Full Idea
The One must be composed of parts, both being a whole and having parts. So on both grounds the One would thus be many and not one. But it must be not many, but one. So if the One will be one, it will neither be a whole, nor have parts.
Gist of Idea
It seems that the One must be composed of parts, which contradicts its being one
Source
Plato (Parmenides [c.366 BCE], 137c09), quoted by Kathrin Koslicki - The Structure of Objects 5.2
Book Reference
Koslicki,Kathrin: 'The Structure of Objects' [OUP 2008], p.95
A Reaction
This is the starting point for Plato's metaphysical discussion of objects. It seems to begin a line of thought which is completed by Aristotle, surmising that only an essential structure can bestow identity on a bunch of parts.