Prologue

There are many who usurp the dignity of the Christian religion; but that form of faith has supreme authority, and has it exclusively, which, both on account of the universal character of the rules and doctrines affirming its authority, and because the worship in which they are expressed has spread throughout the world, is called catholic or universal.

The belief of this religion concerning the Trinity is as follows: the Father is God, the Son is God, the Holy Spirit is God. Therefore, Father, Son, and Holy Spirit are one God, not three Gods.

The nature of their Unity is such that there is no difference. Difference cannot be avoided by those who add to or take from the Unity, as for instance the Arians, who by graduating the Trinity according to merit, break it up and convert it to Plurality.

For the essence of plurality is otherness; apart from otherness plurality is unintelligible.

In fact, the difference between things is to be found in genus or species or number. In as many ways as things are the same, in the same number of ways they are said to be diverse. Sameness is predicated in three ways: by genus; e.g., a man and a horse, because of their common genus, animal. By species; e.g., Cato and Cicero, because of their common species, man. By number; e.g., Tullius and Cicero, because they are numerically one. Similarly difference is expressed by genus, species, and number. But a variety of accidents brings about numerical difference; three men differ neither by genus nor species, but by their accidents, for if we mentally remove from them all other accidents, still each one occupies a different place which cannot possibly be regarded as the same for each, since two bodies cannot occupy the same place, and place is an accident. Wherefore it is because men are plural by their accidents that they are plural in number.