For there is this great difference between things temporal and things eternal, that a temporal object is valued more before we possess it, and begins to prove worthless the moment we attain it, because it does not satisfy the soul, which has its only true and sure resting-place in eternity: an eternal object, on the other hand, is loved with greater ardor when it is in possession than while it is still an object of desire, for no one in his longing for it can set a higher value on it than really belongs to it, so as to think it comparatively worthless when he finds it of less value than he thought; on the contrary, however high the value any man may set upon it when he is on his way to possess it, he will find it, when it comes into his possession, of higher value still.
-Augustine, On Christian Doctrine 1:38
Moreover, if those who are called philosophers, and especially the Platonists, have said aught that is true and in harmony with our faith, we are not only not to shrink from it, but to claim it for our own use from those who have unlawful possession of it.
-Augustine, On Christian Doctrine 2:40
Platonism is an epistemology. It denies there can be real knowledge of mutable objects. The objects of knowledge are therefore immutable. Platonic epistemology differs from the Aristotelian in that it affirms immediate knowledge of these objects, which the latter denies to human knowledge. Nominalism either radically denies that there are permanent objects of knowledge at all or more moderately denies that human kind has real access to them.
The Platonic epistemology has this which commends it to us: it opens up a metaphysics of wonder, as can be seen in the first quotation. That, we can say, is what Platonism essentially is. It begins as an epistemology, one that argues the direct apprehension of the immutable objects of knowledge. This in turn leads to a metaphysics of wonder, which implies an ontology of diminishing and increasing participation in the Good.