§ 2. Sensory Elements.—If I look at grass, I have the sensation of green. If I look at snow, I have the sensation of white. I can assign no psychological reason why in the one case the sensation is that of white and in the other that of green. The difference can only be accounted for by the different way in which my eye is affected by different kinds of light. So in all cases the qualities of sensation must be ultimately accounted for by reference to the nature of the stimulus.

If I do not actually see grass or snow, but summon up mental pictures of them in my mind's eye, the qualities of greenness and whiteness are present in my mental image as they are present in actual perception. Now these qualities would not be present in the mental image unless they had been previously produced by the operation of an external stimulus. For this reason, some writers would apply the term sensation to these qualities even when they appear in the mental imago. Both in actual perception and in the mental image they defy psychological analysis, and can be ultimately accounted for only by reference to external stimulation. There is, however, an objection to applying the word sensation to both cases indifferently. Though greenness appears both in the perception and in the mental image of grass, it appears in a different manner in each instance. The present operation of the external stimulus gives it peculiar intensity, steadiness, and other distinctive characters, which do not belong to it in the mental image. It is better to restrict the term sensation to the special form of consciousness which accompanies the actual operation of the stimulus. The qualities of sensation as they appear in mental imagery may be called sensory elements, but not sensations. The term elements indicates that their peculiar nature cannot be psychologically accounted for,—that ultimately it can only be explained by reference to an external stimulus. The word sensory indicates that their existence presupposes the previous existence of corresponding sensations.