This chapter aims to discuss what psychologists mean by the term stimulus, with the hope of deciding what they ought to mean by it. Fechner, following Weber, conceived the grand scheme of a measurement formula for consciousness, relating its judged intensity to a simple variable of the stimulus. The notion that a stimulus is what excites a cell, and is therefore punctate, seems to many theorists the only rigorous definition. A sort of compromise between the informative stimulus and the empty stimulus is provided by the use of the term cue. According to Woodworth, “a cue, as used in psychology, is a stimulus which serves as a sign or signal of something else, the connection having previously been learned.” The concept of a stimulus in psychology and in common speech is broad, loose, and vague; unlike the strict meaning the term has been given, that is, the energy that triggers a receptor.