Sensation

a) Nature of Sensation — b) The Sentient System — c) The Senses and Their Objects — d) The Sensing Process

a) Nature of Sensation

Sensation is a word of manifold meaning. In what may be called its operative meaning, sensation is the vital activity of the knowing-power of a sentient organism: the activity of sensing which is found, in greater or lesser degree of complexity and perfection, in every animal organism. This is the meaning we employ throughout the present study.

To sense an object is to react consciously to an impression received from that object through bodily organs or sensories. Sensation in this meaning is the conscious reaction, by or through bodily parts, to bodily impression. Sensation is a knowing activity; it is an awareness. It is the awareness in a sentient organism of bodily reality manifested by the qualities (common and proper) of such reality — qualities such as color, sound, shape, hardness, desirability, harmfulness.

There is a further sense in which the term sensation is used: to indicate the result or product of the sensing-activity — the sense-knowledge itself, the sense-image or percept as retained in the mind. Thus we speak of a “vivid sensation” when we really mean a vivid sense-experience or sense-image.

b) The Sentient System

The bodily system by means of which the sentient organism exercises its power of sensation is called the sensing-system or sentient 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. The sentient system has three main parts.

1. 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; it 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 rest of the brain and down the spinal cord to the various parts of the body. 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.

c) The Senses and Their Objects

There are two classes of senses: external senses and internal senses.

The External Senses. The external senses are those which receive their impressions from objects outside the organism. There are five external senses:

1. Sight is the sense by which the organism is aware of the color (and, in consequence, of the visible shape and size) of objects. Its organ is the eye, and its proper object is color.

2. Hearing is the sense by which the organism is aware of sound. Its organ is the ear, and its proper object is sound.

3. Smell is the sense by which the organism is aware of odor. Its organ is the olfactory apparatus in the nose, and its proper object is odor.

4. Taste is the sense by which the organism is aware of flavor. Its organ is the tongue (and partly the palate), and its proper object is flavor.

5. Touch is the most fundamental and most widely distributed of all the senses. The sense of touch is the “bedrock” of all sentient life. Its proper objects include the tangible qualities of solidity or resistance, temperature, and pain.

The Internal Senses. The internal senses are those which receive their impressions from intra-organic sources — from within the organism itself.

1. The central sense or common sense (sensus communis) is the internal sense which unifies the reports of the external senses, and which is the seat of sense-consciousness. It is the sense by which the organism is aware that it is sensing — by which it knows that it sees, or hears, or feels. Its organ is the brain (more precisely, the central nervous system, of which the brain is the center and the chief part).

2. The imagination is the inner sense which retains and reproduces the images that come in from outer sensation. 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.

3. 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; it is the imagination which does that. Sense-memory does not function without the aid of imagination. But when imagination reproduces a past image, the sense-memory recognizes this image as a reproduction of something once experienced — as a record of the actual past.

4. Instinct (also called the estimative sense or the evaluative sense) is the inner sense by which a sentient organism appraises objects as connatural or otherwise, as beneficial or harmful, as to be sought or to be avoided, quite apart from the merely pleasant or unpleasant character of the sensory impression. It is by this sense that the lamb flees from the wolf (not merely because the wolf smells unpleasant, but because the lamb evaluates the wolf as an enemy) and that the ewe recognizes her own lamb (not by mere sensory impression alone, but by a specific instinctive appraisal).

d) The Sensing Process

The process of external sensing may be summed up thus: a 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 act of sensation proper, producing the expressed species or the sense-percept which is the fruit and content of the sensing act.

The object of sense-knowledge is trans-subjective — that is, it exists outside and independently of the knowing subject. An object of knowledge is said to be trans-subjective in matter when it is a real bodily thing existing in itself and not merely as an image in the mind. It is said to be trans-subjective in form when its essential nature as this kind of thing is truly grasped, not merely imagined or assumed. It 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. The external senses require an object that is 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.

Summary of the Article

In this Article we have made a psychological study 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.