He who doubts cannot, while doubting, doubt that he doubts. Doubt, even when generalized, is not the abolition of my thought, it is merely a pseudo-nothingness, for I cannot extricate myself from being; my act of doubting itself creates the possibility of certainty and is there for me, it occupies me, I am committed to it, and I cannot pretend to be nothing at the time I execute it. Reflection, which moves all things away to a distance, discovers itself as at least given to itself in the sense that it cannot think of itself as eliminated, or stand apart from itself. But this does not mean that reflection and thought are elementary facts there to be observed as such. As Montaigne clearly saw, one can call into question thought which is loaded with a sediment of history and weighed down with its own being, one can entertain doubts about doubt itself, considered as a definite modality of thought and as consciousness of a doubtful object, but the formula of radical reflection is not: ‘I know nothing’ – a formula which it is all too easy to catch in flat contradiction with itself – but: ‘What do I know?’ Descartes was not unmindful of this. He has frequently been credited with having done beyond skeptical doubt, which is a mere state, and with making doubt into a method, an act, and with having thus provided consciousness with a fixed point and reinstated certainty. But, in fact, Descartes did not suspend doubt in the face of the certainty of doubt itself, as if the act of doubting were sufficient to sweep doubt away by entailing a certainty. He took it further. he does not say: ‘I doubt, therefore I am’, but ‘I think, therefore I am’, which means that doubt itself is certain, not as actual doubt, but as pure thought about doubting and, since the same might be said in turn about his thought, the only proposition which is absolutely certain and which halts doubt in its tracks because it is implied by that doubt, is: ‘I think,’ or again, ‘something appears to me.’ (464-5)
Merleau-Ponty, Maurice. Phenomenology of Perception. Trans. Colin Smith. Routledge 2005.