6. Sense Faculties and the "Medium"
For a better understanding of sense knowledge, a few points should be made clear regarding the structure and working of sense faculties.
a) The nature of sense faculties. - It is apparent that sense faculties are organic powers and therefore depend not only on the soul in which they have their source and principle, but also on the body, in which they manifest themselves through component parts forming well-defined organs. Even the mere superficial attention to sensation reveals this fact. It follows that in the state of separation from the body the soul retains its sensory powers only in principle, or radically, and cannot perform the acts of sense.
Since, moreover, the senses are not utterly spiritual powers, they cannot reflect on themselves in the strict meaning of the word; which means they cannot have a clear and distinct knowledge of their own activity. Aristotelian psychology does, however, attribute a manner of reflection to one internal sense, the common sense; and to this extent one may speak of a sensory self-awareness or consciousness.18
As for the physiology of sense organs, Aristotle took great interest in it. But it should not surprise us that some of his statements on the imit ter, though not lacking in discernment, stand in need of considerable review and rounding out. One of his more basic and more durable ideas in this connection was that sense had to be devoid of whatever form it was to receive. To illustrate, the pupil of the eye could receive all colors because it was itself colorless, being formed, so to speak, of water.
b) The "medium" in sense knowledge. - That sensation requires both the sense faculty together with its organ and an object to determine the faculty is obvious enough. Perhaps not so immediately evident is the further and universal need of a certain auxiliary agency called the "medium." Nevertheless, the existence of the medium is verified on two grounds.
First, with respect to three senses, sight, hearing, and smell, the medium is simply a fact of experience. In these senses the organ is separated from the sensible object by a greater or lesser gap of air or moisture, which, it is apparent, serves as a medium of transmission. Secondly, it is also a clearly demonstrable fact that the suppression of the medium may entail the suppression of the sensations in question. Placed squarely against the eye, the colored object is no longer perceived as colored; and if the sounding object is brought too close to the ear, what had been a distinct sound becomes a jumbled noise. It would seem that the energy emanating from the sensible object has to be refracted by a medium before it can suitably move the organ. Curiously enough, Aristotle also holds for a medium in the case of touch and taste, where, in contrast to the aforesaid senses, direct bodily contact with the sensible object appears to be necessary. Still, there is a medium for these senses, which is the surface flesh of the body. The organs of touch and taste, on the other hand, are internal to the surface.
Like the organs of sense, the medium must be in a state of neutrality to the forms it is to receive and transmit. Thus, the "diaphanum," which is the medium for sight, is colorless; and in like manner the medium for sound would be soundless. In touch and taste the medium, as we have said, is the surface flesh. Since this is composed of various elements with qualities of their own, the medium in this case consists in a certain balance or "mean" between the tangible qualities inherent in the flesh. When the latter is in this mean condition, the organ of touch is receptive to such tangible qualities in the external object as exceed that mean in either direction. Accordingly, the hand that is in a mean condition can perceive hot or cold in the object it touches.
In the Aristotelian doctrine of sensation, therefore, the medium plays a very important part. As already indicated, its first role is one of transmission with respect to the form received by the senses. The ancients and Scholastics also ascribed to it a protective function in regard to the organs of sense, seeing that direct physical contact with the object might be harmful to them. Certain commentators, moreover, believe that the medium also serves to "immaterialize" the forms to be received by the senses. The medium then would be the instrument whereby forms become actually sensible, or sensible in act.
7. The Number of External Senses
Before the number of external senses can be assigned we need to know the basis on which one sense differs from another. This basis cannot be the organs, since these are relative to the senses and exist for their sake. Much less, on the same ground, can it be the various mediums serving the senses. Nor can it be the different natures of sensible qualities; for, as St. Thomas argues, considered in the abstract these are not sensibles but intelligibles, and only intellect can know the natures of sensible qualities. The basis, however, for the number and distinction of the senses must be found in something that pertains formally to sense, namely, in the qualities that are individually proper to the senses, meaning the proper sensibles 19
Consequently, it is on the basis of the proper sensibles that Aristotle enumerates five different senses, a classification that has become perennial and classical.20 Aristotle gives no further a priori reason for his enumeration, which would seem therefore to rest simply on common observation. St. Thomas, who usually makes it a point to find an intrinsic reason for such groupings, pi ()vides us with two separate interpretations for the number and distinction of the external senses.
