THE NATURE OF SENSATION
Sensation Gives Us Our World of Qualities.—In actual experience sensations are never known apart from the objects to which they belong. This is to say that when we see yellow or red it is always in connection with some surface, or object; when we taste sour, this quality belongs to some substance, and so on with all the senses. Yet by sensation we mean only the simple qualities of objects known in consciousness as the result of appropriate stimuli applied to end-organs. We shall later see how by perception these qualities fuse or combine to form objects, but in the present chapter we shall be concerned with the qualities only. Sensations are, then, the simplest and most elementary knowledge we may get from the physical world,—the red, the blue, the bitter, the cold, the fragrant, and whatever other qualities may belong to the external world. We shall not for the present be concerned with the objects or sources from which the qualities may come.
To quote James on the meaning of sensation: "All we can say on this point is that what we mean by sensations are first things in the way of consciousness. They are the immediate results upon consciousness of nerve currents as they enter the brain, and before they have awakened any suggestions or associations with past experience. But it is obvious that such immediate sensations can be realized only in the earliest days of life."
The Attributes of Sensation.—Sensations differ from each other in at least four respects; namely, quality, intensity, extensity, and duration.
It is a difference in quality that makes us say, "This paper is red, and that, blue; this liquid is sweet, and that, sour." Differences in quality are therefore fundamental differences in kind. Besides the quality-differences that exist within the same general field, as of taste or vision, it is evident that there is a still more fundamental difference existing between the various fields. One can, for example, compare red with blue or sweet with sour, and tell which quality he prefers. But let him try to compare red with sweet, or blue with sour, and the quality-difference is so profound that there seems to be no basis for comparison.
Differences in intensity of sensation are familiar to every person who prefers two lumps of sugar rather than one lump in his coffee; the sweet is of the same quality in either case, but differs in intensity. In every field of sensation, the intensity may proceed from the smallest amount to the greatest amount discernible. In general, the intensity of the sensation depends on the intensity of the stimulus, though the condition of the sense-organ as regards fatigue or adaptation to the stimulus has its effect. It is obvious that a stimulus may be too weak to produce any sensation; as, for example, a few grains of sugar in a cup of coffee or a few drops of lemon in a quart of water could not be detected. It is also true that the intensity of the stimulus may be so great that an increase in intensity produces no effect on the sensation; as, for example, the addition of sugar to a solution of saccharine would not noticeably increase its sweetness. The lowest and highest intensity points of sensation are called the lower and upper limen, or threshold, respectively.
By extensity is meant the space-differences of sensations. The touch of the point of a toothpick on the skin has a different space quality from the touch of the flat end of a pencil. Low tones seem to have more volume than high tones. Some pains feel sharp and others dull and diffuse. The warmth felt from spreading the palms of the hands out to the fire has a "bigness" not felt from heating one solitary finger. The extensity of a sensation depends on the number of nerve endings stimulated.
The duration of a sensation refers to the time it lasts. This must not be confused with the duration of the stimulus, which may be either longer or shorter than the duration of the sensation. Every sensation must exist for some space of time, long or short, or it would have no part in consciousness.
