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The Causal Aspects of Aquinas’s Theory of Perception

The object, the medium, and the faculty of sensation have been discussed extensively; hence it is now opportune to consider what happens when the triadic necessary relation holds and sensation or direct awareness happens. In his ‘Intentionality and Physiological Processes: Aristotle’s Theory of Sense-Perception’,[1] Sorabji argues for a materialist reading of Aristotle and, a fortiori, of Thomas.[2] In response to materialism, however, Aquinas argues explicitly for a twofold ‘immutation’, one of which is intentional. It appears that Sorabji dismisses what Aquinas called intentional change, which produces instances of esse intentionale.[3]

Burnyeat rejected Aristotelian intentionality theory because of the materialism endemic to the new science with its reliance on Cartesian material theory and the rejection of formal causes.[4] It is true that Aquinas provides a causal account, because if there were no object, then there would be no sense knowledge as such. This follows from the triadic necessary relation for sensation, one of whose terms is the proper sensible itself. Furthermore, the ‘sensible species’—a concept whose logic will be analysed later—is caused in the sensing faculty by means of the object. However, a causal theory of perception does not entail atomism. Aquinas emphasizes the role of formal cause in the production of an instance of esse intentionale, not the role of efficient cause.

Considering Aquinas’s causal analysis begins with his own account of why he considers the Democretean theory inadequate. Aquinas argued adamantly against atomism in general and Democritus in particular. He claimed that an atomist account of sensation and perception permitted only a material, physical change in the sense faculty. On the other hand, his account of perception permits both an intentional and a physical immutation. It is evident that Aquinas’s rejection of atomism is based upon the rejection by atomism of the possibility of intentionality theory, with its corresponding esse intentionale. The following passage indicates that the ‘change’ needed to explain the possibility of sensation must be something other than an atomistic ‘discharge’:

Democritus did lay down that no other cause for any of our knowledge is required save the emission of bodily images from things and their entrance into our soul; the process of knowledge is an affair of images and discharges. The absence of any distinction between mind and sense underlies this opinion; the assumption is that all knowledge is like sensation, where objects of sense induce a physiological change. [. . .] Sensation is not the activity of the soul alone, but of the body-soul compound. So also with regard to all the activities of the sensible part. That sensible things outside the soul cause something in the human organism is as it should be; Aristotle here agrees with Democritus that the activities of the sensitive part are produced by the impressions of sensible objects on the senses—not however in the manner of a discharge, as Democritus had said, but in some other way. Democritus, incidentally, held that all action is the upshot of atomic changes. (Summa Theologiae, I q. 84 a. 6; emphasis added)

This passage is important for an adequate causal analysis of Aquinas’s perceptual theory because it denies that a sufficient condition for sensation is explainable solely in


terms of physical or bodily activity. That a strict atomist causal explanation, furthermore, is not acceptable for Aquinas is evident from the following passage taken from the above quotation: ‘the activities of the sensitive part are produced by the impressions of sensible objects on the senses—not however in the manner of a discharge, as Democritus had said, but in some other way. This indicates once again that physicalism and eliminative materialism are foreign to Aquinas’s philosophy of mind. The ‘some other way’ in the above passage is, of course, the crucial phrase; obviously, it needs sufficient explication. This phrase requires a twofold analysis and interpretation for Aquinas. In one sense, it refers to a causal analysis, which is different from what Democritus proposed. In a second sense, Aquinas suggests that sensation, as an instance of knowledge, is partially an intentional process. This is an instance of his principle that intentionality requires an ‘immaterial’ reception of the form of a material thing. In the Summa Theologiae, Aquinas elucidates this twofold consideration:

Some philosophers wish to base the distinction and number of the external senses on the difference of their organs; others on the diverse natures of sensible qualities in the medium of sensation. But neither attempt is convincing. It is the case that faculties are not for organs, but conversely; there are not diverse senses because there are different organs. Instead, nature provides diverse organs to match the diversity of powers. Similarly as regards the media of sensation. The basis for the distinction and number of the external senses should be determined by what is direct and proper to each sense. A sense power is a receptive power, designed by nature to be immuted by the sensible qualities existing in the external world. This external object is what is directly [i.e. per se] perceived by sense, and the senses are diversified according to the diversity found in the objects. (Summa Theologiae, I q. 78 a. 3)

Suttor wrote that in this passage, Aquinas proposes ‘a bold and emphatic claim that structure is for the sake of function, not vice versa’.[5] This claim further supports a naturalistic thrust in Aquinas’s philosophy of mind; the comparison with Gibson once again is striking.

  • [1] Johansen spells out the implications of the Sorabji account quite nicely: ‘On Sorabji’s account, thesense-faculty becomes like F insofar as the sense organ literally becomes F. In seeing red, the eye jelly literally becomes red, in smelling cheese the nose become cheesy, in feeling hot the body literally becomes hot,and so on and so forth. If the notion of the sense-faculty’s becoming like the sense-quality is to be cashedout in terms of material processes, then it is difficult to see what these material process could be other thanthe ones Sorabji points to’: T. K. Johansen, Aristotle on the Sense-Organs (Cambridge: Cambridge UniversityPress, 1998), 21. Johansen appears not to include any medieval Aristotelians in his discussions. However,he appears to be correct in this analysis.
  • [2] Richard Sorabji, ‘Intentionality and Physiological Processes: Aristotle’s Theory of Sense-Perception, inMartha Nussbaum and Amelie Rorty (eds), Essays on Aristotle’s De Anima (Oxford: Clarendon Press,1992), 195-226.
  • [3] In response to Sorabji on the one hand and Burnyeat on the other, Nussbaum and Putnam proposed afunctionalist reading of Aristotle’s account of sense perception. Functionalism is based on Putnam’s earlywork reducing intentional states to computational states. This functionalist account, however, still reducesthe causal action of perception to a material cause. Nussbaum and Putnam’s version of functionalism forAristotle appears to rule out intentional change, a change Aquinas adopts unequivocally.
  • [4] M. F. Burnyeat, ‘Is An Aristotelian Philosophy of Mind Still Credible’?, in Martha Nussbaum andAmelie Rorty (eds), Essays on Aristotle’s De Anima (Oxford: Clarendon Press, 1992), 15-26.
  • [5] Timothy Suttor, ‘Commentary’ Summa Theologiae, vol. 11 (Oxford: Eyre & Spottiswoode; New York:McGraw-Hill, 1969), 130, n. (a). For the two kinds of immutation, see Summa Theologiae, I q. 78 a. 3 ad 3.
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