Fechner's general formula for getting at the number of units in any sensation is "S" = "c" log "R", where "S" stands for the sensation, "R" for the stimulus numerically estimated, and "c" for a constant that must be separately determined by experiment in each particular order of sensibility. Fechner's reasoning has been criticized on the grounds that although stimuli are composite, sensations are not. "Every sensation," says William James, "presents itself as an indivisible unit; and it is quite impossible to read any clear meaning into the notion that they are masses of units combined."