The Pavlovian "principle of strength".

The Pavlovian principle of strength assumed that the magnitude of the conditional response is a linear function of the intensity of the external conditional stimulus. But experiments failed to provide evidence for the universality of the principle. The Pavlovians tried to identify conditions that distorted the linearity of this relationship. Some of the disturbing conditions were external and some were internal intervening variables. It is possible that the relation between the strength of the conditional stimulus and the magnitude of the conditional response is not linear but logarithmic. Pavlov acknowledged the lack of experimental evidence to support the principle of strength in its original form.