This core assumption of yours is false. However to show that to be the case is a deep philosophical discussion that would be entirely off topic in this thread.

Very briefly and borrowing the words of Bruce Charlton who is more articulate on this topic then I am.

"Most mainstream metaphysical assumptions are incoherent wrt. epistemology. As examples:

If natural selection is assumed metaphysically true, as the only and sufficient explanation of Man; then human reason must be a product of natural selection; which means that human reason can never know anything (because natural selection is about differential fitness, not about truth).  

If it is assumed that the universe is assumed to be a combination of randomness and determinism; then we personally could never know this - because we personally would be a combination of randomness and determinism and could never know anything. The universe might actually Be random/ determined - but if so, we personally could never know that."

A more detailed analysis of this point for those interested in this topic can be found here:

Metaphysics comes before Epistemology
http://charltonteaching.blogspot.com/2017/12/metaphysics-comes-before-epistemology.html?m=1