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.
Boethius, De Trinitate (quoted from here)