The tradition that medieval thinkers received presented them with three plausible candidates for the subject matter of metaphysics: being as such (which included the study of the first principles and causes of beings), substance, and God. In practice, it seems that the main struggle was between the conception of metaphysics as the study of being qua being, which was associated with Avicenna, and the conception of metaphysics as the study of eternal substances, which was often attributed to Averroes.
Duns Scotus, like many of his contemporaries, sided with Avicenna. Avicenna’s understanding of the subject matter of metaphysics was not without precedent (compare al-FarabI, The Aims of Aristotle’s Metaphysics, in McGinnis and Reisman 2007:78-81, but see Fakhry 1984). But it was Avicenna’s particular formulation of the claim that metaphysics is the study of being inasmuch As it is being which was to be extremely influential in the thirteenth and fourteenth centuries.
Avicenna observed that metaphysics could not be solely about God or about substance. The subject of metaphysics cannot be God, because the subject of any science is something whose existence has been postulated at the outset of the investigation; a science only studies the states and properties of its subject. But the existence of God cannot be postulated at the outset of metaphysical study (for no other science has established His existence); rather, the existence of God is something that is sought in metaphysics (Avicenna, The Metaphysics of al-Shifa’ (The Healing), 1,1). Likewise, the subject of metaphysics cannot be the first principles and causes of substances. They too are the things that the science seeks after, not what it has already postulated. Finally, if metaphysics were solely about substance, then metaphysics would fail to be fully general. It would only study being under one of its aspects, namely, substance, and not being in general. Metaphysics would not be the highest science; it would be a science that is subordinate to some yet unspecified science that studies being in general.
Avicenna concluded that subject of metaphysics is being inasmuch as it is being: some of the science will study the causes of being, some of it will study the accidents, some of it the First Cause, and some of it the starting points of the other sciences, all of which also study being but under one aspect or another (Metaphysics, I, 2).
Avicenna’s conception of metaphysics was very influential, but it did not go unchallenged. Averroes, for example, attacked Avicenna’s reason for rejecting the notion that God is the subject matter of metaphysics (In I Phys., com. 83 [Aristotelis Opera cum Averrois commentariis, IV, 47v]). Avicenna is mistaken, for the demonstration of the existence of the separate substance, which Averroes identified with the first form and the final cause of everything else, has already occurred by the end of the study of physics (cf. Aristotle, Physics VIII). Accordingly, Averroes seems to have endorsed the opposing claim that the proper and primary subject of metaphysics is the separate substance, or God.
Aquinas agreed with Avicenna that the subject matter of metaphysics was being qua being, or ‘‘being in general’’ (ens commune) (In Metaphys., Proem.). But Aquinas refused to admit that God is included in the subject matter of metaphysics. God and angels are examined in metaphysics, but only indirectly as the principles and causes of being qua being (In De trinitate, q. 5, a. 4; cf. Wippel 2000:15-22). Divine things may be studied in and of themselves, and not merely as principles of other things, but this can only happen insofar as their existence has been revealed to humans in a nonphilosophical way. Hence, Aquinas draws a distinction between philosophical theology (metaphysics) and revealed theology (‘‘sacred doctrine”) (In De trinitate q. 5, a. 4; cf. Wippel 2000:17-18). Aquinas thought that philosophical theology was a part of, and indeed, the culmination of philosophy. Revealed theology studied God and divine things as its proper subject, but its basis must be handed down in Holy Scripture. Since sacred doctrine is known by something that is superior to the natural light of reason, it is superior to metaphysics (cf. Summa theologiae I, q., a. 1).
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