There are many perceptual continua for which an observer can have the experience of perceived magnitude. Sounds varying in amplitude are heard as varying in loudness, and light sources varying in luminosity are seen as varying in brightness. Stevens’s Psycho-physical Power Law describes the relation between stimulus intensity and perceived magnitude for these two examples and for many other perceptual continua as well. This chapter summarizes the evidence supporting the validity of the power law, as well as evidence showing that each such continuum has a distinctive exponent, and distinctive values for its resolving power and its dynamic range. It is proposed that the dynamic ranges for all prothetic continua are subjectively equal. If that hypothesis is verified, and the evidence about the power law summarized here is validated, those conclusions would be consistent with the existence of a single central mechanism that registers the input from any sensory receptor that results in the experience of a perceived magnitude. It is also proposed that such a device has its own distinctive dynamic range, and its own distinctive resolving power. From the viewpoint of psychophysics, it exists as a purely conceptual system. For those engaged in the study of neuroscience, it stands as a challenge: Is there a structure in the brain where such a device can be found?