a) One explanation begins with the degree of immateriality pertaining to a sense, the immateriality being inversely propoi tional to the scope of physical alteration attending the socalled "spiritual," that is, immaterial "immutation" of sense. I lighest in rank, from this point of view, would be sight, since the ancients believed it involved no bodily change at all. Below sight belonged hearing and smell, which entail a modification in the object. Lowest of all would be taste and touch, which imply an alteration not only in the object but also in the organ. Generally speaking, the principle underlying this interpretation, namely, the criterion of immateriality, is certainly as valid today as it was for St. Thomas; the details of the scheme, however, would need to be considerably retouched in the light of present-day knowledge.
b) In the Commentary De Anima 21 St. Thomas classifies the senses according to their usefulness and purpose in animal life. All living things, he observes, must have a nutritive power, but not all need to have faculties of knowledge. Animals of course do, but even among them some require more senses than others. From this approach St. Thomas discovers two broad classes of senses: first, the lower senses, which are so fundamental that without them animal life is simply impossible, namely, touch at the very least and, perhaps, taste; secondly, the higher senses, hearing, smell, and sight, which represent a much more perfect degree of animal life and are capable of perceiving their objects from a distance. This twofold division has its justification in the fact that higher animals must move about from place to place to provide their sustenance, a condition which, plainly, calls for a greater number of senses. Lower animals, on the other hand, find their means of subsistence within immediate reach; not being compelled to forage, as it were, they can do without the ability to perceive objects from a distance. This again, admittedly, is a very ingenious and resourceful explanation, but also one that is not easily verified in every detail.
Problem regarding touch. Is the sense of touch one or many? What has given rise to this question is the great variety of sensory experiences commonly attributed to touch, such as the sensation of muscular effort, of light and heavy, of warm and cold and pain, to mention the more obvious ones. St. Thomas himself had some doubt in the matter, leaning to the view that touch is a kind of genus embracing several species .22 Modern psychology takes for granted the existence of a special kinesthetic and muscular sense as well as a temperature sense; whereas the experience of pain is thought to belong to the emotional side of human experience. Aristotle, on the other hand, showed some inclination to assimilate taste to touch, considering taste as a kind of touch that is restricted to the tongue.23
8. The Aristotelian Theory of Sight
The Aristotelian theory of sight, which in general is adopted by St. Thomas, deserves separate treatment not only for its own sake but also for the special applications that are made of it both in philosophy and theology, such as in the matter of intellectual knowledge and in the doctrine of faith and the beatific vision."' The object of sight is the visible. Within the realm of the visible we find two things, color and the illuminable. Color is visible in itself, whereas the illuminable is only visible through color. Considering the matter more closely, we note, furthermore, that all bodies, whether transparent or opaque, have in common a certain quality, the diaphanum or transparent (perspicuum). Of itself, the diaphanum is a pure potency; it is actualized by fire or the celestial bodies, and when in act, it is light. lint light itself is only a principle of visibility; it does not become visible in fact until it is actualized by color, which then becomes the limit or extremity of bodies. Therefore, an object will be actually visible when its diaphanum (which it has in common with other bodies) is illuminated by light (that is, reduced by fire or the celestial bodies to its act, which is light) and furthermore determined by color. In all this, to be noted, there is no trace of local movement; the whole process is a qualitative alteration.
In De Sensu Aristotle opposes this theory to the emission theory of sight taught generally by Plato, Empcdocles, and Democritus. Sight, according to these philosophers, is to be interpreted as a luminous ray emanating from the eye. Empedodes, if not the others, thought that the eye consists of fire, and therefore is capable of emitting rays that make it possible to see surrounding objects. Another version of the theory is that exterior bodies also send out minute particles, and sight results from the stream of rays encountering the stream of particles. Aristotle rejected the notion that the eye is a sort of luminous center emitting rays of light; nor did he believe it to consist of fire, but rather of water, and its role in relation to the object was one of pure passivity. His basic color, moreover, was white, to which black was opposed as its absence or absolute minimum. The intermediate colors were formed of mixtures of black and white.