Sensation

Article 2. Sensation

a) Nature of Sensation

Sensation is a word of manifold meaning. In what may be called its operative meaning, sensation is the activity of a sense as it lays hold of a suitable object. In another phase of meaning, sensation is the result or fruit of the activity of a sense. Thus the activity of the sense of sight, exercised in apprehending a visible object, is sensation. And the fruit of the activity, the finished product of seeing, is a sensation. To put the matter in another way, sensation, as an activity or operation, is more precisely called sensing; the object sensed, as now apprehended or known, is a sensation. A third meaning of the term sensation,—a meaning which, in some manner, includes the other two,—is this: sensation is the capacity or power of a living body to exercise the sense and to experience in itself the result of this activity.

Contrast sensation and perception. When I sense an object,—say, for example, a warm radiator,—I gather various points of knowledge, I experience several phases of the thing sensed. I sense warmth, smoothness, hardness,—all by the sense of touch or feeling. Each of these is a percept. The sum of the percepts is my sensation (that is, the fruit or product of the sensing) of the warm radiator, acquired by the sense of touch. I sense the object by perceiving that it is warm, and hard, and smooth. Thus a sensation is sometimes the sum of several percepts. Sometimes, however, the sensation is simple and not compound, and then the sensation and the percept are one and the same. Indeed, sensation and perception are only two aspects of one reality. In so far as my senses make me aware of objects, I experience sensation; in so far as the objects present phases for my sensing-activity or grasp, I perceive them and experience perception. I perceive, for example, that the morning cup of coffee is liquid, and hot, and aromatic, and of definite flavor and color. Sight, touch, and taste are the senses employed to gather these percepts which come together to make up my sense-knowledge or sensation of coffee.

Sensation as an operation (that is, as sensing) is the activity of a sense. Now a sense is an organic cognitive faculty; a sense is a bodily knowing-power. While we speak of bodily appetition (tendency to follow and act upon sense-knowledge) and locomotion (movement to carry out the drive of appetition) as sentient, we do not call these powers senses; we call them sentient because they belong to that order or grade of life which is, first and fundamentally, a sensing order, and because appetition and locomotion presuppose the activity of sense. A sense, we repeat, is an organic cognitive faculty. Each sense is exercised by means of the body or by a special bodily part or member, and this is called a sensory, a sense-organ, or simply an organ.

That which sense lays hold of through its organic action is the object of sense. Sense-objects are known as sensile objects, sensiles, or sensibles. An object knowable by one sense alone (as color, for example, is knowable by sight) is called the proper object of that sense. An object that is knowable by two or more senses (as bodily movement, for instance, is knowable by sight and also by touch) is called the common object of the senses concerned. Proper and common objects come together to constitute the object per se of the senses which perceive them. A sensile that is not directly perceivable by any sense (that is, not perceivable per se or in itself), but is sensed by its known association with that which is directly sensed, is called the accidental object or, more commonly, the object per accidens of the sense that indirectly perceives it. Let us illustrate this matter. I can tell whether a wheel is in motion by looking at it, by touching it, and even by listening to find whether it makes the humming sound usually made by whirling objects. Now, bodily movement is the common object of sight and touch, but it is not the object per se of hearing at all; one cannot hear movement itself. But I know by experience that a whirling wheel usually makes a humming sound, and through the sense of hearing I grasp this sound as the proper and per se object of that sense. And through this per se object I (indirectly or accidentally) perceive what experience has taught me to associate with it, namely, the movement of the wheel. And thus I say that bodily movement is, in this instance, the object of hearing: I hear the wheel move,—not indeed per se but per accidens. Similarly, I see that the ground is wet after a shower. Now, I cannot see wetness; it is not the proper object of sight, nor is it the common object of sight and another sense. But I know by experience that wet ground has a certain appearance; this appearance is visible, and, accidentally to this visible object, I perceive the wetness—I see that the ground is wet. I perceive the wetness per se by the sense of touch, and can know that the ground is wet by touching it with my hands or walking barefoot upon it. But I also perceive the wetness per accidens by the sense of sight, and I see that the ground is wet.

