Trying to explore topics in arbitrary depth made me realize that I actually do not know anything.
I know that I know nothing. - Socrates
In an absolute sense, one can be argued not to "know" anything, except for the existence of one's own thoughts, as 17th-century philosopher John Locke pointed out. Even earlier, Descartes addressed when saying cogito, ergo sum (I think, therefore I am). While Descartes was attempting to "prove" logically that the world exists, his legacy in doing so is to have shown that one cannot have such proof, because all of one's perceptions could be false (such as under the evil demon or simulated reality hypotheses). But one at least has proof of one's own thoughts existing, and strong evidence that the world exists, enough to be considered "proof" by practical standards, though always indirect and impossible to objectively confirm.