Over and above beings endowed with mere vegetative life, we find beings in nature that are also provided with sensitive activity. We have already seen that the principle of this activity resides in a special kind of soul, the sensitive soul. Common experience, moreover, testifies to the presence of three basic types of faculties in this soul, namely, those of sense knowledge, those of sense appetite, and the faculty of locomotion. These faculties and their vital activities will be considered in the order mentioned.
Sense knowledge results from the immediate action of material objects on the senses. Following Aristotle, St. Thomas distinguishes two groups of faculties in the realm of sense knowledge, the external senses and the internal senses. The first are moved directly by sensible objects, which cannot be perceive unless they are externally present. The second group obtains knowledge of material objects through the medium of the former. Furthermore, since some of the internal senses retain the species of perceived objects, they can have a certain knowledge of them even in their absence. It should also be noted that the terms "external" and "internal" do not necessarily refer to location of the organ of sense. There may quite possibly be external senses in the body whose organs do not lie on the surface. For example, as understood by Aristotle the external sense of touch and its organ or organs are not on the outside, but within the body.
Some authors begin the study of the senses with a preliminary consideration of the metaphysical principles relating to knowledge in general. Such considerations, we think, are better left for the opening chapter on intellectual life, where they can be more fully applied. The present study of the senses, therefore, begins immediately with the analysis of sensation.
I. THE EXTERNAL SENSES
Our discussion of sensation is based on Aristotle's treatment of the same problem in De Anima 1 and De Sensu et Sensato. In the main, St. Thomas follows the doctrine of Aristotle, though sometimes his procedure and arrangement of the matter are slightly different.2 Later commentators, John of St. Thomas especially,3 have added their own developments to the subject, so that in handling the various sources care must be had to identify the personal contributions of each author.
1. The Problem of Sensation in Aristotle
Almost invariably the first reaction of modern psychologists to the Aristotelian doctrine of sensation is one of bewilderment. This impression arises not only from a difference of tool and technique, but even more from the way the whole problem is conceived. In the Aristotelian discussion of the matter one of the first questions to be raised, if not the first, is whether a faculty of sense knowledge is an active or passive potency. At the very outset, then, the problem revolves around the metaphysical topic of act and potency. Such an approach, it need hardly be said, is very different if not utterly foreign to the way modern psychology goes about its inquiry.
In any event, according to Aristotle sensation in its initial phase is a passivity, that is, a being acted on. To sense is in the first instance to undergo an action (pati), to be altered in some way. In this operation the active principle is the perceived object. This position manifestly is a reaction against the Platonic doctrine of sensation, which minimized the role of the sensible object. In the view of Aristotle the external thing itself somehow moves and determines the sense faculty, and sensation results from the faculty being moved and acted upon, that is, from a passion.4 But the alteration undergone by sense is utterly irreducible to the modification that results from a purely material thing being subjected to an abrasive or frictional action. The sense faculty, at least in normal sensation, is in no way impaired by reason of its passive role; on the contrary, in being thus acted on, it attains its proper perfection. All this is by way of saying that when the sense faculty receives a form, the mode of recept ion is utterly different from receiving a form in the entitalive order. This difference is construed as the capacity of sense to receive the form without the matter. We are about to see how St. Thomas avails himself of this point in developing th doctrine of sensation. For the moment keep in mind that fo his master Aristotle, one of the most striking things about sensation is its passivity.
2. The Passivity and Activity of the Senses According to St. Thomas
In substance, St. Thomas adopts the Aristotelian position set forth in the foregoing paragraphs. Thus, in the Summa he writes: "Now, sense is a passive power, and is naturally immuted by the exterior sensible." 5 The same view is expressed in the Commentary De Anima. "To sense," he asserts, "consists in being moved and acted on. For, when the sense is in act, it undergoes a certain alteration. But when a thing is altered, it is moved and acted on."
Sensation, then, is the result of an object acting on sense, which, from this point of view, must be considered a passive potency. But is it only passive? Do we not also, and as a matter of course, speak of the activity of sense? Certainly we do; and St Thomas was not unaware of it. In some places, even, the more active role in sensation is seemingly attributed to the faculty itself or to the soul, instead of the object. For example, u. the Commentary De Sensu et Sensato, speaking of sight he says: "Sight, considered in its being or reality, is not a corporeal passion,7 but its principal cause is the power of the soul." 8 Despite appearances, there is no discrepancy here. We have but to remember that in the process of sensation there are two phases: the passive phase, in which the sense is informed and determined by the external object; and the active phase, which properly constitutes the act of knowledge, and in which the informed faculty determines itself. This is how the commentators in general explain the matter, and it may be that their interpretation lays more stress on the active side of sensation than does the literal text of Aristotle. Initially and fundamentally, however, sensation is a passivity or passive process. St. Thomas on his part is careful to elucidate the special character of this passivity, which, as we have seen, is not to be confused with the passivity of matter.9 He notes that a subject receiving a form may receive it, and so be affected, in two ways. In other words, the modification of the subject may be of the material order, and this he calls a natural immutation, immutatio naturalis; or, it may be of the immaterial order, and this he calls a "spiritual" immutation, immutatio spiritualis. In the first case, when the form is received the subject is changed in its natural being, esse naturale; in the second case, it is modified in its intentional or objective being, esse spirituale.
