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 poem. After a shorter time we should expect to find the difference greater; after a longer time we should expect to find it less. If the first committing to memory is a very careful and long continued one, the difference will be greater than if it is desultory and soon abandoned.

In short, we have without doubt in these differences numerical expressions for the difference between these subliminally persistent series of ideas, differences which otherwise we would have to take for granted and would not be able to demonstrate by direct observation. Therewith we have gained possession of something that is at least like that which we are seeking in our attempt to get a foothold for the application of the method of the natural sciences: namely, phenomena on the side of the effects which are clearly ascertainable, which vary in accordance with the variation of conditions, and which are capable of numerical determination. Whether we possess in them correct measures for these inner differences, and whether we can achieve through them correct conceptions as to the causal relations into which this hidden mental life enters—these questions cannot be answered a priori. Chemistry is just as little able to determine a priori whether it is the electrical phenomena, or the thermal, or some other accompaniment of the process of chemical union, which gives it its correct measure of the effective forces of chemical affinity. There is only one way to do this, and that is to see whether it is possible to obtain, on the presupposition of the correctness of such an hypothesis, well classified, uncontradictory results, and correct anticipations of the future.

Instead of the simple phenomenon—occurrence or non-occurrence of a reproduction—which admits of no numerical distinction, I intend therefore to consider from the experimental standpoint a more complicated process as the effect, and I shall observe and measure its changes as the conditions are varied. By this I mean the artificial bringing about by an appropriate number of repetitions of a reproduction which would not occur of its own accord.

But in order to realise this experimentally, two conditions at least must be fulfilled.

In the first place, it must be possible to define with some certainty the moment when the goal is reached—i.e., when the