Justin E. H. Smith in his own blog:
By the end of the day yesterday, “I think, therefore I am,” alongside “Je suis Charlie,” had become one of the central slogans of the mass demonstrations, in Paris and around France, against the murder of the cartoonists at Charlie Hebdo. For me this phrase has long been banalized by misappropriation (“I windsurf, therefore I am,” &c.), and routinized by its occurrence in a pedagogical setting: at least once every year for the past fifteen years I have attempted to explain to classes full of undergraduates what it means, and what it does not mean.
Yesterday, however, when I saw the phrase in its original Latin on a placard at the Place de la République, it suddenly came to life for me again. In a flash I was reminded of the full profundity of what René Descartes had meant to say in his 1641 Meditations on First Philosophy. I also understood, as if suddenly, why this slogan had become so politically important in the current moment in France, and how, perhaps, it fails to capture what is truly at stake.
The French philosopher means to establish, in the second of his six Meditations, that thought is an indubitable indicator, indeed the only indubitable indicator, of his own existence as a metaphysical subject. No evil genius, however powerful, could possibly convince you that you exist, when in fact you don't exist, if you are able to think about the question of your own existence. Descartes proceeds to give a short list of various forms of thinking or cogitation (the Latin term is cogitatio): doubting, affirming, willing, denying, and so on. Even if you are simply doubting your own existence, it follows of necessity that you exist, since doubting is a form of thinking.