<P> In Descartes, The Project of Pure Enquiry, Bernard Williams provides a history and full evaluation of this issue . Apparently, the first scholar who raised the "I" problem was Pierre Gassendi . He "points out that recognition that one has a set of thoughts does not imply that one is a particular thinker or another . Were we to move from the observation that there is thinking occurring to the attribution of this thinking to a particular agent, we would simply assume what we set out to prove, namely, that there exists a particular person endowed with the capacity for thought". In other words, "the only claim that is indubitable here is the agent - independent claim that there is cognitive activity present". The objection, as presented by Georg Lichtenberg, is that rather than supposing an entity that is thinking, Descartes should have said: "thinking is occurring ." That is, whatever the force of the cogito, Descartes draws too much from it; the existence of a thinking thing, the reference of the "I," is more than the cogito can justify . Friedrich Nietzsche criticized the phrase in that it presupposes that there is an "I", that there is such an activity as "thinking", and that "I" know what "thinking" is . He suggested a more appropriate phrase would be "it thinks" wherein the "it" could be an impersonal subject as in the sentence "It is raining ." </P> <P> The Danish philosopher Søren Kierkegaard calls the phrase a tautology in his Concluding Unscientific Postscript . He argues that the cogito already presupposes the existence of "I", and therefore concluding with existence is logically trivial . Kierkegaard's argument can be made clearer if one extracts the premise "I think" into the premises "' x' thinks" and "I am that' x"', where "x" is used as a placeholder in order to disambiguate the "I" from the thinking thing . </P> <P> Here, the cogito has already assumed the "I" 's existence as that which thinks . For Kierkegaard, Descartes is merely "developing the content of a concept", namely that the "I", which already exists, thinks . As Kierkegaard argues, the proper logical flow of argument is that existence is already assumed or presupposed in order for thinking to occur, not that existence is concluded from that thinking . </P> <P> Bernard Williams claims that what we are dealing with when we talk of thought, or when we say "I am thinking," is something conceivable from a third - person perspective; namely objective "thought - events" in the former case, and an objective thinker in the latter . He argues, first, that it is impossible to make sense of "there is thinking" without relativizing it to something . However, this something cannot be Cartesian egos, because it is impossible to differentiate objectively between things just on the basis of the pure content of consciousness . The obvious problem is that, through introspection, or our experience of consciousness, we have no way of moving to conclude the existence of any third - personal fact, to conceive of which would require something above and beyond just the purely subjective contents of the mind . </P>

What was the only thing descartes was sure about