We may sum up this important doctrine on sense-objects in the following simple schema:

f proper to one sense alone

perse< r

common to two or more senses Sense-objects-

per accidens

Sometimes the object per accidens is not, in itself, a sense object at all; it may be an object knowable only to the intellect, but manifested or accompanied by sensible characteristics, and through these the sense is said to perceive the object itself. Thus we see that a man is alive. Life is not visible, nor is it the object of any sense. We do not hear life or smell or taste it; we do not know it by sight or by touch; we cannot have a concrete image of it in the interior sense called imagination. But we understand what life is. Our intellect, rightly interpreting the findings of the senses, has discerned properties and attributes in living bodies which enable it to formulate the concept, the understanding grasp, of what life itself really is. And since the essence of reality called life is regularly manifested in organisms by sensible signs, the senses which lay hold of these signs are said to perceive life, not indeed per se, but per accidens. We know that a man is alive because we see him move, we hear his voice, we feel the vibrant grasp of his hand. These things are sensible per se, and by and through them (i. e., per accidens) we sense the life which they manifest.

b) The Sentient System

The chain or connected group of organs which make up the equipment of a living body for a certain type of activity is a system. The sentient system is part of a larger system called the nervous system, through which both vegetal and sentient operations are exercised. The vegetal operations of man (nutrition, growth, and the formation of the reproductive cells) are discharged by nerves and muscles which constitute the sympathetic system. The sentient system is properly called the cerebro-spinal system. It is apparent that the sympathetic system and the cerebro-spinal system together constitute the human nervous system. The cerebro-spinal system has three main parts or branches, to wit, an inner, an outer, and a connecting part. The inner part is the cerebro-spinal axis, and consists of the brain and the spinal cord. The outer part consists of the external sense-organs. The connecting link between inner and outer parts is made of the cerebro-spinal nerves.

i.The cerebro-spinal axis consists of brain and spinal cord. The brain has three main parts. The larger part (called the large brain) fills most of the skull from the forehead back; it is called the cerebrum. It is a soft mass of matter, made of an outer coating of gray cellular substance and an inner body of white nerve-fibers. The cerebrum is, so to speak, folded into its place, and has, in consequence, deep furrows and wrinkled folds; the furrows are called fissures and the folds are convolutions. The chief fissure runs through the center from front to back and lies just under the “part” of hair that is “parted in the middle.” This is called the medial fissure or the median fissure, and it divides the cerebrum into halves which are called respectively the right hemisphere and the left hemisphere. Each hemisphere has a front, a middle, and a rear section, marked off by fissures, and these are called respectively the frontal lobe, the parietal lobe, and the occipital lobe. The cerebrum consists of millions of neurons or nerve-cells which intercommunicate in a marvellous manner and have, in general, a connection with the fibers or chains of neurons called nerves. The nerves reach on through the outer brain and down the spinal cord whence they branch out through the body to the sense-organs, the muscles, the blood-vessels, and the glands.

Below the cerebrum or large brain, and at the back of the head, lies the second main part of the brain: it is called the cerebellum or the little brain. Like the cerebrum, it is divided into two hemispheres.

Under the cerebellum, and connected with it (and through it, with the cerebrum) lies the third main brain-part, which is the widened upper end of the spinal cord: it is called the medulla oblongata or oblong marrow

The cerebrum seems to be the seat of sense-memory and imagination. The cerebellum appears to have much to do with controlling and coordinating bodily movements (locomotion) and may be the seat of sense-appetency (appetition). The medulla oblongata is the seat of the nerves of the face, and of the nerves which control the activity of heart and lungs; it seems also to be the center of the vegetal operations in man.

The spinal cord (which spreads and flattens at the top to form the medulla oblongata) narrows at the base of the medulla and enters the hollow center of the backbone, or spinal column, extending downward to the lower sections of the backbone, thinning as it goes. The spinal cord consists of the various nerves (i. e., nerve-fibers) which are welded into larger units called tracts. The backbone or spinal column, which is the bony case of the spinal cord, is made up of little sections, set with comparative looseness one on the other; the sections are called vertebrae. Nerves branch out from the spinal cord through the space between each pair of vertebrae, and terminate in organs, muscles, blood-vessels, and glands. The cerebro-spinal nerves constitute the connecting link between the outer organism and the cerebro-spinal axis; we shall speak of them in a moment. It is interesting here to notice that the spinal cord is made of an outer shell or coating of white nerve-fibers uniting into tracts, and an inner core of gray cellular matter, while the cerebrum and cerebellum are gray outside and white inside. The medulla oblongata has a structure like that of the spinal cord, and indeed it is the widened upper portion of the cord itself.

