Where does knowledge of what exists come from?

Synthetic a priori 

Knowledge about the way the world is which can be verified without needing sense experience. 

All philosophers agree that analytic statements can be justified a priori because you don't need experience for them. However, when it comes to synthetic statements, there are disagreements. 

Rationalists claim that some synthetic statements can be verified a priori. Empiricists claim there is no such thing and they synthetic statements can only be verified a posteriori (with experience).