St. Thomas Aquinas, Commentary on the Metaphysics of Aristotle, tr. John P. Rowan (Chicago: Regnery, 1961), bk. 4.

546. [...] Every science that deals with many things that are referred to one primary thing is properly and principally concerned with that primary thing on which other things depend for their being and from which they derive their name; this is true in every case. But substance is the primary kind of being. Hence the philosopher who considers all beings ought to consider primarily and chiefly the principles and causes of substances. Therefore, his consideration extends primarily and chiefly to substances.

547. [...] It is the business of the first philosopher to consider all substances. There is one sense and one science of all things belonging to one genus. For example, sight is concerned with all colors, and grammar with all words. Therefore, if all beings somehow belong to one genus, all species of being must belong to the consideration of one science which is a general science, and different species of being must belong to the different species of that science.

He says this because it is not necessary for one science to consider all the species of one genus according to the special notes of every single species, but only inasmuch as they agree generically. But according to their proper notions, the different species of one genus belong to the special sciences, as happens in the present case. For inasmuch as all substances are beings or substances, they belong to the consideration of this science, but inasmuch as they are a particular kind of substance, as a lion or an ox, they belong to the special sciences.

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Return to Lesson 28: Could there be a Theory of Everything?