death-of-dreams has somewhat missed the point of
Descartes'
method of doubt. In Descartes' argument, we are asked to consider the possibility that some deceiving
demon is fooling us. It seems as though we have left hands. But (according to the Method) we cannot rule out the possibility that we are simply, for example,
brains in jars hooked up to some equipment that provides us with the seamless illusion that we play tennis, sneeze and so on. Descartes notes that, even in such cases, we still
think we sneeze, play tennis and have left hands. Even if we adopt Descarte's methodology, and doubt the reality of these things, this doubt is itself a thought! He argues, therefore, that the fact of our own thinking cannot be doubted consistently. On this basis, he chooses to define himself as a 'thinking thing', because at least that much is certain.
Jean-Paul Sartre, in his prologue to Being and Nothingness has a little to say about cogito ergo sum, or simply 'the cogito' as it is known in the trade, arguing along roughly the following lines: since the being of the phenomena is distinct from the phenomenon of being (Sartre spends some time establishing this) then the phenomenon of the self is not identical to the self experiencing that phenomenon - in Descartes' terms we could perhaps say: 'but the I that thinks is not the I it thinks it is.'
Sartre calls this experiencing 'I' the "pre-reflexive cogito", and uses it to clarify his notion of 'the pour soi' or 'being for itself.'