Mathematical philosophy, a study of fate and freedom; lectures for educated laymen, by Cassius J. Keyser.

372 MATHEMATICAL PHILOSOPHY
The third one of the experimenter's fundamental
problems has to do with what occurs within the wall. It
is this: Given a sense department and in it a sensation
corresponding to the least (or greatest) stimulus that
will produce it, to determine how much the stimulus must
be increased (or decreased) to beget a new, or different,
sensation.
Some Technical Terms and Symbols.-In the literature we encounter, as you probably know, the equivalent
terms-Threshold, Linmen, Schwelle-introduced by
Herbart about a hundred years ago, and the symbolsR, L, D, T, RL, TL, DL-whose meanings are easy to
grasp. R comes from the German Reiz, signifying stimulus; L stands for Limen, or threshold; and T for terminal; RL denotes initial threshold,-the least stimulus that
will yield a sensation; TL denotes terminal threshold,the greatest stimulus that will yield a sensation; and DL
denotes difference threshold,-the difference between the
least (or greatest) amounts of stimulus that correspond
to two just discernibly different sensations. The three
fundamental problems may accordingly be restated thus:
To determine in each sense department its RL, TL, and
DL.
The pioneers, Weber and Fechner, were contemporaries but the work of Weber came first. He dealt mainly
with hearing and touch. A rough statement of Weber's
Law,-so named by Fechner,-is this: The increase of
stimulus necessary to produce a change of sensation is not
a constant difference, but is a constant ratio of the preceding stimulus. It is, you notice, concerned with the
DL, the difference threshold. It is often referred to as
Fechner's law or the Weber-Fechner law or the psychophysical law. It is to Fechner,-whose great work,