Full Idea
In addition to an eternal unchanging model and a visible and changing copy of reality, there must be a third part, the receptacle and nurse of all becoming and change.
Gist of Idea
In addition to the underlying unchanging model and a changing copy of it, there must also be a foundation of all change
Source
Plato (Timaeus [c.362 BCE], 49b)
Book Reference
Plato: 'Timaeus and Critias', ed/tr. Lee,Desmond [Penguin 1971], p.67
A Reaction
cf Aristotle's criticism in Metaphysics