As a _conscious_ element, a sensation marks an interruption in a course of action previously entered upon. Many psychologists since the time of Hobbes have dwelt upon what they call the relativity of sensations. We _feel_ or sense cold in transition from warmth rather than absolutely; hardness is sensed upon a background of less resistance; a color in contrast with pure light or pure dark or in contrast with some other hue. A continuously unchanged tone or color cannot be attended to or sensed. What we take to be such monotonously prolonged sensations are in truth constantly interrupted by incursions of other elements, and represent a series of excursions back and forth. This fact was, however, misconstrued into a doctrine about the nature of knowledge. Rationalists used it to discredit sense as a valid or high mode of knowing things, since according to it we never get hold of anything _in itself_ or intrinsically. Sensationalists used it to disparage all pretence at absolute knowledge.
Properly speaking, however, this fact of the relativity of sensation does not in the least belong in the sphere of knowing. Sensations of this sort are emotional and practical rather than cognitive and intellectual. They are shocks of change, due to interruption of a prior adjustment. They are signals to redirections of action. Let me take a trivial illustration. The person who is taking notes has no sensation of the pressure of his pencil on the paper or on his hand as long as it functions properly. It operates merely as stimulus to ready and effective adjustment. The sensory activity incites automatically and unconsciously its proper motor response. There is a preformed physiological connection, acquired from habit but ultimately going back to an original connection in the nervous system. If the pencil-point gets broken or too blunt and the habit of writing does not operate smoothly, there is a conscious shock:--the feeling of something the matter, something gone wrong. This emotional change operates as a stimulus to a needed change in operation. One looks at his pencil, sharpens it or takes another pencil from one's pocket. The sensation operates as a pivot of readjusting behavior. It marks a break in the prior routine of writing and the beginning of some other mode of action. Sensations are "relative" in the sense of marking transitions in habits of behavior from one course to another way of behaving.