beliefs.
Williams describes the statement "I think" as indubitable.
This is because "if anyone believes he is thinking or that he exists then necessarily he has a true belief." Using the Cartesian method, the statement that "I think" is unique because it is the only premise that has the property of having its truth confirmed by the act of doubting it. Descartes demonstrates that he has found a statement that is not defeated by the possibility of a 'malicious demon' when he says "If I am in doubt, if I am entertaining the possibility that I might be deceived, then the very fact that I am around to entertain that doubt shows that I must exist". Thus he is already assuming that his ability to entertain doubt, or "think" is certain, which based on the premise above, is a not unreasonable conclusion.
Before further examining his assertion that he is thinking, the question of what Descartes means...