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Principle F. An ‘X’ is knowable only insofar as it is in act

Given Principles A, B, and D-1, Principle F might be considered a derived proposition. If a knower, which is an existent capable of receiving a form of another immaterially, takes on the act of an object in an immaterial manner, then the object, which is known, must have been in act before it could have been known. This philosophical statement follows from Aquinas’s notion that every potency is affected only by something which is in act. This is a restatement of Principle B.

Aquinas argues for two different kinds of potencies:

  • (a) ontological potencies (which are the materiaprima and materia secunda in the individual);
  • (b) epistemological potencies (which are the cognitive faculties of the external sen- sorium, the faculties of the internal sensorium and the intellectus possibilis).

It follows from Principle F that neither an ontological potency nor an epistemological potency is a direct object of knowledge. An ontological potency is either a mental construct—e.g. materia prima—or known by means of a complicated abstractive process of the intellectus agens working in tandem with the intellectus possibilis—e.g. materia secunda or the set of sortal properties rooted in an essence of a primary substance. Neither kind of ontological potency can have an extra mental referent, which is capable of informing an epistemological potency. Noted above is the claim that if an ontological potency had an extra mental referent—especially in the case of materia secunda—then Aquinas would be a Platonist. Moreover, epistemological potencies are themselves never the direct object of knowledge. Principle F holds that neither faculties of the external or the internal sensorium nor the intellectus possibilis can know themselves by an act of direct awareness.

In Aquinas’s account of perception, reflective awareness is only possible because there is a specific faculty—the sensus communis—by which one can be aware of the acts of the external senses.[1] The sensus communis is that faculty by means of which a perceiver is aware that she is aware. Put differently, the acts of awareness become the object of a second act of awareness of the sensus communis. The sensus communis will be discussed in detail later. Parenthetically, it should be noted that Aquinas argued that the intellectus possibilis is capable of self-reflection. This is due to the complete spirituality or immateriality of this faculty. In other words, it is not tied ontologically to any physiological organ; Aquinas is not a physicalist. This claim, however, requires a much more detailed analysis than can be given here.[2]

Principle F further amplifies Principle D-1. The ontological potency is what gives ‘body’ or ‘fleshing-out’ to the materially existing concretum. In Aquinas’s ontology, weight and extension follow from (in the sense ofbeing ontologically dependent upon) matter. And matter is the ultimate potency of any physical object. An epistemological potency, on the other hand, is the cognitive capacity to receive the form of an existing thing but not to give that form a material embodiment as happens with an ontological potency. In one sense, however, there is a ‘fleshing-out’ with respect to the epistemological potencies. It is by means of an epistemological potency that a ‘piece of knowledge’ is anchored into a particular space-time context, with the result that a cognitive agent now is engaged in an act of knowing.

The ontological force of Principle D-1, together with Principle F, is that if an epistemological potency were exactly the same as an ontological potency in all of its functions, then this identity would rule out a priori the possibility of a knowing agent’s possessing esse intentionale. Thus, Principles D-1 and F ground ontologically the possibility for making the distinction between esse naturale and esse intentionale. Moreover, these two principles elucidate that ‘funny characteristic’ that Chisholm used to ground the distinction between existents capable of intentional acts and those other existents manifesting only physical phenomena. Immanent action is a central item in discussing Aquinas’s account of mental acts.[3] This further indicates that Aquinas adopts the ontological theory of intentionality.

This concludes the elucidation of the seven principles of intentionality, which are presupposed in the philosophy of mind of Thomas Aquinas. These seven principles spell out the metaphysical machinery acknowledged, at least implicitly, in those writings of Aquinas concerned with explaining the possibility of knowing. A clear elucidation of this machinery in terms of principles of intentionality is necessary in order to provide some insight into a rather difficult bit of philosophy of mind theory. That the Aquinian account is difficult no one will deny.

After this discussion of Aquinas’s account of intentionality, the next item for consideration is an inquiry into how Aquinas fits into the general categories of the classical rationalism/empiricism debates. It is difficult to classify either Aristotle or Aquinas within the rigid categories for differing philosophy-of-mind theories commonly used by historians of modern and contemporary philosophy. Chapter 3 begins these discussions.

  • [1] In order to avoid confusion over various pre-philosophical discussions of ‘common sense’ it seemspreferable in this study to leave the terms of this internal sense faculty in the Latin customary to the university discussions of Thomas’s time. Hence, the terms ‘sensus communis5 are used throughout this book asthe mode of referring to what Aquinas refers to as the ‘common sense’ faculty. This term is not coextensivewith what Reid or Moore meant by common sense.
  • [2] On this issue, Kenny and Haldane differ on the need of a material vehicle for the intellectus possibilis.See John Haldane, ‘Kenny and Aquinas on the Metaphysics of Mind’, in John Cottingham and Peter Hacker(eds), Mind, Method, and Morality: Essays in Honour of Anthony Kenny (Oxford: Clarendon Press, 2010),119-39.
  • [3] Aquinas distinguishes between a transient action and an immanent action. A transient action is aphysical change, while an immanent action is intentional in character. The immanent action remainswithin the agent while the transient action passes out to another thing. It follows from this distinction thatintentional acts are not esse naturale because every change bringing about an esse naturale would be atransient action. Furthermore, every actio humana would be immanent while every actio hominis would betransient. In the Prima Pars of the Summa Theologiae, Aquinas writes: (Sentire et intelligere sunt actionesimmanentes’
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