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 first reproduction were everywhere an invariable objective sign of an equally invariable fixedness of the series, it would be of real value to us. This, however, is not actually the case. The inner conditions of the separate series at the moment of the first possible reproduction are not always the same, and the most that can be assumed is that in the case of these different series these conditions always oscillate about the same degree of inner surety. This is clearly seen if the learning and repeating of the series is continued after that first spontaneous reproduction of the series has been attained. As a general thing the capacity for voluntary reproduction persists after it has once been reached. In numerous cases, however, it disappears immediately after its first appearance, and is regained only after several further repetitions. This proves that the predisposition for memorising the series, irrespective of their differences of a larger sort according to the time of day, to the objective and subjective conditions, etc., is subject to small variations of short duration, whether they be called oscillations of attention or something else. If, at the very instant when the material to be memorised has almost reached the desired degree of surety, a chance moment of especial mental clearness occurs, then the series is caught on the wing as it were, often to the learner’s surprise; but the series cannot long be retained. By the occurrence of a moment of special dullness, on the other hand, the first errorless reproduction is postponed for a while, although the learner feels that he really is master of the thing and wonders at the constantly recurring hesitations. In the former case, in spite of the homogeneity of the external conditions, the first errorless reproduction is reached at a point a little below the level of retention normally connected with it. In the latter case it is reached at a point a little above that level. As was said before, the most plausible conjecture to make in this connection is that these deviations will compensate each other in the case of large groups.

Of the other source of error, I can only say that it may occur and that, when it does, it is a source of great danger. I mean the secret influence of theories and opinions which are in the process of formation. An investigation usually starts out with definite presuppositions as to what the results will be. But if this is not the case at the start, such presuppositions form