The Triadic Relation
This analytic reconstruction of Aquinas’s position on visual sensation proposes an account consisting essentially of a three-term necessary relation; this triadic relation itself can be designated as the necessary conditions for sensation by using the following terms:
- (a) the sensible object;
- (b) the adequate medium;
- (c) the properly disposed sense faculty.
These three categories as the terms of the necessary relation are commensurate with Aquinas’s rejection in principle of representationalism. Accordingly, one of the terms of the triadic relation is the presence of an object, which Aquinas refers to as a ‘sensible’ In addition to the presence of an object, there must be both a proper medium and a sensing faculty or power itself. These three entities constitute the terms of the triadic relation, which is a necessary condition for Aquinas’s analysis of sensation. Furthermore, each of these entities is itself a necessary component of the necessary triadic relation. It follows, then, that if any term is missing, there will be no triadic relation. In other words, each term is a necessary condition for the existence of the triadic relation, which relation itself is the necessary condition for sensation/percep- tion. This necessary triadic relation might be symbolized as follows: NC[O-M-F]. In De Veritate, Aquinas indicates this relation: ‘the sense always apprehends a thing as it is, unless there is an impediment in the organ or in the medium’ (De Veritate I, ii). The discussion of sight in the preceding section indicated the textual foundations for these claims. The present task is to reconstruct a theory that is fair and consistent with the texts of Aquinas on sensation. Visual perception will continue to be used as the paradigm case for this analysis of Aquinas’s theory of sensation. Aquinas himself considers this act of Sensation-Ia to be the highest or ‘best’ of the acts of sensation of the external senses, in that it acquires a status near to the ‘spiritual’ awareness of the intellect: ‘Now sight, which is without natural immutation either in its organ or in its object, is the most spiritual, the most perfect, and the most universal of the senses’ (Summa Theologiae, I q. 78 a. 3).
With visual perception, there is an object, which is colour, and Aquinas defines the primary visible as colour. Thus he considers these sensible properties—what have classically been called the secondary qualities, which he calls the proper sensibles—to be in some way or other existing as qualitatively distinct causal factors in the world independent of minds. This existence of colour in a truncated world does not entail, however, that there can be no difference in the actual perception of a colour—a proper sensible—in different perceptual situations. Aquinas does not argue for a simple- minded position of direct realism, sometimes referred to as ‘naive realism’. In other words, he would be the first to admit that a colour will be perceived differently in the bright sunlight of high noon and in the dimness of twilight. The facet of visual perception that forced Locke to admit that secondary qualities are mind-dependent was faced by Aquinas. However, the variability of the proper sensibles will be explained by other factors than by Locke’s gambit, which was to make this category of perceptual objects completely dependent on a mind. For Aquinas, perceptual conditions do differ. This is not because the object has changed, however. Rather, it happens because the medium has been affected differently. The diaphanum or medium changes due to the intensity of the light actually present. Accordingly, it is not the object alone that determines what is perceived but also the intensity of the transparent medium. The diaphanum or medium becomes transparent, Aquinas argues, only because of light. Therefore, the change in the medium is due to the intensity of the light actually present in the medium. The effect of this medium on visual perception further substantiates his claim that secondary qualities are not mind-dependent. In other words, he claims that the medium contributes substantially to the way something is perceived. Accordingly, the proper sensible—in this case, colour—can exist independently of a mind as a qualitatively distinct causal factor, but nonetheless be perceived differently in differing circumstances partly because of the different intensity of light in the medium. This is a another instance justifying the claim that Aquinas is an objective relativist in his theory of sensation; furthermore, he remains throughout an epistemological realist.
Two of the three necessary conditions for the triadic relation, namely, the object and the medium, have been noted. The third necessary condition for perception is the perceiving organ and the perceiving faculty; Aquinas distinguishes between the organ and the faculty. The organ is the physiological machinery—the vehicle—and the faculty is the ability to receive intentionally the form of another thing in a non-entitative manner—i.e. in esse intentionale. Therefore, ‘sensation apparatus’ refers to both the sense faculty and the physiological organ of that faculty.
The ‘primary sensitive part’, i.e. the organ of sense, is that in which a power [i.e. a sense faculty] of this sort resides, namely a capacity to receive forms without matter. For a sense organ, e.g. the eye, shares the same being with the faculty or power itself, though it differs in essence or definition, the faculty being as it were the form of the organ. [. . . ] A bodily organ is ‘what receives sensation’, i.e. is the subject of the sense faculty, as matter is subject of form, and yet the magnitude and the sensitivity of sense differ by definition, the sense being a certain ratio, i.e. proportion and form and capacity, of the magnitude [i.e. of the organ]. (Commentary on the Soul, no. 555)
Not only must the organ exist along with the faculty, but also the organ and faculty must be properly disposed so that they may perform a certain perceptual function, which is to exercise or perform a certain type of mental act. There are two senses of ‘being disposed properly’ for the sense apparatus, but it is too early now to consider them in detail. At any length, neither the perceiving organ nor the perceiving faculty is to be considered as a complete tabula rasa. Rather, they are structured mental capacities geared to undertaking certain mental acts. The activity of the organ and sense faculty occurs only because the structure reacts in a certain manner with the proper and appropriate sensible object as found in an appropriate medium. The constitutive structure of the perceiving apparatus, therefore, is in itself found in a condition of Disposition-2/Actuality-1 by its makeup or ‘ratio’. This position is a necessary component to the claim that Aquinas opts for a ‘structured mental act’ both in sensation and perception and in concept formation. This explanatory account, furthermore, entails that the perceiving apparatus could not be what it is without its structure or ratio.
The faculty and organ are not diaphanous in any sense of the term. A structured sensation apparatus with a certain dispositional order is necessary for perception to occur. Accordingly, the claim is put forward that the existence of a properly disposed perception faculty/organ is one term of the necessary triadic relation which itself constitutes a necessary condition for the act of perception.
Because the organ and the faculty are not identical in Aquinas’s philosophy of mind, one might further distinguish the disposed sense organ from the disposed sense faculty. In the schema NC[O-M-F], one might divide ‘F’ into ‘DO for ‘disposed sense organ’ and ‘DF’ for ‘disposed sense faculty.’ Hence, NC[O-M-DO/DF]. Thus, ‘DO’ refers to the physiological structure that makes an eye pupil, for instance, colour-blind. The structure of ‘DO’ is related directly to the structure of ‘DF’. The disposition of the faculty is dependent on the dispositional situation of the organ. This is an essential feature of Aquinas’s naturalistic epistemology.
-  For a different perspective of the innate mental endowment characteristic of Aristotle’s theory of perception, see Judith M. Baumrin, ‘Aristotle’s Empirical Nativism, American Psychologist 30(4) (1975),486-94.