Theologicae regulae

No partitive name, or pronoun, is fittingly joined to an essential name.

For it is less fitting to say: Some God, or a certain God, or that God, since a partitive expression divides among things named which, in act or by nature, are many. Whence it is fittingly said: Some phoenix; for although there are not several in act, there can nevertheless be several by nature: but neither in act nor by nature can we understand there to be several gods: and although there are several persons of whom this name God is the appellation, nevertheless with regard to this name partitive distinction has no place, because just as it is divided among the things named, so also among the natures of the things named. For it would not truly be said: Peter is a different man from Paul, unless he were a man by a different humanity than Paul. This name, however, "one," although it is sometimes held partitively, yet because it is sometimes held exclusively, it is admitted that it be said, one God, that is, not many. And if anywhere a universal or particular sign is found joined to an essential name, let that universality, or partition, be understood to refer to the plurality of person, not of essence, as when it is said by Boethius in the book On the Trinity: God differs from God in nothing, the sense is: No thing which is God differs, in that it is God, from the thing which is God.