[§ 14]. Organic Sensations.—There are still other sensations, coming to us from the internal bodily organs; from various parts of the alimentary canal, from the organs of sex, from heart and blood-vessels, from the lungs, from the sheathing membrane of the bones; but it is doubtful if they are of new kinds; probably they consist simply of pressure, cold, warmth, and pain. The dull deep-seated pains that we call aches are, perhaps, different from the bright pains of the skin; but most of the differences among pains, differences that we express by the terms lancing, throbbing, piercing, stabbing, thrilling, gnawing, boring, shooting, racking, and so on, are either differences of time (steady, intermittent) or space (localised, diffused) or degree (moderate, acute), or else are differences due to the blending of pain with various other sensations.
The organic sensations, like the kinæsthetic, tend thus to occur in groups or complexes, and we have as yet no very sure means of disentangling them. It is, nevertheless, quite clear that in their case, as in that of the touch-blends, we have to distinguish between experience and meaning. Hunger and nausea seem, for example, to be very different; yet the core of both turns out on analysis to be the same dull pain; and we know that the onset of a bilious attack is often heralded by an unusually keen appetite, so that the beginnings of nausea are in fact confused with a growing hunger. The difference between hunger and nausea is due partly to a difference in the processes which ordinarily accompany the central pain,—motor restlessness or lassitude in the case of hunger, and dizziness in that of nausea; but more especially to a difference of meaning or interpretation; hunger stands for want of food, and nausea for indigestion.
We shall see later that organic sensations play a large part in emotion, as kinæsthetic sensations do in perception. Plato set the ‘spirited’ or ‘passionate’ part of the soul in the breast; the Psalms abound in phrases that suggest the same idea; we speak to-day of the heart coming up to the mouth, or dropping to the boots. So we read in the Old Testament that Joseph’s bowels yearned upon his brother, and in the New Testament of bowels of compassion; and the inner stir that the writers have in mind is familiar to everybody.
[§ 15]. Sensation and Attribute.—We have been talking all this while about sensations, but we have not yet said what sensations are. They make up, as you will have guessed, one class of the mental elements, the elementary mental processes of § 4, that we reach by analysis of our complex experiences. They are therefore simple and irreducible items of the mental world. How shall we define them?
We can define them, in strictness, only by writing down a complete list of what we have called their characters. Every sensation shows itself to us under various aspects, or, as we are accustomed to say, possesses a number of attributes. We have been dealing, so far, with the qualitative aspect of sensations. This may itself be single; the quality of lights is just their lightness or darkness; or it may be manifold; the quality of colours can be properly described only if we take account of hue, tint, and chroma; that of tones only if we take account of pitch, volume, and tonality, perhaps also of vocality. Quality is the natural thing to start out from, because it is what interests us most in everyday life, and has therefore been named; so that, when we speak of sensations, we speak of them by their qualities. There are, however, several other attributes; sensations possess intensity, and vividness, and duration, and some of them possess extension. We shall discuss these aspects later on.
Does it seem strange, now, that an elementary hit of experience should turn so many sides to the observer? Think then of chemistry, and of the chemical elements. Sodium is a chemical element; but it has many aspects or properties; physically regarded, it is soft, it is fusible, it volatilises at high temperatures; chemically, it combines with oxygen, it decomposes water, it is univalent, it has a low atomic weight, it is electropositive, and so forth. Sodium cannot be reduced, chemically, to anything simpler than itself, but it is nevertheless many-sided. The same thing is true of sensations.
So a complete list of the aspects or attributes of sensation is as near as we can come to a definition. But since that sort of statement is clumsy; since we cannot make it complete till we have observed the sensations under all their possible aspects; and since we know that mental processes are correlated with processes in the nervous system; we may adopt another plan, and define sensation by reference to the special bodily organ with which it is connected. Sensations are then elementary mental processes that come to us by way of skin, muscle, ear, and the rest of the sense-organs.
[§ 16]. The Intensity of Sensation.—A sensation may remain the same in quality, and yet vary in strength or intensity. A pressure may be the pressure of an ounce or of half-a-pound; it is always pressure, the same quality, but its intensity differs. The tone you get by blowing across the mouth of a bottle may be loud or faint, though it is still the same pitch, the same tone. The weight you carry may strain the arm very little or a great deal; the sensation of strain from the tendons is the same in both cases, but its intensity is different.
The study of this attribute of sensations has led to the discovery of a psychological law, which has much practical importance. Suppose that we are working with intensities of noise, the noise made by the drop of an ivory ball upon an ebony block. Suppose that, by varying the height from which the ball falls, we have found a series of intensities of sensation a, b, c, d, e, which may be represented by the numbers 1, 2, 3, 4, 5; a series, that is, in which the difference between the two noises a and b is equal in sensation to the difference between b and c, or between c and d, or between d and e. That sounds a little difficult; but the series may really be established without much trouble. Now, what about the stimuli, the heights of fall? Must the ball drop twice as far for b as for a, three times as far for c as for a, and so on? No: equal differences in intensity of sensation do not correspond with equal differences in intensity of stimulus. Equal differences in intensity of sensation correspond rather with relatively equal difference in the intensity of stimulus. In other words,