The cardinal assumption of Plato's metaphysic is, that the real is definitely thinkable and knowable in proportion as it is real; so that the further the mind advances in abstraction from sensible particulars and apprehension of real being, the more definite and clear its thought becomes.
Locke next inquired to what extent knowledge - in the way either of intuitive certainty, demonstrative certainty, or sense perception - is possible, in regard to each of the four (already mentioned) sorts of knowable relation.
The second sort of knowable relation is sometimes intuitively and sometimes demonstrably discernible.
Turning from abstract mathematical and moral relations to concrete relations of coexistence and succession among phenomena - the third sort of knowable relation - Locke finds the light of pure reason disappear; although these relations form " the greatest and most important part of what we desire to know."
The metaphysic is the epistemology from another point of view - regarded as completing itself, and explaining in the course of its exposition that relative or practical separation of the individual knower from the knowable world, which it is a sheer assumption to take as absolute.