This leads to a key principle in classical metaphysics: being and unity are convertible. This means they have the same extension – wherever there is being, there is unity, and wherever there is unity, there is being. They differ only conceptually, not in reality. The concept of “being” emphasises the fact that something exists; the concept of “one” emphasises the fact that this existing thing is internally undivided. Both point to the same reality, but from different angles.
This idea of convertibility extends to the other transcendentals as well. Every being, insofar as it is, is also true (intelligible to a mind) and good (desirable by a will). But unity is foundational in a particular way. Without internal coherence, a thing cannot be grasped by the intellect as a definite object nor desired by the will as a specific good. Unity, in a sense, makes the other transcendentals possible.