2. The cerebro-spinal nerves which connect the organs, muscles, blood-vessels, and glands, with the central axis, reach from the spinal cord through the spaces between the vertebrae and extend through the organism. Some nerves carry impressions inward from organ to brain; these are sensory nerves (called also afferent nerves). Other nerves carry an impulse (which is a response to the stimulus carried in by the sensory nerves) outward to organs, muscles, vessels, and glands; these are efferent nerves or motor nerves. Efferent is the better name for these nerves; it means “carrying outward”; and motor nerves are specifically those efferent nerves which carry impulse to the muscles of the organism so that some movement results. Sometimes the sense-stimulus is carried by the sensory nerves all the way to the brain, and the response comes from the brain along the track of the efferent nerves. But often the sensory nerves do not carry their stimulus the whole distance to the brain; their “message” or stimulus jumps across to the motor-process, and the reaction or response goes to muscle and organ by a shortened route. When this occurs we have what is called a reflex. A reflex is an involuntary response to sense-stimulus. Coughing, sneezing, winking the eyelids many times a minute as we all do,—these are reflexes. A cough that is forced is not, of course, a reflex; nor is a deliberate blinking of the eyes. A stimulus that goes all the way to the brain arouses consciousness which, in man, is an awareness of the stimulus which gives him some choice in deciding what to do about it. But in reflexes the will has no play. We may be conscious of the reflex after it has occurred, and indeed so immediately afterwards that our awareness of it is almost concomitant with it. But in reflexes the stimulus and response are over and done before any choice or decision can be exercised. The time-interval between a stimulus and its response is usually very brief, and it varies for the various senses and for different conditions and circumstances which affect the organism. This time-interval is known as reaction-time or psy~ chologicdl time. Some success has been achieved by psychologists in measuring it experimentally. The interplay of sensory nerves and efferent nerves is wondrously complex. Some appreciation of it may be gained from the consideration of the nerve-connections possible within the brain itself. On this point, Professor H. C. Warren (in Human Psychology, ed. 1920, pp. 41, 42) says, “We may liken the brain to a telephone exchange, in which any one of thousands of subscribers may be joined up with any other. The analogy is not quite correct, since sensory neurons are never joined with other sensory neurons in complete circuit. If we suppose our telephone wires divided into two groups, those which receive messages but do not respond, and those which make calls but never receive messages, the analogy will hold.”

5. The sense organs or sensories are those external bodily parts which are structurally adapted for various sentient functions, and which receive impressions from their respective objects and so take on the stimulus which is then conveyed to the central axis (or through a reflex circuit) by the sensory nerves, and knowledge (i. e., sentient knowledge) results or may result. We shall gain a fuller knowledge of the sensories and their operation from the study of the various senses and their respective objects which is now to follow.

c) The Senses and Their Objects

There are in man five external senses and four internal senses. The external senses are: sight, hearing, smell, taste, touch. The sense of touch (which is what one ordinarily means by bodily feeling) includes what some physiologists call the resistance-sense, the temperature-sense, the sense of pleasure, the sense of pain, and the muscular sense. The internal senses are: the imagination, the sentient memory, the central sense, and instinct. The imagination is sometimes called the fancy. Instinct is often called by the old name of the estimative sense or the estimative power, or simply the estimative.

i. Sight or vision is the sense by which bodily objects are perceived inasmuch as they have colored surface or surfaces. We judge many things on the basis of what we see,—distance, for example, and motion. But the proper object of the sense of sight is colored surface. Things wholly colorless cannot be seen, nor can they be judged as to size, distance, or motion. It is because a bodily object is colored (i. e., has colored surface) that we are able to see it, and to perceive its size, shape, movement or rest, nearness or remoteness. We see size, shape, etc., as per se objects of sight, but not as the proper object of this sense; these objects are common to sight and to another sense, mostly touch. But the common objects of two or more senses are perceived respectively by these senses only in and through the respective proper objects of the senses in question. Thus, we repeat, the objects which sight has in common with another sense or other senses, are perceived by sight (and perceived per se) only inasmuch as such objects exist in a bodily reality which is colored, or, to be precise, which has colored surface.

