Theologicae regulae

Monad is that by which any thing is one.

God is said to be not only one, but indeed a monad, that is, unity: and this for many reasons. For he is called one, or unity, by reason of simplicity, or singularity. For from God all plurality is banished. Now all plurality is threefold: plurality of parts, which is found in corporeal things, which are made up of parts; plurality of properties, which is found at least in corporeal things: for in them many properties are combined by concretion, as in spirits; there is a plurality of effects, which is found in properties: for any property, though it may not have a joining together of parts, or a concretion of natures, nevertheless has a plurality of effects. Whence whiteness makes a thing white, makes it colored, makes it qualified: and every such diversity is banished from the celestial monad. For God is not diverse by reason of parts, because he is incorporeal; he is not diverse by reason of properties, as Augustine says; because in him there is no number, no plurality: he is not diverse by various effects, because he is not a formal cause. Wherefore he is truly one, indeed unity. Whence Boethius in the book On the Trinity says, speaking of God: "Truly, then, one, in which there is no number, or plurality." He is also called one, or unity, by reason of immutability: for, as is found in the gloss upon the book of Kings, where it is said: "There was a certain man in Ramathaim-zophim," and so on, by this term one is predicated of God immutability. He is also called one by reason of exclusion, so that the sense is: God is one, that is, not many. He is also called one by reason of likeness, because he has a manifold likeness with unity. Unity descends from no one: all plurality flows from unity; unity of itself begets unity: of itself it brings forth equality. So God is from no one, and whatever is from himself; so of himself he begets another self, that is, the Son; of himself he brings forth one equal to himself, that is, the Holy Spirit. Whence you can draw another rule from what has been said.