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Cognitive Possibility and the Internal Senses in Thomas

Aquinas posits the internal senses because human awareness of the external world is not explained adequately only in terms of an awareness of the ‘sensibles’—both proper and common—of objects in the world. Referring to what might be taken as an early twentieth-century paradigm for these questions, Aquinas claims that the awareness of sense data or impressions of qualia alone do not exhaust human perception. The awareness of sense qualities in the mind is not sufficient to explain human perception. Aquinas appeals to the pre-analytic data of sense awareness. The need for positing additional sense faculties comes about because of the inadequacy of the external senses alone to provide a sufficient explanatory account of human awareness. The following passage from the Summa Theologiae suggests this mode of operating with the internal senses:

Because nature does not fail in necessary things, there must needs be as many actions of the sensitive soul as may suffice for the life of a perfect animal [i.e. a perceiver]. If any of these actions cannot be reduced to one and the same principle, they must be assigned to diverse powers. This is because a power of the soul is nothing else than the proximate principle of the soul’s operation. (Summa Theologiae, I q. 78 a. 4)

If the preceding analysis of Aquinas’s theory of sensation is correct, then the intentional awareness of an object in the external world is an ‘awareness of P, where ‘P’ is a sensible object, either a proper sensible (colour, sound, taste, etc.) or a common sensible (shape, motion, number, etc.) in the external world. Moreover, the object ‘P’ is perceivable only in conjunction with the other two necessary conditions for sensation, namely, a sufficient medium and a properly disposed faculty. Aquinas’s contention is, however, that this analysis alone does not explain sufficiently what a perceiver is aware of during an act of awareness. There is more to sense knowledge than an awareness of empiricist primary and secondary qualities—the proper and the common sensibles. Aquinas parts company radically with the British empiricists.

The force of Aquinas’s argument is that if knowers manifest knowledge behaviour that cannot be explained sufficiently through external sensation alone, then this epistemological datum requires an additional account, which provides an ontological underpinning for the mental acts of sensation. Like the external sense analysis, Aquinas provides an explanation and not a foundationalist position. In considering the external senses, Turnbull posed the question:

I think it is helpful to suppose that both Plato and Aristotle put to themselves the following question: Granted the existence of the unperceived material world, how is it possible for sentient beings, qua sentient, to become aware of it? Or, if you please, how is it possible for them to perceive it, or rather, parts of it? And I think the answer that both gave to this question is: Only by way of perceiving colors, sounds, odors, hards and softs, and the like.[1]

Turnbull’s claim is commensurate with Aquinas’s metaphilosophy articulated by Haldane, placing emphasis on explanation and not on criterial justification; this same method is used in the analysis of the inner senses. In his Summa Theologiae, Aquinas sets forth the clearest and shortest account for what he takes to be the ontological need for the internal senses:

We must observe that for the life of a perfect animal [i.e. one with sense knowledge], the animal should apprehend a thing not only at the actual times of sensation, but also when the thing is absent. Otherwise, since animal motion and action follow apprehension, an animal would not be moved to seek something absent; the contrary of which we may observe especially in perfect animals, which are moved by progression, for they are moved towards something apprehended and absent. Therefore, through the sensitive soul, an animal must not only receive the species of sensible things, when it is actually affected by them, but it must also retain and preserve them. [. . .] Therefore [. . .] the power [external sense faculties], which receives the species of sensible things, must be distinct from the power [internal sense faculties] that preserves them. (Summa Theologiae, I q. 78 a. 4)

Next, Aquinas affirms that not only do the internal senses retain sensible species, but also the three faculties of inner sense engage in other types of mental awareness. This is based, he believes, on the pre-analytic data of mental awareness experienced by ordinary perceivers: a naturalistic, explanatory motif directs this inquiry.

Again, we must observe that if an animal were moved by pleasing and disagreeable things only as affecting the sense, there would be no need to suppose that an animal has a power [sense faculty] besides the apprehension of these forms which the senses perceive, and in which the animal takes pleasure, or from which it shrinks with horror. But the animal needs to seek or to avoid certain things, not only because they are pleasing or otherwise to the senses, but also because of other advantages and uses, or disadvantages; an example is the sheep that runs away when it sees a wolf, not because of its colour or shape, but because it is a natural enemy. So too, a bird gathers together straws, not because they are pleasant to the sense, but because they are useful for building its nest. (Summa Theologiae, I q. 78 a. 4)

These examples lead Aquinas to postulate the need for additional faculties and corresponding functions for the internal senses: Animals, therefore, need to perceive such intentions, which the external sense does not perceive. Now some distinct principle is necessary for this, since the perception of sensible forms comes by an immutation caused by the sensible, which is not the case with the perception of the above intentions’ (Summa Theologiae, I q. 78 a. 4 ad 2; emphasis added).

These passages indicate the mode of explanation that Aquinas adopts as he seeks to establish a complete explanatory theory of sensation and perception involving the external and the internal senses.

  • [1] Robert G. Turnbull, ‘The Role of the “Special Sensibles” in the Perception Theories of Plato andAristotle’, in Robert G. Turnbull and Peter Machamer (eds), Studies in Perception (Columbus: Ohio StateUniversity Press, 1978), 7.
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