The organ of sight is the eye. The eyeball is somewhat like a camera. The pupil is the widening and contracting “shutter” over the lens, and, at the back of the eye, in place of film or plate, is the retina. The space between lens and film (pupil and retina) is not empty, but is filled with a clear jelly-like mass called the vitreous humor. The retina is no lifeless film, but is capable of receiving endless successive impressions in a vital manner; it is connected with the optic nerve (or seeing nerve), the terminations of which constitute what are called the rods and cones of the retina. A visible object (i. e., an object presenting colored surface under due conditions) falling within the range of vision, casts its image on the retina, whence by the rods and cones (i. e., the optic nerve) it is carried to the brain, and the response is the active seeing and the knowing of the object as a thing seen.

The medium through which colored surfaces are carried, so to speak, to impress their images upon the organ of sight, is ether in waves or in wavy motion. The length of the waves and their frequency vary for the various colors. (Modern psychologists speak sometimes of “colorless” sensations of sight, in which blinding white, gray, and black affect the visual or seeing sense).

2. Hearing or audition is the sense by which bodily objects are perceived inasmuch as they have sound, that is noise, or tone with intensity and pitch. The proper object of hearing is sound, and it is carried to the sense through the medium of air-vibrations. The organ of hearing is the ear, particularly the membrane of the inner ear, in which the terminations of the acoustic nerve (or hearing nerve) are located. Bodily objects which come together or strike on one another in such wise as to set up suitable air-vibrations are the originators of sound; the vibrations vary in rapidity and volume for the different sounds. These vibrations pass through the air and are caught by the outer ear and directed into the channel which guides them to the membrane of the inner ear where they stimulate the terminals of the acoustic nerve, and hearing results.

5. Smell or olfaction is the sense by which bodily objects are perceived inasmuch as they have odor. Odor is the proper object of this sense. The organ of smell is the nose, especially the membrane which lines the upper nostrils and which contains the terminations of the olfactory nerve (i. e., the smelling nerve). Minute particles of odorous substance are breathed in through the nostrils with the air, and when these come in contact with the nasal membrane, they stimulate the terminals of the olfactory nerve, and the operation of smelling results.

4. Taste or gustation is the sense by which bodily objects are perceived inasmuch as they have flavor or savor. The proper object of taste is savor in a sapid substance which comes, while in solution in the saliva, into contact with the terminations of the gustatory or tasting nerve on the tongue and palate. The terminals of the gustatory nerve are called “taste buds.” When these are stimulated in the manner indicated, the operation of tasting results.

5. Touch or feeling is the sense by which bodily objects are perceived inasmuch as they are hot or cold, hard or soft, rough or smooth, pleasurable or painful, taut or relaxed, wet or dry or “sticky,” etc. Touch is a most complex sense; its proper object must be discerned in its definition just given. The organ of touch is, first and foremost, the dermis or underskin, particularly the papillae or buds of the dermis, for these are the terminations of the tactual nerve or touching nerve. Such papillae seem to exist also within the organic tissues or flesh throughout the body; or it may be more accurate to say that the tactual nerve sends its fibers nearly everywhere throughout the body-mass as it reaches on to the dermis, and wherever these fibers are stimulated, throughout their length at any point, the sense of feeling is exercised. The dermis-papillae are most numerous in the finger tips. Contact with tangible objects stimulates the tactual nerve and gives rise to tactual sensation, that is, to the operation of touching or feeling. The sense of pleasure is regularly a result of high and pleasing stimulation of a well-conditioned tactual nervous structure; the sense of pain is regularly the excessive stimulation of the tactual nerve, or the stimulation which an ill-conditioned nervous structure is not fitted to receive. The muscular sense is the sense of touch inasmuch as it perceives movements within the body or of the body, especially movements of the joints and the loosening and tautening of skin and body-structure which often accompanies such movements.

6, The imagination or fancy is the inner sense which is fitted to perceive, preserve, and reproduce in concrete image, the findings of the outer senses; it can also rearrange, reconstruct, exaggerate, minimize, cartoon, and commingle the images once formed upon external sensation. The organ of the imagination is in the brain; probably this organ is located in the outer part of the cerebrum, but its exact location in the brain-structure is not known. All inner senses have their respective organs in the brain, but the particular part of the brain to which each internal sense should be organically ascribed is a matter of dispute among scientists. The student will please notice that the imagination is, first and foremost, a sense which retains and reproduces the images that come in from outer sensation. Only in its secondary function is it the faculty of poet and dreamer, which we ordinarily indicate by the name imagination or fancy. Nor can this faculty evoke any image except that which has somehow come from actually experienced sensation. Wild and extravagant as imagination may sometimes be, its most startling pictures are still the product of things once actually sensed. For while imagination can rearrange, reconstruct, exaggerate, minimize, expand, reduce, commingle, cartoon, and transform the sense-images and parts of sense-images once actually experienced in fact, it remains ever true that the elements of the imagination-images have actually been experienced. In a word, the imagination is not a creating faculty. In its first and fundamental service it is a reproducing faculty which faithfully records, retains, and, upon due stimulus, evokes, the images of things sensed. In its secondary service, the imagination may be called a rearranging faculty.

