§ 1. Sensation and Stimulus.—One characteristic mark of what we agree in calling sensation is its mode of production. It is caused by what we call a stimulus. A stimulus is always some condition, external to the nervous system itself and operating upon it. This stimulus may consist in physiological change originating in the organism itself, as in the case of organic sensations, or in physical conditions external to the organism, which act on the peripheral organs of sense, and by means of afferent nerves affect the central nervous system. The change in the internal state of the body which gives rise to organic sensation may be initiated, in the first instance, by an external stimulus acting on peripheral organs as in the case of tickling. We have also to count among the various modes of stimulation the irritant effect of certain variations in the nature and the distribution of the bloodsupply within the brain, leading to hallucinations. Causal dependence on some kind of external condition is essential to the conception of sensation.

It is above all things important to distinguish the cause of sensation from the object of senseperception. A man examining a material thing present to his senses may successively or simultaneously see it, feel it, weigh it in his hand, hear the sound it makes, smell it, and taste it. In so doing he perceives its sensible qualities, such as colour, hardness, weight, odour, and flavour. He does so by means of the sensations which are produced in him by the varying relations of his sensitive organism to the object. But the sensible qualities perceived are by no means identical with the cause of sensation. The coloursensation, for instance, is due to a vibratory motion of the particles of the luminiferous ether, giving rise to certain chemical or physical changes in the organ of vision, and so to a certain modification of connected parts of the nervous system. But these conditions are not what a man sees when he perceives the colour red or blue. Similarly, the weight of the object as perceived is by no means to be identified with the changes produced by it in the skin, muscles, tendons, etc., which occasion the sensations necessary to the perception of the weight.

Sensations are essential to the perception of things and their qualities; but in the conception of what constitutes a sensation we abstract from the cognitive function which belongs to it as an element in the perception of an object. The vital point on which we fix attention is that a sensation is a mode of consciousness produced by a specific mode of stimulation, and having its own specific nature ultimately determined by the conditions which produce it. We have noted that the producing conditions may, in the first instance, be external to the organism. But they can only affect the nervous system by first operating on those parts of the organism, which we call the organs of sense.

Thus the changes in the organ of sense, and the subsequent processes by which these changes affect the nervous system, constitute the essential antecedents of the sensation.