Monologium, on the Being of God78 Chapter LXXVIII. The supreme Being may in some sort be called Three.

And so it is evidently expedient for every man to believe in a certain ineffable trinal unity, and in one Trinity; one and a unity because of its one essence, but trinal and a trinity because of its three—what? For, although I can speak of a Trinity because of Father and Son and the Spirit of both, who are three; yet I cannot, in one word, show why they are three; as if I should call this Being a Trinity because of its three persons, just as I would call it a unity because of its one substance.

For three persons are not to be supposed, because all persons which are more than one so subsist separately from one another, that there must be as many substances as there are persons, a fact that is recognised in the case of more men than one, when there are as many persons as there are individual substances. Hence, in the supreme Being, just as there are not more substances than one, so there are not more persons than one.

So, if one wishes to express to any why they are three, he will say that they are Father and Son and the Spirit of both, unless perchance, compelled by the lack of a precisely appropriate term, he shall choose some one of those terms which cannot be applied in a plural sense to the supreme Being, in order to indicate what cannot be expressed in any fitting language; as if he should say, for instance, that this wonderful Trinity is one essence or nature, and three persons or substances.

For these two terms are more appropriately chosen to describe plurality in the supreme Being, because the word person is applied only to an individual, rational nature; and the word substance is ordinarily applied to individual beings, which especially subsist in plurality. For individual beings are especially exposed to, that is, are subject to, accidents, and for this reason they more properly receive the name sub-stance. Now, it is already manifest that the supreme Being, which is subject to no accidents, cannot properly be called a substance, except as the word substance is used in the same sense with the word Essence. Hence, on this ground, namely, of necessity, that supreme and one Trinity or trinal unity may justly be called one Essence and three Persons or three Substances.