Physical Science applies to subjects having in themselves the principle of mobility or change, and investigates, principally and for the most part, the Essence or Form thereof; yet not exclusively the Form, for the Form must always be joined with Matter. The subject of Physics includes Matter in its definition, like hollow-nosed, not like hollow (p. 1025, b. 33). All the animal and vegetable world is comprised therein; and even some soul, as far as soul is inseparable from Matter (περὶ ψυχῆς ἐνίας θεωρῆσαι τοῦ φυσικοῦ, ὅση μὴ ἄνευ τῆς ὕλης ἐστίν — p. 1026, a. 5).
Mathematics is another branch of theoretical science; applying to subjects immovable and in part inseparable from Matter; that is, separable from Matter only in logical conception (p. 1026, a. 7-15).
Theology, or First Philosophy, or Ontology, is conversant with subjects self-existent, immovable, and separable from Matter (p. 1026, a. 16).
Now all causes are necessarily eternal; but these more than any other, because they are the causes active among the visible divine bodies; for, clearly, if the Divinity has any place, it must be found among subjects of that nature; and the most venerable science must deal with the most venerable subjects (p. 1026, a. 19). The theoretical sciences are more worthy than the rest (αἱρετώτεραι), and First Philosophy is the most worthy among the theoretical sciences (a. 22). A man may indeed doubt whether First Philosophy is distinguished from the other theoretical sciences by being more universal, and by comprehending them all as branches; or whether it has a separate department of its own, but more venerable than the others; as we see that Mathematics, as a whole, comprehends Geometry and Astronomy (a. 27). If there exist no other distinct Essence beyond the compounds of Nature (παρὰ τὰς φύσει συνεστηκυίας — a. 28), Physics would be the first of all sciences. But if there be a distinct immovable Essence, that is first; accordingly the science which deals with it is first, and, as being first, is for that reason universal (καὶ καθόλου οὕτως ὅτι πρώτη — a. 30). It is the province of this First Philosophy to theorize respecting Ens quâ Ens — what it is and what are its properties quâ Ens (a. 32). (Alexander says the First Philosophy is more universal than the rest, but does not comprehend the rest: πρώτη πάντων καὶ καθόλου ὡς πρὸς τὰς ἄλλας, οὐ περιέχουσα ἐκείνας, ἀλλ’ ὡς πρώτη — Schol. p. 736, a. 27.)
Now Ens has many different meanings:—
1. Ens κατὰ συμβεβηκός.
2. Ens ὡς ἀληθές — Non-Ens ὡς ψεῦδος.
3. Ens κατὰ τὰ σχήματα τῆς κατηγορίας (decuple).
4. Ens δυνάμει καὶ ἐνεργείᾳ.
1. Respecting the first, there can be no philosophical speculation (p. 1026, b. 3). No science, either theoretical, or practical, or constructive, investigates Accidents. He who constructs a house, does not construct all the accidents or concomitants of the house; for these are endless and indeterminate. It may be agreeable to one man, hurtful to a second, profitable to a third, and something different in relation to every different Ens; but the constructive art called house-building is not constructive of any one among these concomitants (b. 7-10). Nor does the geometer investigate the analogous concomitants belonging to his figures; it is no part of his province to determine whether a triangle is different from a triangle having two right angles (b. 12). This is easy to understand: the Concomitant is little more than a name — as it were, a name and nothing beyond (b. 13). Plato came near the truth when he declared that Sophistic was busied about Non-Ens; for the debates of the Sophists turn principally upon Accidents or Concomitants, such as, Whether musical and literary be the same or different? Whether Koriskus or literary Koriskus, be the same or different? Whether everything which now is, but has not always been, has become; as in the case of a man who being musical has become literary or being literary has become musical? and such like debates (see Alexander, Schol. p. 736, b. 40). For the Concomitant or Accident appears something next door to Non-Ens (ἐγγύς τι τοῦ μὴ ὄντος, p. 1026, b. 21), as we may see by these debates. Of other Entia there is generation or destruction, but of Accidents there is none (b. 23).