But immediately upon this I observed that, whilst I thus wished to think that all was false, it was absolutely necessary that I, who thus thought, should be somewhat; and as I observed that this truth, I think, therefore I am (COGITO ERGO SUM), was so certain and of such evidence that no ground of doubt, however extravagant, could be alleged by the sceptics capable of shaking it, I concluded that I might, without scruple, accept it as the first principle of the philosophy of which I was in search.

[Mais aussitôt après je pris garde que, pendant que je voulois ainsi penser que tout étoit faux, il falloit nécessairement que moi qui le pensois fusse quelque chose; et remarquant que cette vérité, je pense, donc je suis, étoit si ferme et si assurée, que toutes les plus extravagantes suppositions des sceptiques n’étoient pas capables de l’ébranler, je jugeai que je pouvois la recevoir sans scrupule pour le premier principe de la philosophie que je cherchois.]

René Descartes (1596-1650) French philosopher, mathematician

Discourse on Method [Discours de la méthode], Part 4 (1637) [tr. Veitch (1850)]
    (Source)

(Source (French)). Alternate translations:

But presently after I observ’d, that whilst I would think that all was false, it must necessarily follow, that I who thought it, must be something. And perceiving that this Truth, I think, therefore, I am, was so firm and certain, that all the most extravagant suppositions of the Scepticks was not able to shake it, I judg’d that I might receive it without scruple for the first principle of the Philosophy I sought.
[Newcombe ed. (1649)]

But immediately afterwards I noticed that whilst I thus wished to think all things false, it was absolutely essential that the "I" who thought this should be somewhat, and remarking that this truth "I think, therefore I am" was so certain and so assured that all the most extravagant suppositions brought forward by the sceptics were incapable of shaking it, I came to the conclusion that could receive it without scruple as the first principle of the Philosophy for which I was seeking.
[tr. Haldane & Ross (1911)]

But immediately upon this I noticed that while I was trying to think everything false, it must needs be that I, who was thinking this, was something. And observing that this truty "I am thinking, therefore I exist" was so solid sceptics could not overthrow it, I judged that I need not scriple to accept it as the first principle of philosophy that I was seeking.
[tr. Ascombe & Geach (1971)]

But immediately I noticed that while I was endeavoring in this way to think that everything was false, it was necessary that I, who was thinking this, was something. And observing that this truth, "I am thinking, therefore I exist" was so firm and sure that all the most extravagant suppositions of the sceptics were incapable of shaking it, I decided that I could accept it without scruple as the first principle of the philosophy I was seeking.
[tr. Cottingham, Stoothoff (1985)]