Well, we do know 1 thing without making at least one leap of faith, courtesy of Descartes:
If nothing existed, there wouldn't be anything to have these thoughts. Therefore, since I'm thinking, there must be something that exists, and at least part of that is me. It might be an algorithm, a boltzman brain, some weird universe of thought, whatever. I might even be this singular thought and what I assume to be my memories and nothing else exists. But I know I exist in some kind of way.
Beyond that, you need to make assumptions, like whether reality is logical, whether your senses and and memories have any relation to reality, and so on and so forth. It makes sense to assume these assumptions are correct, but you can't know or prove they are true without relying on other assumptions that you can't know or prove independently either. Heck, without assuming that reality is logical, the concept of a proof doesn't even exist. You can choose to reject those assumptions, but that's a useless philosophical deadend.