"I will go forward until I know something certain -- or, if nothing else, until I at least know for certain that nothing is certain." (P 147b)

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.