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 when at the same time we may assume that the separate cases belong to the same causal system, whose elements, however, are not limited to exclusively constant values, but may run through small circles of numerical values symmetrical around a middle value.

Our question, however, is not answered conclusively by the statement just made. Suppose we had in some way found satisfactorily constant mean values for some psychical process, how would we go about it to learn whether we might or might not assume a homogeneous causal condition, necessary for their further utilisation? The physical scientist generally knows beforehand that he will have to deal with a single causal combination, the statistician knows that he has to deal with a mass of them, ever inextricable despite all analysis. Both know this from the elementary knowledge they already possess of the nature of the processes before they proceed with the more detailed investigations. Just as, a moment ago, the present knowledge of psychology appeared to us too vague and unreliable to be depended upon for decision about the possibility of constant experimental conditions; so now it may prove insufficient to determine satisfactorily whether in a given case we have to deal with a homogeneous causal combination or a manifold of them which chance to operate together. The question is, therefore, whether we may throw light on the nature of the causation of the results we obtain under conditions as uniform as possible by the help of some other criterion.

The answer must be: This cannot be done with absolute certainty, but can, nevertheless, be done with great probability. Thus, a start has been made from presuppositions as similar as possible to those by which physical constants have been obtained and the consequences which flow from them have been investigated. This has been done for the distribution of the single values about the resulting central value and quite independently of the actual concrete characteristics of the causes. Repeated comparisons of these calculated values with actual observations have shown that the similarity of the suppositions is indeed great enough to lead to an agreement of the result. The outcome of these speculations closely approximates to reality. It consists