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8 hard to analyse? I shall first discuss the second difficulty in connection, of course, with memory, since that is our present concern.

If we consider once more the conditions of retention and reproduction mentioned above, but now with regard to the possibility of computation, we shall see that with two of them, at least, a numerical determination and a numerical variation are possible. The different times which elapse between the first production and the reproduction of a series of ideas can be measured and the repetitions necessary to make these series reproducible can be counted. At first sight, however, there seems to be nothing similar to this on the side of the effects. Here there is only one alternative, a reproduction is either possible or it is not possible. It takes place or it does not take place. Of course we take for granted that it may approach, under different conditions, more or less near to actual occurrence so that in its subliminal existence the series possesses graded differences. But as long as we limit our observations to that which, either by chance or at the call of our will, comes out from this inner realm, all these differences are for us equally non-existent.

By somewhat less dependence upon introspection we can, however, by indirect means force these difference into the open. A poem is learned by heart and then not again repeated. We will suppose that after a half year it has been forgotten: no effort of recollection is able to call it back again into consciousness. At best only isolated fragments return. Suppose that the poem is again learned by heart. It then becomes evident that, although to all appearances totally forgotten, it still in a certain sense exists and in a way to be effective. The second learning requires noticeably less time or a noticeably smaller number of repetitions than the first. It also requires less time or repetitions than would now be necessary to learn a similar poem of the same length. In this difference in time and number of repetitions we have evidently obtained a certain measure for that inner energy which a half year after the first learning still dwells in that orderly complex of ideas which make up the