Prior to a century ago the approach to psychological problems consisted primarily of philosophical speculation. The transition of psychology from a philosophical to a scientific discipline was greatly facilitated when the German physicist G. T. Fechner introduced techniques for measuring mental events (1860). The attempt to measure sensations through the use of Fechner’s procedures was termed psychophysics and constituted the major research activity of early experimental psychologists. Since this time psychophysics has consisted primarily of investigating the relationships between sensations (Ψ) in the psychological domain and stimuli (ϕ) in the physical domain.