This is a seeming consequence of Descartes' statement, but I think it's not his conclusion. It is not the case that we cease to exist if we are not thinking, but that because we think we know that what we are is, in essence, I thinking thing, or what we call today a conscious being.

Yes, thinking and knowing are circular, but that's not fallacious reasoning because thinking and knowing are mutually dependent.

Very nice article.