Divine simplicity
The doctrine that God is not made of parts of any kind — not physical parts, and not a bundle of separable attributes either. It does real work in arguments about the Trinity: if there is no substrate for a mere aspect to sit in, then any real distinction in God has to be someone rather than something.
If God had parts, the parts would be prior to God, and something prior to God would be more ultimate than God. So classical theology holds that God is simple: no physical parts, no composition of matter and form, and no real distinction between God and God's attributes.
The doctrine bites on this question at one specific point. In an ordinary thing you can have a real internal distinction without two individuals, because there is a substrate for aspects to inhere in — a statue's shape is not its bronze, and neither is a separate thing. Simplicity removes that option in God. So if there really are three distinct somethings in God, they cannot be mere aspects; they have to be someones. That is the strongest available defence of the step from "three roles" to "three persons", and it is the step most versions of the argument skip.
Where this comes up
Sources
- Debby research packet OTC-002, Argument E step 7