 One of the main catalysts for the development of mathematical theories of measurement was an ongoing debate surrounding measurability in psychology. The debate is often traced back to Gustav Fechner’s (1860) Elements of Psychophysics, in which he described a method of measuring intensities of sensation. Fechner’s method was based on the recording of “just noticeable differences” between sensations associated with pairs of stimuli, e.g., two sounds of different intensity. These differences were assumed to be equal increments of intensity of sensation. As Fechner showed, under this assumption a stable linear relationship is revealed between the intensity of sensation and the logarithm of the intensity of the stimulus, a relation that came to be known as “Fechner’s law” (Heidelberger 1993a: 203; Luce and Suppes 2004: 11–2). This law in turn provides a method for indirectly measuring the intensity of sensation by measuring the intensity of the stimulus, and hence, Fechner argued, provides justification for measuring intensities of sensation on the real numbers.