Once finished with his discussion and analysis of the Empedoclean principle, Aquinas next considers the first proposition expressed above. ‘To sense is to be moved or acted upon in some way, for the act of sensation involves a certain alteration of the subject’ (Commentary on the Soul, no. 350). This proposition asserts that sensation is an instance of ‘being moved’ or ‘being acted upon’ in some manner or other. Once again, the analysis of this proposition is in terms of the concepts of potency and act. This is consistent with the Aristotelian definition of motion as found in the Physics. If sensation is motion in some way, then it too will utilize the concepts of act and potency as required by the definition of motion. It is important to notice the ‘qualified’ remark that sensation is an instance of motion. Note that part of the text of the proposition is the following: ‘To sense is to be moved or acted upon in some way . . . The phrase ‘in some way’ is important for Aristotelian philosophy of mind. Both Aristotle and Aquinas claim that knowledge is only analogous to a motion. It follows, therefore, that knowledge is neither identical nor coextensive with a motion or an action. At any length, one must note that knowledge is not reducible to motion. Hence, any reduction of Aquinas’s philosophy of mind to atomism, materialism, physicalism, or any form of a causal theory of perception as espoused by either the early modern philosophers or contemporary physicalists, is a category mistake.
Following Aristotle’s procedure, Aquinas lists the various contexts in which the terms ‘act’ and ‘potency’ are used in any philosophy of mind analysis of the first proposition. Aquinas begins his analysis with a discussion of these concepts as applied to the intellect, where both concept formation and concept exercise occur. This discussion is germane to this inquiry because the analysis of the terms ‘act’ and ‘potency, when used in relation to the acquisitions of concepts, is modified so that it might apply equally to instances of sensation and perception.
In explicating the concepts of potency and act, Aquinas uses a twofold approach insofar as these concepts apply to the intellect. His first approach is to explain what types or classifications of acts and potencies are used in epistemological contexts. Secondly, he proposes an analysis for a potency being reduced to an act in the knowing process. These two considerations are interrelated in that the first discussion pertains to a description of what types of potencies or dispositions and acts or perfections are necessary for a philosophy of mind analysis. The second discussion is concerned with the development or process from a disposition to an actuality. These terms lead to a philosophy of mind account of how a non-knower becomes a knower. ‘Non-knower’ is used in this context to refer to a cognitive agent capable of knowing but not yet knowing. It does not refer to a being incapable of knowing at all, for example, a table, a chair, or a beer can.
The use of disposition in this context is important. By means of a disposition or capacity, Aquinas argues that only certain kinds of existents are capable of knowing. Simply put, it is by the gambit of intentional dispositions that Aquinas maintains that a stone cannot form a concept (a ‘conceptus’) whereas a human being can. The discussion of principles of intentionality affirmed this distinction. Aquinas, through the use of a dispositional analysis, widens the philosophical gap between human knowers and non-knowers rather than lessening this gap. By means of ontological dispositions, Aquinas offers an elucidation of Chisholm’s reference to ‘that funny characteristic’ which all knowers possess. Kenny argued much the same way:
Again, rats can see, and discriminate between circles and triangles; but no amount of gazing at diagrams will make a rat a student of geometry. The specifically human ability to acquire complicated concepts from experience, and to grasp geometrical truths presented in diagrams, will perhaps be what Aquinas has in mind when he speaks of the agent intellect.
In regard to the first point—i.e. to explain the different types of dispositions used in an epistemological analysis—Aquinas provides the following illustrations of various uses of act and potency:
Aristotle distinguishes act and potency in the intellect in the following way:
- (a) We speak, he says, in one sense of potency when we say that a person is able to be a knower, referring to the natural capacity of the person for knowledge. Human beings, we say, belong to that class of existents who know or have knowledge, meaning that this nature can know and form habits of knowing.
- (b) In another sense, however, we say of a person that she knows, meaning that she knows certain definite things; thus we say of one who has the habit of some science—e. g., grammar—that the person is now one who knows. (Commentary on the Soul, no. 359)
Aquinas explicates further this difference:
Now, obviously, in both cases the capacities of the human person are implied by calling the person a knower. But not in the same way in both cases.
(a) In the first case, a human person is said to be ‘able’ through belonging to a certain genus or ‘matter’; i.e., the specific nature had a certain capacity that puts the person in this genus. Thus, the person is in potency to knowledge as matter is to its form.
- (b) But the second person, with the acquired habit of knowing, is called ‘able’ because she can, when she wishes, reflect on the knowledge attained—unless, of course, one is accidentally prevented, for example, through exterior preoccupation or by some bodily indisposition.
- (c) A third case would be when a person is actually thinking about something here and now. This person is the one who most properly and perfectly is a knower in any field; e.g., knowing the letter ‘A, which belongs to the above-mentioned science of grammar. (nos 359, 360; emphasis added)
Aquinas next sums up nicely his position:
Of the three, then, the third knower is simply in act; the first knower is simply in potency; while the second knower is in act as compared with the first, and in potency as compared with the third. [. . .] Clearly then potentiality is taken in two senses (i.e., the first and second knower) and actuality also in two senses (the second and the third knower). (no. 360)
In elucidating the logic of the concepts of act and potency, Aquinas suggests that epistemological dispositions (potencies or powers) and their corresponding perfections (acts) can be used in at least two distinct senses. These passages suggest once again a family resemblance methodology similar to the use theory of meaning. Aquinas denies the possibility of a singular use or unified meaning for the concepts involved in his philosophy of mind. On the contrary, these concepts have different senses and nuances depending upon the ‘context’ in which they are being used. This is probably an additional use of Aquinas’s theory of analogy. For Aquinas, the concept of ‘being’ (ens) is an analogous concept. Since the principal division of Being is ‘potency and act, these terms too have analogous uses in differing contexts.
-  Geach stressed this gap in Mental Acts, where his analysis is similar to what Aquinas argued; heemphasizes the difference between animal knowledge and human knowledge. ‘What is at issue hereis not just the way the term “concept” is to be used, but the desirability of comparing these achievements of rats and dogs with the performances of human beings who possess a concept of triangle; thepsychologists I am criticizing want to play down the differences between human and animal performances, and I want to stress them’: Peter T. Geach, Mental Acts (London: Routledge & Kegan Paul,1957), 17.
-  Anthony Kenny, ‘Intellect and Imagination in Aquinas’, in Kenny (ed.), Aquinas: A Collection ofCritical Essays (Garden City, NY: Doubleday Anchor, 1969), 279.