Of God and His Creatures

Ratione unum. Ratio here is not opposed to res: it means first the definition of a thing, and then that which is specially denoted by definition, the essence of nature. This meaning of ratio is not uncommon in the Contra Gentiles. The word may often be rendered 'essential notion,' meaning the object of such notion. See note 2.48i.

Of God and His Creatures: 2.56