without evil, no labor and no progress.

Fechner's "psycho-physics," a science which was founded by him in
continuation of the investigations of Bernoulli, Euler, and especially
of E.H. Weber, wears an entirely different aspect from that of his
metaphysics (the "day view," moreover does not claim to be knowledge,
but belief--though a belief which is historically, practically, and
theoretically well-grounded). This aims to be an exact science of the
relations between body and mind, and to reach indirectly what Herbart
failed to reach by direct methods, that is, a measurement of psychical
magnitudes, using in this attempt the least observable differences in
sensations as the unit of measure. Weber's law of the dependence of the
intensity of the sensation on the strength of the stimulus--the increase
in the intensity of the sensation remains the same when the relative
increase of the stimulus (or the relation of the stimuli) remains
constant;[1] so that, _e.g._, in the case of light, an increase from a
stimulus of intensity 1 to one of intensity 100, gives just the same
increase in the intensity of the sensation as an increase from a stimulus
of intensity 2 (or 3) to a stimulus of 200 (or 300)--is much more generally
valid than its discoverer supposed; it holds good for all the senses. In
the case of the pressure sense of the skin, with an original weight of 15
grams (laid upon the hand when at rest and supported), in order to produce
a sensation perceptibly greater we must add not 1 gram, but 5, and with an
original weight of 30 grams, not 5, but 10. Equal additions to the weights
are not enough to produce a sensation of pressure whose intensity shall
render it capable of being distinguished with certainty, but the greater
the original weights the larger the increments must be; while the
intensities of the sensations form an arithmetical, those of the stimuli
form a geometrical, series; the change in sensation is proportional to the
relative change of the stimulus. Sensations of tone show the same
proportion (3:4) as those of pressure; the sensibility of the muscle sense
is finer (when weights are raised the proportion is 15:16), as also that
of vision (the relative brightness of two lights whose difference of
intensity is just perceptible is 100:101). In addition to the
investigations on the threshold of difference there are others on the
threshold of stimulation (the point at which a sensation becomes just
perceptible),