7. The sentient memory or sense-memory is the inner sense by which sensations once experienced, and now reproduced in imagination, are recognized as once had, as of the actual past. Sense-memory does not call up the past, nor does it reproduce images once experienced; we have just seen that the sentient evoking and reproducing faculty is imagination. It is the proper function of sense-memory to recognize the past as reproduced in imagination. Imagination is stirred to the reproducing of images by experience (i. e., by sensation) identical or similar to that of the past, and by countless chains of relation and circumstance which constitute the marvellously complex “association” of recorded sensations. Imagination and sentient-memory work together; indeed, without imagination, sense-memory cannot function at all, and, without sentient memory, the service of imagination would be largely futile and illusory. But, despite their close connection and their interdependence, the imagination and the sentient memory are two distinct faculties, each with its own proper operation.

8. The central sense is the internal sense which perceives, distinguishes, unites, and divides the findings of the other senses. It is a kind of “central telephone operator.” For the several senses are not reflective; they do not perceive their own operation, but directly lay hold of trans-subjective objects. But sentient knowledge supposes a consciousness back of perception, and this is furnished by the central sense. To illustrate this point. The eye does not see that it sees, nor is the ear aware of its own operation of hearing. Yet when we see a thing we are sentiently aware, not only of the object seen, but of the fact that we see it. When we hear a concourse of sweet sounds, we are aware, not only of the sounds, but of our experience, our hearing, of the sounds. It is the central sense which operates to provide this awareness. Another name for the central sense is simply sense-consciousness. Still another name is the common sense, that is, the sense which is the common background, basis, and correlating power required by all the other senses.

p. The instinct or estimative power is the internal sense which apprehends externally sensed objects as useful or harmfuldesirable or undesirable. This sense makes the cat avoid the dog, and the mouse avoid the cat; it stirs the bird to arrange and build its nest and to care for its young. In man many actions are “instinctive,” but, since man has intellect, instinct is not developed in him, nor is it needed, in such degree as it is found in merely sentient creatures. The action of throwing out one’s arm to break a fall, or of bracing oneself against a sudden strong wind, may be called instinctive, although some psychologists regard these actions as reflexes. A human parent, however timid, will often be found brave in the face of a serious danger which threatens his children, and this in an unreasoning instinctive manner.

d) The Sensing Process

We have discussed the physiological part of sensation; here we study the sensing process in its psychological aspect.

All sensing begins with the external senses. When an object falls within range and notice of a sense equipped to perceive it, it is said to be impressed upon the sense through the instrumentality of the sense-organ. Reacting to this impression the sentient organism is aware not only of the impression (indeed, not directly of the impression at all) but of the object, the trans-subjective reality, the external sensile being there actually present. To study the mode or manner of the sensing-process, it will be well to consider the progressive steps of the activity as these are exemplified in the operation of one definite sense. We choose the noblest of the external senses, which is that of sight.

Let us suppose that we are a horse grazing in a grassy field. Under due conditions of light and distance, the normal vision cannot fail to see this object as soon as it falls under notice. If our view falls upon the grazing horse, we infallibly see it. And the process of our seeing is as follows. An image of the horse is impressed upon the retina of the eye. This is just such an image as would fall upon the film or plate of a camera set for making a picture of the horse. The image itself is the product of the object, light, the physical constitution and the chemical composition of the eye. It is a photo-chemical image. When such an image is impressed upon a camera-plate the picture is “taken,” the photographic process is done. But when such an image is impressed upon the retina of the eye, the seeing-process is just ready to begin. Here at once we notice the vast difference between “taking a picture” and exercising the sentient operation of seeing.

