Relationis oppositio
The principle that in God everything is one except where relations stand opposed to one another. It is how Aquinas gets three persons out of four real relations, and it is the formula the Council of Florence used to license the filioque.
Latin, “opposition of relation”.
Aquinas counts four real relations in God — paternity, filiation, active spiration, procession — and three persons. The mismatch is not an embarrassment he has to explain away; he explains it directly. Relations distinguish persons only when they stand *opposed* to one another. The Father bears two relations, one to the Son and one to the Spirit, and because those two are not opposed to each other they do not make two Fathers.
This is the most rigorous Trinitarian theology on offer explicitly denying that relation-count equals person-count. It gets three out of four.
It also has costs that should be stated. The formula is the Florentine one, deployed in the West to license the *filioque*, so an Eastern Orthodox reader cannot simply borrow it. And it does not by itself yield a number: getting to exactly three still needs the separate claim that there are exactly two processions.
Where this comes up
Sources
- Thomas Aquinas, Summa Theologiae Ia q.36
- Council of Florence, bull Cantate Domino (1442)