We shall distinguish several different artifices in the process of transition from Weber's experiments, or from any other series of similar observations, to a psychophysical law like Fechner's. It is first of all agreed to consider our consciousness of an increase of stimulus as an increase of the sensation S: this is therefore called S. It is then asserted that all the sensations ΔS, which correspond to the smallest perceptible increase of stimulus, are equal to one another. They are therefore treated as quantities, and while, on the one hand, these quantities are supposed to be always equal, and, on the other, experiment has given a certain relation ΔΕ = ∫(E) between the stimulus Ε and its minimum increase, the constancy of ΔS is expressed by writing ΔS = C ΔE/∫(E), C being a constant quantity. Finally it is agreed to replace the very small differences ΔS and ΔΕ by the infinitely small differences _d_S and _d_E, whence an equation which is, this time, a differential one: _d_S = C _d_E/∫(E). We shall now simply have to integrate on both sides to obtain the desired relation[24]: S=C ∬_d_E/∫(E). And the transition will thus be made from a proved law, which only concerned the _occurrence_ of a sensation, to an unprovable law which gives its _measure._ Without entering upon any thorough discussion of this ingenious operation, let us show in a few words how Fechner has grasped the real difficulty of the problem, how he has tried to overcome it, and where, as it seems to us, the flaw in his reasoning lies. [Sidenote: Can two sensations be equal without being identical?]