The impressed image,—considered now, not in its photo-chemical character but in its completed reality as a physical thing—stirs or excites the optic nerve and is taken into the organ of sight; it is now the intra-orgahic object of the operation of seeing. By the very fact of its being intra-organic, the object is sensed; it is seen. By and through the intra-organic object, the extra-organic object (the grazing horse itself) is seen. Thus what we see is, first and foremost, the intra-organic object, and this is the ‘medium by which we see the extra-organic object which is the trans-subjective reality that comes to our sentient knowledge by the seeing-operation. Still, we are not aware of the intra-organic object as such; by its unobserved mediation (i. e., its office and service as a medium) we directly sense the extra-organic object, the trans-subjective object. Because the mediation of the intra-organic object is not sentiently observed, we say that this is a wholy objective mediation.

The intra-organic object is an impressed species. Here species means image or likeness. The intra-organic object is a likeness or image of the trans-subjective object, impressed upon the sense and received within the sense-organ. The intra-organic object is also called an intentional image to indicate that this species or likeness is formed according to the intent, that is, the connatural tendency, of the knowing-power, the faculty, which forms it. The word intentional has, in this instance, nothing to do with the ordinary meaning of intention or purpose, except in so far as the term may suggest the intention and purpose of the Creator in framing our senses and their organs.

Summing up our findings thus far made, we understand that external sensation begins with an impression made by an external trans-subjective object upon a sensory or sense-organ. The sensory takes in the impression by reacting to it, and the impression thus taken in is now the impressed species or the intentional image of the object. The impressed species is the object taken into the organ (as intra-organic object). And the reaction of the sensing-power or sense-faculty to the intra-organic object is the operation of sensing whereby the trans-subjective reality (the extra-organic object) is sentiently known. We know the objects of the external senses through the entirely objective mediation of the intra-organic object.

The extra-organic object of the external senses is said to be trans-subjective. The term means that the extra-organic object is something which is really there in the world of knowable realities, and that the knowledge of it in the sentient subject is not something “made up,” not a figment of the knowing-power itself. Some philosophers use the term “objective” to express the factual quality in the extra-organic object of sensation, but this term in not so accurate as trans-subjective. For the sensile object has its physical existence as a thing that does not depend upon the sentient knowledge of any creature; it exists whether it is sentiently known or not. Yet.it is such a thing as may come across or over (Latin, trans) to the subjective world and take on a new existence,—a cognitional existence, a knowledge-existence,—in the sentient subject. The trans-subjective world is the world of knowable realities which do not depend upon the knowing-creature for existence in fact but only for existence in knowledge; not for being but for being known. The grazing horse exists in fact, whether seen or unseen by any creature; its physical existence in no wise depends upon, or is affected by, its being sentiently known. But for the horse to have cognitional existence,—existence in the knowledge of one who sees it or otherwise sentiently knows it,—it must be actually sensed; such existence actually does depend upon the operation of sensing. Therefore, the horse in its physical existence is, as an object of sense-knowledge, a reality that is non-subjective; it is independent of the knowing-subject. Yet it is a knowable object: it can, so to speak, come over or across (trans) the chasm which separates the non-subjective from the subjective realm, and take on a new (cognitional) existence in the knowledge of the creature that sentiently knows it. Thus it is accurately said to be trans-subjective.

All objects of sense must be trans-subjective. Now, there are three points upon which trans-subj ectivity stands. These are called matter, form, and presence. An object is trans-subjective in matter, or in content, when it is knowable as the precise reality that it is in fact; that is, when the knowledge of it in the knower is knowledge of that thing, that reality, and not some figment of the knower’s own making. Every direct object of knowledge (of intellect as well as of sense) must be trans-subjective in matter. Secondly, an object of knowledge is said to be trans-subjective in form, or in mode or manner, when its cognitional existence corresponds in mode to its physical or real existence among knowables. When I see the horse grazing, I see an individual, concrete object, and the sense of sight gives me knowledge of it in a species or image that is also individual and concrete; the physical existence of the object is marked by individuality and concreteness, and so also is its cognitional existence: therefore, the object so known is trans-subjective in form. We shall presently see that the objects of intellect, while trans-subjective in matter, are not trans-subjective in form, for the intellect knows things in the abstract and in universal, whereas the things known have existence in individuality and concreteness. Thirdly, an object of knowledge is said to be trans-subjective in presence when it is actually there for the grasp of the knowing-power, and is not merely evoked in image. Intellect and the inner senses of imagination and sentient memory can evoke or view their objects when these are no longer present; but the external senses cannot do so. We may think of a horse, or imagine a horse, or sentiently recognize the recalled image of a horse once seen, even when the horse is not present to the actual here-and-now grasp of the eye or other external sense-organ. But the external senses require an object that is trans-subjective in presence as well as in matter and in form. The external sense-object must be trans-subjective on all three points. For the inner senses of imagination and memory, the object must be trans-subjective in matter and in form. For intellect, the object of direct knowledge must be trans-subjective in matter.

