Section 9. Resumé
Two fundamental difficulties arise in the way of the application of the so-called Natural Science Method to the examination of psychical processes:
(1) The constant flux and caprice of mental events do not admit of the establishment of stable experimental conditions.
(2) Psychical processes offer no means for measurement or enumeration.
In the case of the special field of memory (learning, retention, reproduction) the second difficulty may be overcome to a certain extent. Among the external conditions of these processes some are directly accessible to measurement (the time, the number of repetitions). They may be employed in getting numerical values indirectly where that would not have been possible directly. We must not wait until the series of ideas committed to memory return to consciousness of themselves, but we must meet them halfway and renew them to such an extent that they may just be reproduced without error. The work requisite for this under certain conditions I take experimentally as a measure of the influence of these conditions; the differences in the work which appear with a change of conditions I interpret as a measure of the influence of that change.
Whether the first difficulty, the establishment of stable experimental conditions, may also be overcome satisfactorily cannot be decided a priori<. Experiments must be made under conditions as far as possible the same, to see whether the results, which will probably deviate from one another when taken separately, will furnish constant mean values when collected to form larger groups. However, taken by itself, this is not sufficient to enable us to utilise such numerical results for the establishment of numerical relations of dependence in the natural science sense. Statistics is concerned with a great mass of constant mean values that do not at all arise from the frequent repetition of an ideally frequent occurrence and therefore cannot favor further insight into it. Such is the great complexity of our mental life that it is not possible to deny that constant mean values, when obtained, are of the nature of such statistical constants. To test that, I examine the distribution of the separate numbers represented in an average value. If it corresponds
