Plato
(c. 428-c. 348 BC):
The reality described by a true definition of justice is not one located in
any particular place or time, it does not change, and it is in every
respect (is fully) just. In the absence of any adequate description of
this reality Plato refers to it as "the just itself" and writes of similar realities
as Forms...
The insistence that the objects of knowledge must be Forms is the result of an exaggeration of the requirement that what is known must be true, helped along by a Greek idiom that suggests that characteristics are substances. If what is known must be true without qualification (the exaggeration) then it must be possible to say it is without qualification, but one can only say this of Forms. No object of sensory experience can be described as just, beautiful, large, or heavy without qualification. Such things will always be unjust, ugly, small, or light in comparison with something else. Hence there can be no knowledge of the justice, beauty, size or weight of sensory objects - at best only true belief.
A Dictionary Of Philosophy. Second edition. (Pan Books 1984).