Principle which expresses the motivation underlying Plato's theory of forms and similar doctrines.
Where there are a number of objects of the same kind, or sharing a single property, it seems that there must be a single something which is this kind or property, and which therefore gets treated as an abstract non-material substance.
Strictly, the principle could be seen simply as giving a motive for postulating universals (see Platonism), whatever the status those universals then have; but the term is mainly used in a Platonic context.
In the work of Plato (c.427-c.347 BC) himself, the principle is hardly explicit, the nearest reference to it being in his Republic (§596). It appears more explicitly in the lost but reconstructed work De Ideis (On the Forms) by Aristotle (384-322 BC).