Monologion: Chapter 79
BASIC WRITINGS
Chapter 79 What the supreme essence can in a certain sense be said to be three of Behold! Clearly it is beneficial for every human being to believe in an ineffable threefold Unity and unified Trinity. They are indeed one and a Unity in virtue of one essence, but I do not know what it is in virtue of which they are three and a Trinity. For although I can say 'Trinity' because of the Father, the Son, and the Spirit of both, who are three, I cannot express by any one word what it is in virtue of which they are three, as if I were to say [they are a Trinity] in virtue of being three persons in the same way that I would say they are a Unity in virtue of being one substance. For they are not to be thought of as three persons, since whenever there are two or more persons, the persons subsist* separately from each other in such a way that there must be as many substances as there are persons. This is known to be true in the case of two or more human beings, who are as many individual substances as they are persons. Therefore, just as there is not more than one substance in the supreme essence, so also there is not more than one person.76
And so if anyone wishes to talk about this with someone, what will he say the Father, the Son, and the Spirit of both are three of, unless perhaps for lack of a strictly appropriate word he is forced to choose one of those words that cannot be said in the plural of the supreme essence in order to signify that which cannot be said by a suitable word? For example, he might say that this wondrous Trinity is one essence or nature and three persons or substances.77
For these two words are more fittingly chosen to signify the plurality within the supreme essence, since 'person' is said only of an individual rational nature, and 'substance' is principally said of individuals, which especially constitute a plurality. For it is individuals that especially stand under (that is, underlie) accidents,* and therefore they more properly take the name of 'substance'.78
Hence it had already become evident above79 that the supreme essence, which does not underlie any accidents, cannot properly be called a substance unless
'substance' is being used instead of 'essence'.80 Therefore, by this argument of necessity that supreme and unified Trinity or threefold Unity can be irreproachably called one essence and three persons or three substances.
76 The standard definition of 'person' was from Boethius, Contra Eutychem 4.4–5:"an individual substance of a rational nature."Anselm is arguing that in this sense of the word God cannot be three persons, since if he were, he would be three substances and therefore three gods.
77 See the Glossary entry for substance* (sense 3).
78 See the Glossary entry for substance* (sense 2).
79 In Chapter 36.
80 See the Glossary entry for substance* (senses 1 and 2).