Full Idea
The Platonic Parmenides is more exact [than Parmenides himself]; the distinction is made between the Primal One, a strictly pure Unity, and a secondary One which is a One-Many, and a third which is a One-and-Many.
Gist of Idea
Plato's Parmenides has a three-part theory, of Primal One, a One-Many, and a One-and-Many
Source
report of Plato (Parmenides [c.366 BCE]) by Plotinus - The Enneads 5.1.08
Book Reference
Plotinus: 'The Enneads', ed/tr. Mackenna,Stephen [Penguin 1991], p.357
A Reaction
Plotinus approves of this three-part theory. Parmenides has the problem that the highest Being contains no movement. By placing the One outside Being you can give it powers which an existent thing cannot have. Cf the concept of God.