I'd contend that at least some of the fundamental metaphysical truths of the world are knowable.  We are limited in that our senses deal with material reality, so to know unchanging things we have to reason—to extrapolate, to some degree—from what we can perceive.  However, the ancient and medieval philosophical traditions agreed that such rational discovery of truth was possible, albeit limited.

To know unchanging truths more fully, we would have to have those truths revealed to us.

One of the reasons I persist in practicing Christianity is because it supplies that revelatory truth in a way that is consistent with reason.  Christian theology (some parts of it, anyway) feels like an extension of philosophy to revealed truths.  This consistency lends credence to the proposition that there is a God who reveals himself to us in a personal way.