Colour and Sight
In commenting upon Aristotle’s theory of sensation, Aquinas needs to explain how colour actually affects sight: ‘Aristotle says that we are now clear that what is seen in light is colour, and that colour is invisible without light and this because, as has been explained, colour of its nature acts upon a transparent medium, and it does this in virtue of light, which is the latter’s actuality. Hence light is necessary if colour is to be seen’ (Commentary on the Soul, no. 431). Aquinas continues with his analysis:
An indication of this is the actual fact that if a coloured body is placed upon the organ of sight, it cannot be seen. For then there remains no transparent medium to be affected by the colour. The pupil of the eye is indeed some such medium, but so long as the coloured body remains placed upon it, it lacks actual transparency. There has to be a medium, for example, air or something of the kind, which, being actualized by colour, itself acts upon the organ of sight as upon a body continuous with itself. For bodies only affect one another through actual contact. (no. 432; emphasis added)
The above passage indicates that a diaphanous medium between the object and the perceiving organ itself is a necessary condition for any visual awareness. This diaphanous medium, however, is not a vacuum in the manner postulated by Democritus with his theory of atomism, which reduced all entities to material particles moving in the vacuum or void. Both Aristotle and Aquinas seem aware of this likely interpretation. Accordingly, both philosophers reject any identification of their theory of sensation with the atomism of Democritus. This is, moreover, conclusive evidence that Aquinas is not an atomist in his theory of perception.
Next in order, Aristotle sets aside an erroneous view. The atomists were wrong in thinking that if the medium between the eye and the thing seen were a vacuum, any object, however small, would be visible at a distance; e.g. an ant in the sky. This cannot be. For if anything is to be seen, it must actually affect the organ of sight. Now it has been shown that this organ as such is not affected by an immediate object—such as an object placed upon the eye. Hence, there must be a medium between organ and object. But a vacuum is not a medium; it cannot receive or transmit effects from the object. Through a vacuum, therefore, nothing would be seen at all. (no. 433; emphasis added)
In the following passage, Aquinas criticizes further the position defended by Democritus and the cohort of atomists because their theory of perception depends on the postulation of a void:
Democritus went wrong because he thought that the reason why distance diminished visibility was that the medium is of itself an impediment to the action of the visible object upon sight. But it is not so. The transparent medium as such is not in the least incompatible with luminosity or colour. On the contrary, it is proximately disposed to their reception; a sign of which is that it is illumined or coloured instantaneously. (no. 434)
Here Aquinas rejects atomism on the grounds that it would admit a vacuum into his ontology. Aquinas, following Aristotle, refused to admit the existence of a vacuum. While arguing against atomism, however, he does not reject a causal theory of perception. His unequivocal rejection of atomism including the reference to Democritus is important considering the contrary account found in Hamlyn’s Sensation and Perception. Hamlyn affirms rather than denies that a structural affinity exists between the theories of sensation and perception developed by Democritus and by Thomas. Hamlyn proposed the similarity of Thomas with the atomist account of perception: ‘The Thomist theory [of sensation and perception] looks like a combination of the Aristotelian point of view with one such as that put forward by the Atomists. Aquinas makes frequent reference to Democritus.’ The passages from Aquinas’s Commentary on the Soul provide substantial evidence refuting Hamlyn’s interpretation of Aquinas. While Aquinas makes frequent reference to Democritus, these passages often criticize the atomist position on sensation rather than endorsing atomism as a plausible foundation for perception theory. Aquinas utilizes a theory of formal causation in his theory of intentionality, for both sensation and intelligence. He does not accept the efficient causal analysis characteristic of theoretical classical atomism as a sufficient condition for explaining sensation and perception. In contemporary perception discussions, Putnam appears to be in a quandary as to how one would determine the specific efficient cause with an individual idea or concept. In Realism and Reason, he writes:
Even if one is willing to contemplate such unexplainable metaphysical facts (as that some correspondence intrinsically just is reference) the epistemological problems that accompany such a view seem insuperable. For assuming a world of mind-independent, discourse-independent entities, [. . .] there are [. . .] many different ‘correspondences’ which represent possible or candidate reference relations.
In this treatise, Putnam seems to defer to an anti-realist account of intentionality. In an analysis of Putnam’s argument, Haldane counters with the proposition that an intentionality theory as proposed by Brentano and strengthened with suggestions from Aquinas can meet Putnam’s anti-realist thrust. Putnam’s concerns lead Haldane to emphasize the importance of formal cause in understanding how human persons perceive the world veridically. Haldane also responds to Putnam’s analysis in terms of requiring a realist background in order to render coherent an intentionality theory. Haldane, in the end, suggests that Putnam rejects ontological realism in too facile a manner, and that an intentionality theory rooted in Aristotelian formal causality renders a coherent position of realist metaphysics. Formal cause is the ‘metaphysical skull’.
-  Hamlyn appears to assume that because Aquinas adopts a causal theory of perception, it must followthat Aquinas is an atomist: D. W. Hamlyn, Sensation and Perception: A History of the Philosophy ofPerception (London: Routledge & Kegan Paul, 1969), 47.
-  McDowell and Haldane reject the reduction of causality in perception to efficient cause alone. SeeJohn Haldane, ‘A Return to Form in the Philosophy of Mind’, Ratio 11(3) (1998), 253-77.
-  Hilary Putnam, Reason, Truth and History, vol. 3: Realism and Reason (Cambridge, Mass.: HarvardUniversity Press, 1981), 47-8.
-  John Haldane, ‘Putnam on Intentionality’, Philosophy and Phenomenological Research 52(3) (1992),671-82.
-  John Haldane, ‘Realism with a Metaphysical Skull’ (with response by Hilary Putnam), in J. Conantand Urszula Zeglen (eds), Hilary Putnam: Pragmatism and Realism (London: Routledge, 2002).