Perhaps more than anything else Plato's name is associated with a theory of reality. Platonism is virtually another name for realism; and is generally applied to those thinkers who maintain, with regard to some philosophically disputed class of things, that they really exist. Numbers, values and universals are salient examples. Down the ages there has been great debate about whether there are such things and, if so, how they are connected with the parts of the world with which we are more familiar. Debate of this kind is prominent in the work of Plato. Moreover there is much prima facie evidence to support the claim that Plato was a realist of this kind. This form of realism is oft en contrasted with nominalism, which maintains that even though we have names for such things as universals or values, we form what is in fact nothing more than an illusion that these are proper names of real objects.

“Realism” as a term of philosophical art has acquired a second sense during the past century. According to this it concerns the status of some truth in terms of our ability to know it. Something may be true but still lie beyond our human power to know it; an example might be the answer to a calculation involving very large numbers, or some trivial fact about the distant future or remote past.