Problems in interindividual comparisons of perceptual intensities are discussed both from a philosophical and a psychophysical point of view. A theoretical model (the range model) is presented. In this model maximal perceptual intensities are set equal for different individuals in spite of the fact that the corresponding physical intensities may vary greatly. The intensity of a perception is evaluated depending upon its position in the total range from zero (or a minimal intensity) to a maximal subjective intensity (also considering the type of growth function in question). An empirical test of the model has been performed in the area of effort and exertion. High correlations have then been found between estimations of perceived exertion and corresponding heart rates. The range model may also have a more general application in psychology and physiology for most kinds of "interprocess' comparisons.