Known and unknown are relative. I know my first name. I don't know yours. Your theory suggests that the two names will certainly be different. Is that always the case?

The mechanisms of human brain is indeed unknown to us. So how can we say with confidence that its underlying workings are this way or that way with certainty? We can only observe the inputs and outputs and do our best to compare the two. Which is the whole point of AI - to simulate the human brain (or what we can observe about it) to the best of our ability.