C O N T E N T S .

Introductory Matters.

I. General considerations on the relationship between body and soul . . . . 1
II. Concept and task of Psychophysics The scientific study of the relationship between stimulus (physical) and sensation (mental) . . . . . . . . . . . . . . . . . . . . . . . 8
III. A preliminary question . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
IV. Conceptual definitions regarding sensation and stimulus . . . . . . . . . . . . . . . . . 15

External Psychophysics.

The Psychophysical Theory of Measurement.

V. Measurement of bodily activity. Living force original: "Lebendige Kraft"; a historical term for kinetic energy, which Fechner uses to quantify physical stimulus energy . . . . . . . . . . . . . . 20
VI. Measurement principle of sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
VII. Measurement principle of sensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
VIII. Methods of measuring sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . 69

1. Measurement methods for differential sensitivity The ability to detect the smallest difference between two stimuli . . . . . . . . . . . . . . . 71
   a) General presentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
   b) General considerations and precautions . . . . . . . . . . . . . . . . . 76
   c) Considerations regarding the time and space conditions of the
   experiments. Constant errors . . . . . . . . . . . . . . . . . . . . . . . . . 88
   d) Specifics on the method of right and wrong cases A procedure where a subject compares two stimuli and the researcher records the frequency of correct identifications of the difference . . . . . 94
   e) Specifics on the method of average error A procedure where the subject adjusts a variable stimulus until it matches a standard one . . . . . . . . . . . . 120
   f) Mathematical relationship between the methods . . . . . . . . . . . . . . . 128
2. Measurement methods for absolute sensitivity . . . . . . . . . . . . . . . . 130

Fundamental Laws and Facts.

IX. Weber's Law The rule stating that the change in a stimulus needed to produce a noticeable difference is a constant ratio of the original stimulus . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
Weber’s own reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
1) Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
2) Sound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
3) Weights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
4) Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201
5) Extensive magnitudes (Visual and tactile measurement) . . . . . . . . . . . . 211
6) Moral and physical wealth original French: "Fortune morale et physique"; an analogy to Daniel Bernoulli’s theory that the psychological value of money decreases as one’s total wealth increases . . . . . . . . . . . . . . . . . . . . . . . . . . . 226