Relatio rationis
Aquinas's name for the ordering of a thing to itself under two different descriptions — which he classifies as not a real relation at all. It is, thirteen centuries early, exactly the case Peirce's teridentity makes formal.
Latin, “a relation of reason”, as opposed to relatio realis, a real relation.
Aquinas divides relations into the real and the merely conceptual. A relation of reason is one that exists only because we have described a thing in two ways and set the two descriptions side by side — the ordering of a thing to itself, as he puts it, according to two aspects.
He raises against himself the exact objection at issue here: every real relation needs two really distinct extremes. He accepts it. His answer is not that self-relations are fine, but that self-relations are not real relations at all, and that the divine relations are not self-relations.
In modern terms he has named Peirce's case and ruled it out by a substantive claim about which relations are real. The scholastic tradition saw the problem, had vocabulary for it, and never once tried to solve it by counting places.
Where this comes up
Sources
- Thomas Aquinas, Summa Theologiae Ia q.28 a.1 ad 2