The process of external sensing may be summed up thus: trans-subjective object (in matter, form, and presence) falls within range of activity of some suitable sense-organ under due conditions for its operation. The object is then impressed upon the sense-organ, becomes intra-organic, and is so impressed upon the sense itself, and is sensed or sentiently known. The species or image which comes through the organ to become intra-organic and thus to arouse sense-knowledge is an impressed species; and the reaction of sense to the impression of this species is the knowing-grasp of the actual physical object there present. Now, in internal sensation (notably that of imagination and sentient memory) the object must be trans-subjective but need not be so in point of presence; we can image an object or recognize an image of past experience, when the reality imaged or recognized is now no longer present to us in physical, external, trans-subjective fact. But the object, to be known at all, must somehow be set before the knowing power. Imagination calls up or evokes its image, and, so to speak, projects it upon a screen for its own contemplation or for the recognizing-activity of memory. This projected image is said to be expressed by the knowing-power, and is called the expressed species. The external senses have no expressed species, but terminate their knowing-operation by grasping the object actually present in its physical existence. The inner senses know their object (or can know it) in its absence, but only by evoking or projecting its species or image; thus the inner senses terminate their operation by laying hold of an expressed species,—a species expressed within the knowing-power itself, but not a figment of that power, at least in its essential elements.

The species (impressed and expressed) which we have been discussing are sensible or sensile species. Presently we shall study the operation of the intellect, and we shall then find that there are species of a higher order, namely, intelligible species, both impressed and expressed. We shall learn that the idea or concept of the mind is an expressed intelligible species.

We shall complete our study of the sensing-process by determining where sensation takes place. The center of sentient life (in man and the higher animals) is the cerebro-spinal axis. Impressions from sense-objects are carried from sense-organs to this center, and sensing or sensation results. Now, just where does sensation take place—in the brain (or somewhere in the central axis) or in the sense-organ ? When I touch a hot object with a finger-tip, is the sensation of heat experienced in the brain or in the finger-tip? We answer that sensation takes place in the organ. The sense-organs are marvellously well constructed for their purpose of sensing, and nature would surely have acted to no purpose in arranging such amazingly balanced and complex parts of the body if the organ were merely to serve as a kind of “push button” for signalling the brain to go into action and do the sensing. The brain (central axis) is required, of course; it is the central station and the “exchange.” But it is a fact that the actual sensing,—as most scientists now admit,—is done by and in the organs fitted for such operation. External sensation, therefore, takes place, first and foremost, not in the brain, but in the organs of the body. We may borrow an analogy,—admittedly every imperfect,—from the electric light. The lamp on my study-table shows its brightness here. A power-plant is, of course, required, and if it did not send electrical impulses along the lengthy wires which connect my house with the dynamo, I should not have the light. But the fact that the electrical power required to make light must come from the dynamo does not alter the further fact that the actual illumination exists here in a specially constructed bulb suited to receive the electrical impulses in a way that will produce illumination. Somewhat similarly, the fact that the brain and cerebro-spinal axis are necessary for sensation does not alter the other fact that external sensing is done in the organ fitted for such special operation, and not in the “central plant” itself. The manifest weakness of this analogy lies in its suggestion that all the activity considered comes from within,—from the “dynamo.” In sensing, there is another essential element in the process, namely, the trans-subjective object which is of a nature suitable to give a true impulse and impression to and through the organ of sense.

Summary of the Article

In this Article we have defined sensation and have studied its nature. We have contrasted sensation and perception as two phases of one reality. We have defined the object of sense, and have distinguished object per se and per accidens, and have listed the per se objects as common and proper. We have made a detailed study of the physiological structure and function of the sensing-system, (cerebro-spinal axis, cerebro-spinal nerves, external sense-organs). We have studied the senses severally, describing their action, their organs, and their respective proper objects. We have made a psychological study of the sensing-process, and have discovered that the object of sense must be trans-subjective. We have described and defined the species (impressed and expressed) involved in external and internal sensation. Finally, we have discussed the locus of external sense-experience.