Aquinas Day By Day


Aquinas’s topic:  genus and species

Scripture:  “O God, hear my prayer; listen to the words of my mouth.” Psalm 54: 5

Aquinas’s text:  Summa theologiae 1-2.18.7

Here Br. Thomas approaches universals by comparing the wider universal called genus with its subdivision called species.  He compares the wrong way and the right way to divide a genus into its species.

As the Philosopher says in Metaphysics 7, it is necessary that the differences that divide some genus and constitute the species of that genus divide that genus essentially.  If they divided it accidentally, the division would be incorrect.  For example, if one were to say: “of animals, some are rational and some are irrational; and of irrational animals some have wings and some do not have wings.”  For “having wings” and “not having wings” are not essential determinations of what is irrational. But it is necessary to divide them like this:  “Of animals, some have feet and some do not have feet; and of those that have feet, some have two feet, others four, and still others many feet.”  For these determine the prior difference essentially.

Oportet autem, ut philosophus dicit in VII Metaphys., quod differentiae dividentes aliquod genus, et constituentes speciem illius generis, per se dividant illud. Si autem per accidens, non recte procedit divisio, puta si quis dicat, animalium aliud rationale, aliud irrationale; et animalium irrationalium aliud alatum, aliud non alatum, alatum enim et non alatum non sunt per se determinativa eius quod est irrationale. Oportet autem sic dividere, animalium aliud habens pedes, aliud non habens pedes; et habentium pedes, aliud habet duos, aliud quatuor, aliud multos, haec enim per se determinant priorem differentiam.

[Introductions and translations © R.E. Houser]