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The general psychophysical differential equation (GPDE) for calculating ‘intrasubjective’ relations, is reviewed and it is argued that: (a) the use of calculus and Weber functions are unnecessary for the determination of such relations: simple algebraic methods are available, (b) Taylor's expansion in the derivation of GPDE is not necessary: a short cut method is suggested and applied instead, (c) GPDE is untenable as an empirical model, for it is based on one definition and one assumption which are both arbitrary and purely mathematical, (d) The sources of error in the validation procedures used by Eisler are pointed out; (e) by means of a validity test and the due treatment of a dozen sets of experimental data, the Fechnerian integration as well as GPDE are invalidated.

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On the basis of previous experimental findings, it is argued that the power law is not based on a true (subjective) ratio scale, but on a quasi‐ratio scale. Hence, and in spite of the results of the cross‐modality matching, the validity of the power law, as well as the validity of models and equations in which a subjective scale is involved, is doubted. It is further indicated that the validity of Brown's law can, under special conditions, be tested by the relation between subjective variables of velocity, length and time. Finally, the relation between the power law and Luce's theorem is discussed briefly.