In what conceiveable (which does not imply logicality) universes would Rationalism not work in the sense of unearthing only some truths, not all truths? Some realms of truth would be hidden to Rationalists? To simplify it, I mean largely the aspect that of empiricism, of tying ideas to observations via prediction. What conceivable universes have non-observational truths, for example, Platonic/Kantian "pure apriori deduction" type of mental-only truths? Imagine for convenience's sake a Matrix type simulated universe, not necessarily a natural one, so it does not really need to be lawful nor unfold from basic laws.

Reason for asking: if you head over to a site like The Orthosphere, they will tell you Rationalism can only find some but not all truths. And one good answer would be: "This could happen in universes of the type X, Y, Z. What are your reasons for thinking ours could be one of them?"