The Need for a Medium
The discussion of an appropriate medium as a necessary condition for visual perception is illustrated by similar accounts developed by Aquinas of the faculties of hearing and smelling in the external sensorium. Taste and touch, however, are analysed somewhat differently; nonetheless, in this text it appears that some type of medium is a necessary condition for all the faculties of the external senses. Yet taste and touch inherently are different from sight, hearing, and taste. In the following passage, Aquinas begins his discussion of the role the medium plays for the faculties of the external sensorium:
Aristotle shows how the case of the other senses is similar to sight. No sound or odour, for example, is perceived if there is immediate contact with the organ in question. There must be a medium affected by sound or odour, which itself then affects our sense of hearing or of smell. A sounding or odorous body placed upon the organ is not perceived as such. The same is true even of touch and taste, though, for a reason to be given later, this is less evident. (Commentary on the Soul, no. 437; emphasis added)
Aquinas continues this discussion:
Finally, he states what is the medium in hearing and smelling. That of hearing is air, and that of smelling is something common to air and water—just as both of these provide a medium for colour insofar as each is a transparency. There indeed is no name for the quality in air and water, which provides the medium for odour; but it certainly is not transparency. And that both air and water are conductors of smell he shows from the fact that marine animals have a sense of smell. A human person, however, and other animals that walk and breathe, only smell by breathing; this proves that air is the medium of smell. This fact will be explained later. (no. 438)
Regarding sound, he writes that ‘the actuality of sound involves the medium and the faculty of hearing’ (Commentary on the Soul, no. 441). In discussing sound, he makes an important distinction about the ‘actuality’ of sense objects:
The actuality of sound involves the medium and the faculty of hearing. For we can speak of a sense object as actual in two ways:
So far as the object is actually being sensed, i.e. when its likeness is affecting the sense organ. In this way, a sound is actual when it is heard.
So far as the object actually is such that it can be sensed, but is such simply in its own objective being, outside the senses.
And in this way, the other sense objects, colour, odour, savour, etc., exist actually in coloured, or odorous or savourable bodies. But not so sound; for in a sound-producing body, there is sound only potentially: actual sound exists only when the medium is affected by a disturbance from that body. Therefore the act of sound exists, he says, in the medium and in the hearing, but not in the audible body. (Commentary on the Soul, no. 441)
This is Aquinas’s method for avoiding Berkeley’s aporia regarding the old philosophical chestnut about ‘the tree falling in the forest’. It also again indicates the epistemological realism rooted in Aquinas’s account of sensation.
On the question of the need for a medium for the three faculties of sense awareness considered above, a discrepancy exists when comparing the texts of his Commentary with Aquinas’s treatment of sensation in the Summa Theologiae. In the Summa Theologiae, Aquinas asserts that a medium is not needed for all the external sense faculties. The following text illustrates this discrepancy:
Sight, which is without natural immutation [abseque immutatione naturali] either in its organ or its object, is the most spiritual, the most perfect, and the most universal of all the senses. After this comes the hearing and then the sense of smell, which require a natural immutation on the part of the object; while local motion is more perfect than, and naturally prior to, the motion of alteration, as the Philosopher proves. (Physics, VIII, 7-260a28)
Touch and taste are the most material of all. [. . .] Hence, it is that the three other senses are not exercised through a medium united to them, which makes unnecessary any natural immutation in their organ, which happens in regard to these two senses. (Summa Theologiae, I q. 78 a. 3; emphasis added)
Aquinas writes that Aristotle ‘observes that the origin of smell is affected by the odorous through a medium, i.e. air or water’ (Commentary on the Soul, no. 491). Later in the Commentary, Aquinas writes: ‘the tasteable is something tangible, i.e. discerned by touch [and] that is why it is not sensed through a medium extraneous to the body’ (no. 502). Also, ‘touch does not perceive through an extraneous medium, but through one that is conjoined with the subject, i.e. through flesh’ (no. 502). But later still, apparently to the contrary, he writes that ‘we perceive all sense objects through an extraneous medium’ (no. 542). He then notes that ‘this is not noticeable in taste and touch’ (no. 542). There is no apparent resolution for this discrepancy of theory on the need for a medium between that found in the Summa Theologiae and that articulated in the Commentary on the Soul. This may be an instance illustrating that Aquinas at times is somewhat careless in keeping all of his philosophical arguments straight.
The next chapter spells out in some detail the necessary conditions for sensation using the external senses, especially involving a proper sensible. This is analysed in the venue of a three-term necessary relation involving the object, the medium, and the faculty. All three of these terms refer to real relations that are necessary conditions for the act of sensation. An alteration in any of the terms entails a differing awareness. This theory may offer an alternative to the standard ‘arrow of consciousness’ mode first articulated by Moore and later adopted by several twentieth-century philosophers. Hints of this ‘arrow of consciousness’ mode are found in Plato’s texts and in the writings of some of the early modern philosophers. Appendix 1 discusses in some detail the role of light, colour, and the medium. This is an aspect of medieval science that may not be necessary in coming to terms with Aquinas on sensation and perception; but it is important for seeing how Aquinas develops his position while rooted in Aristotle’s De Anima. A second appendix indicates how Aquinas treated the tenets found in classical atomism.
-  Because this present inquiry is limited to the act of visual sensation, the above passage is included forthe sake of informative completeness rather than for analysis. Aquinas, therefore, views the concept of amedium as a necessary ontological structure for an act of awareness with at least three sense organs andfaculties of the external sensorium. Nonetheless, all of the sense organs require a medium.