Proclus
The last great systematic Neoplatonist. His Elements of Theology argues that every effect stays in its cause, comes out from it, and turns back toward it — three stages, and the turning-back closes the circuit so no fourth stage is needed.
Proclus ran the Platonic Academy in Athens in the fifth century and left the most systematic statement Neoplatonism ever received. The *Elements of Theology* proceeds like a geometry text: propositions, each proved from the ones before.
Proposition 35 is the one that matters here. Every effect remains in its cause, proceeds from it, and reverts upon it — and Proclus does not simply assert the three stages, he eliminates the alternatives one at a time. Remaining without procession leaves no distinction. Proceeding without reversion leaves no connection. Reverting without having proceeded is impossible. A companion proposition supplies the closure: the reversion joins the end back to the beginning, so the structure is a circuit and needs no fourth stage.
It is a real argument and the best formal engine any version of the exactly-three case has. What it establishes is that a *derivation* has three stages.
Where this comes up
Sources
- Proclus, Elements of Theology, propositions 33 and 35