The publication in 1957 of S.S. Stevens' famous paper, On the psychophysical law, ignited a controversy which has continued ever since relating to people's subjective judgements of physical reality. Why is it that the perception of sensation can diverge so sharply from the magnitude of the stimulus? How should sensation be measured? Donald Laming brings together a diversity of ideas and a wealth of experimental evidence, and provides a challenging new perspective on the question which has fragmented the research community for nearly 40 years.