"I will go forward until I know something certain -- or, if nothing else, until I at least know for certain that nothing is certain." ("I shall ever follow in this road until I have met with something which is certain, or at least, if I can do nothing else, until I have learned for certain that there is nothing in the world that is certain." (IP3 45b)

But this can't be serious! (Although there is no evidence that Descartes caught the joke.)

If I knew for certain that "nothing is certain," then something would be certain and what I would know would be false, so I wouldn't know it. It is a contradiction for me (or anyone) to know for certain that nothing is certain.

I might go forward until I know something for certain, but what I come to know for certain surely won't be that nothing is certain.