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.

Central to psychophysics is the concept of a sensory threshold. The
philosopher Herbart ( 1824) had conceived of the idea of a threshold by
assuming that mental events had to be stronger than some critical amount
in order to be consciously experienced. Although measurement is not a part
of this description of the threshold, scientists eventually were able to see
the implication of such a concept for psychological measurement. In the
early nineteenth century, for example, German scientists such as E. H.
Weber and G. T. Fechner were interested in the measurement of the
sensitivity limits of the human sense organs. Using measurement tech-
niques of physics and well-trained human observers, they were able to
specify the weakest detectable sensations in terms of the stimulus energy
necessary to produce them. The absolute threshold or stimulus threshold (RL
for the German Reiz Limen) was defined as the smallest amount of stimulus
energy necessary to produce a sensation. Since an organism's sensitivity to
external stimuli tends to fluctuate somewhat from moment to moment,

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