Actually, not much.
How do we know that we are not brains in vats? That we aren't software simulations in an advanced super-computer? That the entire universe of which we think we are a part wasn't created intact (complete with all our memories) last Thursday?
We don't. We can make a basic Cartesian assumption that "thinking" of some sort is going on somewhere, by the fact that we use thinking to make that assumption. But apart from that, we really don't know.
This is a scary thought. (But it is, at least, a thought.)
So where do we go from here? If we can't know anything, what's the basis of doing anything? Why go on, in the face of such uncertainty?
We have senses that seem to show us the world in a generally consistent way, so in the absence of certainty we can proceed in what appears to be a pragmatic manner. In the absence of certainty we can construct a worldview based on probability. We can live a life of induction, but we must first assume that induction works. Our senses may subsequently tell us that such an assumption appears to be consistent with reality as we perceive it. And so we build. Of such pragmatism is civilisation wrought — in sweat, in toil, in reasoned argument, in compassion. It is, we find, worth striving for.
For some people this isn't enough. They're unhappy with such uncertainty, and demand to be told in no uncertain terms, how things really are.
Such people will search for certainty, and believe they find it in religious scripture. They will seize upon the sacred text that purports to reveal in no uncertain terms the secrets of the universe. They will base their lives upon it, proclaiming its inerrancy. It must be inerrant — it's the truth they've been seeking, and if someone is so crass or unwise to question the inerrancy of scripture they'll explain, at tortuous length, why it contains not a single contradiction, and how it grounds their very existence.
But they're wrong. They're no better off than the rest of us — in fact they're worse off, having deluded themselves into thinking they are certain, in a world where certainty is impossible.
Teller on how magicians "alter your perceptions"
17 minutes ago