Further note that number deals with things that differ, and it is quite impossible to enumerate things that differ from one another in no respect: and just so far as they differ are they enumerated: for instance, Peter and Paul are not counted separately in so far as they are one. For since they are one in respect of their essence they cannot be spoken of as two natures, but as they differ in respect of subsistence they are spoken of as two subsistences. So that number deals with differences, and just as the differing objects differ from one another so far they are enumerated.