Home Economics Aquinas’s theory of perception: an analytic reconstruction
Aquinas versus Avicenna
Throughout this discussion of the faculties of both the external and the internal sensorium, one must note that Aquinas is not pulling sense faculties arbitrarily out of a cognitive magician’s hat. In a manner of proceeding somewhat similar to what Strawson proposed at mid-century, Aquinas submits to philosophical analysis what he believes perceivers know in a pre-analytic fashion. Furthermore, he offers epistemological categories in order to provide an explanatory account for the totality of human perception. To interpret this experience, which Strawson once referred to as ‘the massive central core of human thinking’, Aquinas makes use of his epistemological principle that a knowing power is determined by its acts and objects. If there is a specific type of mental activity, then there must be a corresponding capacity or ability necessary to explain these mental acts. This epistemological principle permits him to posit consistently the various internal sense faculties. If there is a characteristic activity, this activity requires an explanatory account, which Aquinas believes is best offered through the naturalist philosophy of mind common to his faculty psychology. He suggests that some capacity or disposition is required as a necessary condition in order to explain the mental acts under discussion.
Using this method to determine the principles of establishing the internal senses, Aquinas rejects Avicenna’s account of the number of the faculties necessary to explain the functioning of the internal senses.
Avicenna, however, assigns between the aestimative and the imaginative a fifth power, which combines and divides imaginary forms. As when from the imaginary form of gold, and the imaginary form of mountain, we compose the one form of a golden mountain, which we have never seen. But this operation is not to be found in animals other than humans, in whom the imaginative power suffices for this purpose. [. . .] So there is no need to assign more than four interior powers of the sensitive part, namely the sensus communis, the imagination, the aesti- mative (or the vis cogitativa) and the memorative powers. (Summa Theologiae, I q. 78 a. 4)
This passage exhibits clearly how Aquinas develops his method for establishing the specific sense faculties for the internal senses. His claim is that the activity of the fifth internal sense faculty postulated by Avicenna is not sufficiently distinct from the activity of the imagination. It follows then that there is no need in this case to postulate an additional faculty of inner sense other than the imagination; the imagination itself can account for the activity of combining various elements of sensation in order to form complex images of things, which have never been observed in the external world: ‘There are two operations in the sensitive part. (a) One is limited to immutation, and thus the operation of the senses takes place when the senses are impressed by the sensible. (b) The other is formation [i.e. image construction], insofar as the imagination forms for itself an image of an absent thing, or even of something never seen’ (Summa Theologiae, I q. 85 a. 2 ad 2).
In the preceding text, the term ‘image’ will be classified as one use of phantasm in the analysis of this much-used but somewhat muddled term in Aristotelian epistemology. Furthermore, in this discussion Aquinas makes use of what later philosophers would call the more familiar ‘Occam’s razor’. That Aquinas was aware of this methodological device is clear from what he writes in the Summa Theologiae: ‘quod potest compleri per pauciora principia, non fit per plura’ (I q. 2 a. 3). Hence, one must not multiply the number of entities postulated if fewer can resolve the aporia under discussion. By a judicious use of his own methodological principle of Occam’s razor, Aquinas claims that Avicenna’s theoretical position lacks warranted philosophical grounds for positing a fifth internal sense faculty. Aquinas’s gambit is that the imagination itself is the faculty which can form images of things not directly perceived—the mermaids, unicorns, winged horses, and leprechauns—which caused twentieth-century philosophers who pondered the writings of Meinong so much anguish.
Much analysis must be undertaken in order to elucidate the functions of the faculties of the internal senses. The greater part of the remainder of this study consists in completing that task. Furthermore, the argument will be put forward that the internal senses, especially the vis cogitativa and the sense memory, play critical roles in the explanation of concept formation through the intellectus agens. The internal senses do not have for their obj ects only the ‘faint’ impressions of the external senses, as Hume proposed for the nature and scope of inner sense. Quite the contrary: the internal senses are distinctly creative and serve as the locus of a perceiver’s awareness of an individual as an individual of a natural kind and not merely as a bundle of sensations. Both the classical British empiricists and the Continental rationalists blurred the workings of the internal senses with the functioning of the external senses. In the history of Western philosophy, therefore, these muddles caused the sophistication of the internal sensorium found in Aquinas to be misunderstood or neglected completely.8
Inner sense for Aquinas is not reducible only to an awareness ‘through the mind’s eye, as it were. This dimension of ‘inner sensation’ begins with Locke and Descartes. In his The Metaphysics of Mind, Kenny argues that philosophical confusions on the nature of the self grow out of the view of inner sense when equated with introspection found
One task of this book is to help set the historical and structural records straight.
in the writings of modern philosophers, especially Locke and Descartes. Often when Kenny uses the concept of ‘inner sense’ in discussing Aquinas, he appears to be concerned with the philosophical problems associated with ‘introspection’ as coextensive with ‘inner sense’. Kenny is concerned that what he considers to be an erroneous view of the self as the subject of inner sensation grew out of this conflation of inner sense with introspection. Kerr also considers this non-introspective dimension of Aquinas’s philosophy of mind. 
The Sensus Communis
Following Aristotle, the first of the internal sense faculties that Aquinas considers is the sensus communis. This term is often translated as ‘common sense’. In order to avoid confusion, it must be noted that Aquinas uses this term to denote an internal sense faculty. It does not refer to some type of ‘common-sense knowledge’ acquired by wise old folks with plenty of experience. Nor, with this faculty of inner sense, is Aquinas advocating any type of ‘common-sense criterion’ for the admission of philosophical truths, which Moore introduced into early twentieth-century English philosophy in his battle with the absolute idealists.11 Furthermore, Aquinas’s use of ‘sensus communis’ is not to be confused with the Scottish common-sense realism of Reid. While Aquinas accepted, at least in principle, some of the presuppositions found in both Reid and Moore, nonetheless his internal sense faculty of the sensus communis transcends the limited use put forward by both Reid and Moore. Hence, Aquinas affirms the existence of a special faculty of the internal senses. In order to avoid misunderstanding, the Latin term for this internal sense faculty will not be rendered into English.
Translators of Aquinas often have been ingenious in attempts to render ‘sensus communis into English. Given all of these English renderings, the Latin term, ‘sensus communis’ will appear in this analysis, rather than its accustomed translation, ‘the common sense’. In particular, this practice will keep front and centre the claim that this is a cognitive faculty of the internal senses.
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