Now the compound or composite may be supposed to be naturally capable of being dissolved in like manner as being compounded; but that which is uncompounded, and that only, must be, if anything is, indissoluble.... And the uncompounded may be assumed to be the same and unchanging, where the compound is always changing and never the same?... Is that idea or essence, which in the dialectical process we define as essence of true existence—whether essence of equality, beauty, or anything else: are these essences, I say, liable at times to some degree of change? or are they each of them always what they are, having the same simple, self-existent and unchanging forms, and not admitting of variation at all, or in any way, or at any time?

Plato - Phaedo