Teleology and Metaphysics
The first use of teleology—i.e. that potencies are related to acts—is rooted fundamentally in Aquinas’s metaphysics. To reject this teleological structure entails refuting the act/potency dichotomy; this is a metaphysical use, which is an account of what a power or potency does. The power to do A is merely ‘the power to do A. Accordingly, if one is to understand the power, one must understand the act—and also the object of the act. Given Aristotelian metaphysics, what is central to that ontological realist account is the act/potency distinction. In refuting teleology in matters of the philosophy of mind, it is not teleology alone that one rejects but the entire ontological scheme articulated by Aristotle and Aquinas. Put differently, this use of teleology is connected intricately with Aquinas’s metaphysical system. Hence, he explains his teleological account of sense faculties, mental acts, and objects only in the context of his metaphysical scheme as a whole. His claims regarding the ontological structure of the sense faculties, in effect, are consistent with the propositions of Aristotelian ontology. The following passage from Aristotle’s Metaphysics illustrates this point:
It is clear that actuality is prior to potency; and I mean not merely that it is prior to that definite potency that is described as a source or principle of change in something else or in itself qua something else, but that it is prior in general to every principle of movement and rest [ . . . ] That 
it is prior in definition is evident, for it is only because there is a possibility of its being actual that what is potential in the primary sense is potential; for instance, what is capable of building is what can build, what is capable of seeing is what can see, and what is visible is what can be seen. The same argument applies to everything else, so that the definition of the actual must precede that of the potential, and knowledge of it must precede knowledge of the potential. (Metaphysics, bk IX, ch. 1 (1049b12-18))
This passage from Aristotle suggests that ontologically actuality is prior to potency. If the sensation faculties are potencies, that which will render them ‘in act’ is prior to the potency itself. Thus, the objects of sensation—the reds, the C-sharps, and the sweets of the world—must be considered first. It is only because of them that perception potencies are what they are. One cannot consider the faculties of perception unless one knows what is in act, which will in turn actualize these potencies. With this ontological commitment to the priority of actuality to potency, in attempting to explain the possibility of sensation, Aquinas considers it necessary to treat first the sensible object- in-act. This statement, therefore, is an instantiation of his general philosophical maxim that acts are prior to potencies. This analysis is grounded in the account of principle (b) above. Furthermore, this example illustrates again the centrality of the explanatory method in Aquinas’s philosophy of mind.
In discussing the role of object in Aquinas’s philosophy of mind, Pilsner writes that Aquinas provided the Latin term ‘propria objecta’ for Aristotle’s ‘ta idia’. ‘Propria objecta’ appears to have been part of the philosophical coin of the realm in the middle thirteenth century. Pilsner also emphasizes that Aquinas refers to both a formal and a material aspect of a sensible object. This distinction is often overlooked by commentators on Aquinas’s account of sensation. In his Quaestiones Disputatae de Virtutibus, Aquinas renders an account of how this distinction holds for sensation. He distinguishes between the formal aspect of the sensible object and the primary substance— the hoc aliquid—in which the sensible object belongs as an accidental or incidental quality. This important text illustrates the difference between proper sensibles and the individual primary substance.
In the sensible object, there is one thing considered as formal and another considered as material. What is formal in the object is that according to which the object is referred to the sensible power or habit; the material aspect on the other hand, is that in which this formal aspect is founded or grounded; in other words, if we speak of the object of the power of vision, its formal object is colour, because insofar as something is coloured, it is visible. On the other hand, what is material in the object is that body in which the colour is found. From this it is clear that a power or habit is referred to the formal aspect formalis ratio] of the object per se, and to that which is material in the object per accidens. And since what is per accidens does not differentiate something but only what is per se, it follows therefore that the material diversity of an object does not diversify the power or habit; this differentiation, however, is accomplished only by the formal aspect. For the visual power by which we see stones, men, and the heavens is one, because this diversity of objects is material, and not according to the formal aspect [formalis ratio] of the visible. (Quaestiones Disputatae de Virtutibus, q. 2 a. 4)
In this text, Aquinas considers both the sensible quality and the primary substance in which the accidental, sensible quality is found. The sensible power of sight—the power of vision—is determined by the colour existing in the primary substance; the primary substance is only the placeholder, as it were, for the sensible quality. Yet the unanswered question in this text is how the primary substance—the individual hoc aliquid—might be known on the perceptual level. In the passage from the Quaestiones Disputatae de Virtutibus quoted above, the three substances—stones, men, and ‘heavens’—are each known by sight because each has the incidental quality of colour. Nothing is said about how an individual person—Megan or Elin, for example—or a specific stone is perceived as an individual primary substance and not as a ‘cluster’ or ‘heap’ of proper and common sensibles.
-  Roderick Chisholm, The Problem of the Criterion (Milwaukee, Wis.: Marquette University Press,1973), 12-14.
-  Pilsner suggests that Aquinas appropriated this terminology from the translation of William ofMoerbeke, who in 1268 provided Thomas with a translation of Aristotle’s De Anima in what one might call‘transliterated’ Greek. See Joseph Pilsner, The Specification of Human Actions in St Thomas Aquinas (Oxford:Oxford University Press, 2006), 93-4.
-  The last three chapters focus attention on the structured mental act of the vis cogitativa as a means toaccomplish this act of perception of the individual thing.
-  One needs to cut Aquinas a little slack with his example of ‘the heavens’. He probably means the various objects that are observable in the heavens—stars, planets, etc.