Home Economics Aquinas’s theory of perception: an analytic reconstruction
Direct Realism in Aquinas
According to standard procedure for epistemological classification, theories of perception are divided into direct realism and representationalism or representative realism. This distinction emphasizes whether the mind has for its direct object the ‘thing’ in the external world or an ‘idea’ of the thing. On the former view, the mind is directly aware of an object in the external world; on the latter, there is an ‘epistemological wedge, a tertium quid, driven between what is directly perceived and the object existing in the external world.
Several (but not all) medieval philosophers give evidence of direct realism leanings, while many modern philosophers, following Descartes and Locke, tend towards rep- resentationalism. Proponents of representationalism normally put the foundational questions as primary in epistemological discussions. In his Essay Concerning Human Understanding, Locke provides the following explicit account of representationalism: ‘It is evident that the mind knows no things immediately but only by the intervention of the ideas it has of them. Our knowledge therefore is real only so far as there is conformity between our ideas and the reality of things.’ Locke next suggests an obvious difficulty with representationalism: ‘But what shall be here the criterion? How shall the mind, when it perceives nothing but its own ideas, know that they agree with things them- selves?’ Locke notes the necessity for a foundationalist epistemology. The search for a criterion becomes the driving hallmark for undertaking epistemology; Descartes argues in a similar manner. Elsewhere Locke discusses the relation between ideas and things.
In placing Aquinas in one category rather than another, it is important to realize from the outset that he considers sensation and perception as an awareness of particular things in the world rather than as an awareness of ideas of those things. In his epistemology and philosophy of mind, Aquinas stresses the possibility of ‘thing consciousness, which is reducible to an externalist position in the nature of knowing. ‘The sense objects, which actualize sensitive activities—the visible, the audible, etc.—exist outside the soul; the reason is that actual sensation (or perception) attains to the individual things, which exist externally outside of the mind’ (Commentary on the Soul, #375; emphasis added); also: ‘Sense is a passive power and is naturally immuted by the exterior sensible. Hence, the exterior cause of intentional immutation is what is per se perceived by the sense’ (Summa Theologiae, I q. 78 a. 3). To the contrary, Aquinas denies that he is a representationalist: ‘The proper active principle in external sensation is a thing existing outside the soul and not an intention existing in the imagination or reason’ (Summa Theologiae, Supp. to III, q. 82 a. 3). Furthermore, Aquinas supports a distinction regarding the ‘object’ of awareness that a representationalist could never affirm: ‘In discussing the kinds of sight, we note this difference: (a) Bodily sight terminates at the body itself. (b) The sight of the imagination, on the other hand, terminates at the image of the body as its object’ (De Veritate, q. 10 a. 3). These passages provide evidence that Aquinas rightly adopted direct realism and not representationalism; he is a realist epistemologist and an externalist. In the Latin texts, Aquinas often used ‘immutatio’ to denote the ‘in actu status of the sense faculty when acted upon by the external sense object. Accordingly, as translated, he argues: ‘the sense objects [. . .] actualize’ and also ‘sense [. . .] is naturally immuted by the exterior sensible’. Both propositions refer to the same epistemological issue.
Aquinas did write about the possibility of representationalism, which he appears to dismiss rather quickly. The following extended passage demonstrates that he rejected representationalism as a possible theory of perception:
Some have suggested that our cognitive powers know only the impressions made on them. For example, that sense knows only the alteration of its organ. According to this reading, mental states are the objects of knowledge. (This would be reducible to an internalist position on knowing.).
This opinion seems to be false for two reasons. First, if the objects of intelligence and science were merely mental states, it would follow that science does not deal with non-mental things, but merely with impressions in consciousness.
Secondly, it would revive the ancient error asserting that whatever seems so is truly so, and that contradictions are simultaneously tenable. Were a cognitive faculty able to perceive no more than its own proper state, then it could judge only about those. Because an object appears according to the manner the faculty is affected it follows that were modes of consciousness the round, the powers to produce those ideas in us as they are in the snowball, I call qualities; and as they are sensations or perceptions in our understandings, I call them ideas; which ideas, if I speak of them sometimes as in the things themselves, I would be understood to mean those qualities in the objects which produce them in us’: ibid., bk 2, ch. 8.
only data, a faculty could judge about merely its own proper impressions. Hence, every judgment would be true. When a person with a healthy-tasting tongue judges honey to be sweet, then it would judge truly; and thus when a person with an ill-tasting tongue judges it to be bitter, then also would it judge truly. In each case, the person would be going on the direct impressions. It follows that every opinion would be equally valid; so also in general would be whatever was fancied. (Summa Theologiae, I q. 85 a. 2)
In the Supplement to the Tertia Pars, Aquinas suggests that his epistemological realism is proper for a theory of sensation and perception:
Every passive power, according to its specific nature, is determined to some special active principle, since a power as such bears a relation to that with respect to which it is said to be the power. Thus, since the proper active principle in external sensation is a thing existing outside the soul and not an intention thereof existing in the imagination or reason, if the organ of sense is not moved by external things, but only by the imagination or other higher powers, there will be no true sensation. Therefore, we never say that madmen or other witless persons (in whom there is this kind of outflow of species towards the organs of sense, because of the very powerful influence of the imagination) indeed have real sensations, but that it seems to them that they have sensations. (Summa Theologiae, Supp. to III, q. 82 a. 3; emphasis added)
These passages taken together illustrate the manner and force with which Aquinas voices his objections to representationalism and internalism.
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