Both types of alteration may be present in sensation; but it is the so-called spiritual immutation that gives the immediate and proper determination to the act of knowledge. Indeed, the unique passivity that characterizes the fact of knowledge corresponds precisely to this second modification or informing of the knowing potency.")
It may be observed, moreover, that in the view of the ancients and Scholastics hot h types of passivity are in fact met with in the operations of the lower senses, touch and taste, since the organs of these senses arc modified in their natural being. Whe the hand touches a warm object, it is actually and physically warmed; at the same time the sense of touch has a cognitive experience of warmth. Smell and hearing, they believed, did not involve a physical modification in the sense faculty, only in the object. The bell, for example, vibrates when ringing. As for sight, this, they thought, resulted from an intentional or immaterial reception alone, without any physical change either in the organ or object of sight. Today, with more accurate methods of investigation, we should doubtless find that in every sensation there is also a physical change in the corresponding organ of sense.11
3. The Sensible "Species"
Sensation, to repeat, means the receiving of a form by a pssive subject. What, exactly, is this form? In Aristotelian tclininology it is called a "species." Sometimes it is further qualified as impressed species, species impressa, to distinguish it from the expressed species, species expressa. The first designates the form that is the principle of knowledge, whereas the second denotes the form that is the term of knowledge, or the object as known. St. Thomas himself speaks only of "species," without the qualifications "impressed" and "expressed," and by species he means the form that constitutes the principle, or initiatory phase, of knowledge. For form as known he uses other expressions. We shall do likewise.
a) The reason for the species. - The proper function of a species, whether sensible or intelligible, is to cause the exterior object to be present to the faculty of knowledge. With the exception of the divine essence in respect of the beatific vision, and the angelic substance in respect of the angelic intellect, a knowable object of any kind cannot form a cognitional union with a knowing faculty except through the medium of a species form. This is twice true of the sense object, which is material and therefore has to be raised to a preliminary degree of immateriality even for sense knowledge. In the species the object possesses this prerequisite mode of immateriality and can therefore determine the act of sensation in its initial stage, but it is the sense faculty itself, after being informed by the species, that actively elicits the act of sensation.
b) The production of the species - As Descartes rightly suggested, species must not be thought of as elf-like creatures, flitting unceasingly to and fro. In other words, the production of the species does not consist in dislodging a form from the object to the faculty of knowledge, but simply in the actuation of the faculty through the influence of the object. It may be asked, however, whether this influence can be brought to bear on the faculty in a direct manner and by the sole power of the thing perceived. This point does raise something of a difficulty. Before the object can specify a potency in its own order of operation, the object itself must be in act in the same order. In intellectual knowledge, for example, whose object at first is not intelligible in act, a special power, the agent intellect, is required to render it actually intelligible. Is such a special power necessary for sense knowledge? Is there need, in other words, of an agent sense, as it were? St. Thomas does not think so. In contrast to objects of the intellect, objects of sense can be considered as already in act, that is to say, as already existing on a level in which they can actuate the senses immediately and thus determine the formation of the sensible species.
4. The Object of Sense Knowledge
a) What, strictly speaking, is the object of sense knowledge? How much, that is, of the reality of external things do we reach through sensation? Certainly not their total being. Like every faculty of knowledge, the senses can have a direct apprehension of forms only:
objectum cujuslibet potentiae sensitivae est forma prout in materia corporali existit.
More precisely, it is not the substantial form or the essence of things that is perceived by sense, but only accidental forms; and not even all of these. The senses, it seems, grasp only accidental forms that are external to the object:
sensus non apprehendit essentias rerum sed exteriora accidentia tantum.
In a word, the object of the senses is the third group or species of qualities, properly known as sensible qualities, together with the quantitative determinations of bodies.
b) In a well-known passage of De Anima Aristotle groups the objects of sensation into three main classes, naming them proper, common, and per accidens.12
The proper sensibles are the particular objects of each of the five external senses, namely, color, sound, odor, taste, and the aggregate of qualities perceived by touch, such as warmth, cold, weight, resistance, and the like. They are called proper because they pertain each to one sense only. Moreover, since the proper sensibles specify each its own sense, they must be specifically distinct one from another. Accordingly, each sense perceives its own sensible and cannot be affected by the sensible of another. The common sensibles, as their name implies, can be perceived by more than one sense. By common agreement there are five such sensibles: size, shape, number, movement, and rest. Sight, touch, and perhaps hearing, have a certain perception of these qualities. The common sensibles, however, are not cornpletely independent and separate objects of sense; they always presuppose the knowledge of the proper sensibles, upon which they impose a further modification. Thus, when I see a colored surface, properly speaking it is the color of the surface that specifies my sight; but the size or extension is also known by sight, and it could be known by some other sense as well. The sensible per accidens is not directly apprehended at all by the senses, but is joined to things that are sensed. I may, for instance, see a colored object, which turns out to be an animal; so I say that I see an animal. Not being directly perceived by sense, these sensibles, it is clear, are not a real issue in any given doctrine of external sense knowledge.