Note 1: Zabarella is here referring to a section in his own commentary where he discusses the opening lines of De Anima III.5 (430a10ff.) where Aristotle writes,
Ἐπεὶ δ’ [ὥσπερ] ἐν ἁπάσῃ τῇ φύσει ἐστὶ [τι] τὸ μὲν ὕλη (10) ἑκάστῳ γένει (τοῦτο δὲ ὃ πάντα δυνάμει ἐκεῖνα), ἕτερον δὲ τὸ αἴτιον καὶ ποιητικόν, τῷ ποιεῖν πάντα, οἷον ἡ τέχνη πρὸς τὴν ὕλην πέπονθεν, ἀνάγκη καὶ ἐν τῇ ψυχῇ ὑπάρχειν ταύτας τὰς διαφοράς·
Since [just as] in the whole of nature there is [what is] on the one hand the material principle for each kind of thing (since this is what is potentially all of them), and on the other, there is what acts as a cause and is a productive principle whereby it makes all things, just as a craft is related to material undergoing change, so it must also follow that these differences should pertain to the soul.
In Zabarella’s commentary on the passage, he begins the main part of his explication by writing that Aristotle’s intent was to show the need for an agent intellect since the existence of a passive element implies the existence of an agent (see col. 1 p. 58). He structures Aristotle’s text in the form of an argument (syllogismus) as follows:
 Quoniam in omni natura reperitur aliquid, quod est tanquam materia, & patiens, &  aliquid, quod se habet, ut agens respectu illius patientis,  necesse est in anima intellectiva reperti has duas differentias, videlicet intellectum patientem & intellectum agentem. (col. 1, p. 58: the numbers here are mine and correspond to the “major,” and “minor” premises and to the “conclusion” respectively)
Since in the whole of nature there is found  something that is like matter and a passive element and something  that is so disposed that it is like an agent with respect to that patient,  it is necessary that in the these differentiations should be found in the intellective soul, which is to say, the enactive and passive intellects.
He goes on to write that a controversy has emerged concerning the word “nature” (as well as specifically regarding the minor premise ( above) that applies directly to this chapter and finally to the central debate over the nature of the intellect: whether it should be considered as something human or as something separate from the human soul and perhaps even divine. The discussion has relevance to Aristotle’s conception of the relationship between the mind and body, as well as to the immorality of the soul.
According to Aquinas, the use of the term “nature” (species naturae: see quotation below) is applicable to the intellect as a whole and thus provides the evidence for thinking that Aristotle intended the passive intellect to be something that, like all other natural things, requires an agent of some kind in order to fully actualize its potential (again, see the reference to the “minor premise”  above). The term “natural” refers to the natural world, and Aristotle clearly applies the term to the intellect in some way, but Aquinas took Aristotle’s words to apply to an intellect that is part of our soul and is in us just as the “sensitive” and “vegetative” aspects of the soul are a part of the total human person. In fact, it might be noticed that Aquinas makes both aspects of the intellect part part of our mind, though he states that neither operates with a bodily organ and concludes that both intellects together are immortal and eternal when separate from the body (see Aquinas’ Commentary on the De Anima, Lectio X, of book III and especially sections 728, where he comments on the relationship between the intellect and nature and 734 where he discusses separability as it applies to the agent intellect). Here is Zabarella’s summation :
Aliqui nanque ut Sanctus Thomas talem minorem sumit, in qua anima accipiatur, vt species naturae, ut minor sit haec, at anima intellectiva est quaedam natura, ideo Sanctus Thomas per naturam, intelligit omne id, quod est imperfectum, & ad perfectione tendit, & exit de potentia ad actum, nam etiam intellectus
For, in fact, some, such as Saint Thomas, have taken up the aforesaid minor, in which the soul is understood as a natural form, as though the minor read as follows, “but the intellectual soul is a kind of nature….” Thus, Saint Thomas understands by “nature” everything that is imperfect and that tends to perfection, and that proceeds from potency to actuality: for even the intellect is this sort of thing. (Italics placed for emphasis)
Aristotle and Aquinas on the Agent-Patient relationship in the Natural World
The tie to “nature” that Aquinas has in view here can be linked explicitly to Aristotle’s Physics II.1 where he distinguishes two senses of nature, two factors involved in motion and rest one corresponding to a thing’s matter; the other to its form. In the first case matter by itself provides an impetus, as fire tends upward and stones downward. In the case of thing’s “form,” it acts as a “goal” for its natural growth or corresponds to the end point of any natural process that all substances at any stage of coming-to-be have naturally within them, inasmuch as they are “natural” substances.
Finally, these strands may now be tied together. As Physics II.1 says, there are material and formal principles in all natural things. But furthermore, it has been seen that for Aristotle, these material and formal principles relate to the potentiality and actuality of a thing in such a way that the material principle relates to its natural potential, whereas the realization of its form, or its arrival at the end of a process might be said to be the manifestation of that natural potential. In a manner of speaking, one might say that the natural impetus to become an apple tree is in the material nature of an apple seed prior to becoming an apple tree; however, in another way, the flowering apple tree represents a complete notion of what it means to be an apple tree, and perhaps helps us understand the nature of apple trees better than anything else. Each case represents a sense in which the term “nature” might be used.
These considerations relate directly to Aquinas’ argument here: that the agent-patient relationship is one that Aristotle holds to applicable to all natural things. Therefore, it makes sense to think that Aristotle might be taking the passive intellect to be the material element with the potential to understand (perhaps due to its inner impetus to understand, as Aristotle writes in Metaphysics I.1) whose end is to understand particular things in the proximate sense, and even all things in a more ultimate and final sense.
The agent intellect, on the other hand, ought to represent actuality itself and so does not become, being incapable of being acted upon (the sun in Aristotle’s analogy is perfectly apt as a reference point). Being in act, it represents being-at-its-end, being fulfilled and complete with respect to its own potentiality so that the two are united in a state of pure actualization. Yet, unlike the case of a fulfillment of a process, the agent intellect is, in its fulfillment also pure activity–as if its natural potency were to be active–so that being always active, it subsists in state of pure actualized potency. The sense, finally, that the two intellects interact is then applicable to an agent-patient relationship that mirrors the craft-material relationship.
On the other hand, a view opposite to this was put forward by Averroes’, Aquinas’ clear opponent on this subject. According to Zabarella’s account, all the Greek commentators shared his opinion that Aristotle did not intent that the passive intellect should be understood as something properly speaking “natural” (sub natura tanquam speciem)(See the bottom of the column 1 on page 58). However, these interpreters understood Aristotle’s reference to the natural realm to be only an analogical one. A primary text in favor of the Averroist view, and one that Zabarella cites as a passage used by Averroists to support their view (see pdf p. 532, col. 2 of the De Mente Agente, ch. 12) is De Partibus Animalium, I, 1 641a32-b10, where Aristotle appears to say that the natural science should not include the study of the intellect, with the implication that it is not something natural. His argument includes the point that the intellect is not itself the source of motion in animals, and as may be seen in the quotations from Physics II.1, the “natural” pertains properly to what has within itself a source of motion or rest. Such attributes as being a source of growth apply to what might be called the “vegetable” aspect of the soul, while attributes such as sensation or locomotion apply to the “animal” aspects of the soul and these pertain to natural science as subjects of discussion, as can be seen in the content of his zoological treatises-but neither of these apply to the intellect.
[To be continued…]