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 has hardly ever been misunderstood by anybody so far as to come in here. But this theoretical constancy is of little value: How shall I find it when the circumstances under which I am actually forced to make my observations are never the same? So I must rather ask:—Can I bring under my control the inevitably and ever fluctuating circumstances and equalise them to such an extent that the constancy presumably existent in the causal relations in question becomes visible and palpable to me?

Thus the discussion of the one difficulty which opposes an exact examination of the causal relations in the mental sphere has led us of itself to the other. A numerical determination of the interdependent changes of cause and effect appears indeed possible if only we can realise the necessary uniformity of the significant conditions in the repetition of our experiments.

Section 6.&emsp;The Possibility of Maintaining the Constancy of Conditions Requisite for Research

He who considers the complicated processes of the higher mental life or who is occupied with the still more complicated phenomena of the state and of society will in general be inclined to deny the possibility of keeping constant the conditions for psychological experimentation. Nothing is more familiar to us than the capriciousness of mental life which brings to nought all foresight and calculation. Factors which are to the highest degree determinative and to the same extent changeable, such as mental vigor, interest in the subject, concentration of attention, changes in the course of thought which have been brought about by sudden fancies and resolves—all these are either not at all under our control or are so only to an unsatisfactory extent.

However, care must be taken not to ascribe too much weight to these views, correct in themselves, when dealing with fields other than those of the processes by the observation of which these views were obtained. All such unruly factors are of the greatest importance for higher mental processes which occur only by an especially favorable concurrence of circumstances. The more lowly, commonplace, and constantly occurring processes are not in the least withdrawn from their influence, but we have it for the most part in our power, when it is a matter of consequence, to make this influence only slightly disturbing. Sensorial perception, for example, certainly occurs with greater