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 at different times of day. When too great changes in the outer and inner life occurred, the tests were discontinued for a length of time. Their resumption was preceded by some days of renewed training varying according to the length of the interruption.

The guiding point of view in the selection of material and in determining the rules for its employment was, as is evident, the attempt to simplify as far as possible, and to keep as constant as possible, the conditions under which the activity to be observed, that of memory, came into play. Naturally the better one succeeds in this attempt the more does he withdraw from the complicated and changing conditions under which this activity takes place in ordinary life and under which it is of importance to us. But that is no objection to the method. The freely falling body and the frictionless machine, etc., with which physics deals, are also only abstractions when compared with the actual happenings in nature which are of import to us. We can almost nowhere get a direct knowledge of the complicated and the real, but must get at them in roundabout ways by successive combinations of experiences, each of which is obtained in artificial, experimental cases, rarely or never furnished in this form by nature.

Meanwhile the fact that the connection with the activity of memory in ordinary life is for the moment lost is of less importance than the reverse, namely, that this connection with the complications and fluctuations of life is necessarily still a too close one. The struggle to attain the most simple and uniform conditions possible at numerous points naturally encounters obstacles that are rooted in the nature of the case and which thwart the attempt. The unavoidable dissimilarity of the material and the equally unavoidable irregularity of the external conditions have already been touched upon. I pass next to two other unsurmountable sources of difficulty.

By means of the successive repetitions the series are, so to speak, raised to ever higher levels. The natural assumption would be that at the moment when they could for the first time be reproduced by heart the level thus attained would always be the same. If this were the case, i.e., if this characteristic