 We can formulate [this law] as follows: A difference between any two stimuli is estimated [geschätzt] to be equal if the relationship between the stimuli is equal. Or: If in our apprehension [Auffassung] the intensity of the sensation is to increase by equal amounts, then the relative stimulus-increase must remain constant. This latter statement may also be expressed as follows: The strength of a stimulus must increase geometrically if the strength of the apperceived sensation is to increase arithmetically. (PP I: 359)