Evidently, in all these cases, the effect of a difference between two stimulations was to introduce certain changes in sensation during the interval which they limited, owing to the fact that the subject expected the difference to occur.  Thus in the third group of experiments there were, very likely, in all cases changes from sensations of high tension to sensations of lower, or vice versa.  It is probable that, in the experiments of the second group, there were also changes in muscular sensations, partly those of eye muscles, partly of chest and arm muscles, introduced by the change of attention from one point to another.  At any rate, it is certain that there were certain sensation changes produced during the intervals by changes of locality.

If, then, we assume that the introduction of additional sensation change into an interval lengthens it, we are led to the conclusion that psychological time (as distinguished from metaphysical, mathematical, or transcendental time) is perceived simply as the quantum of change in the sensation content.  That this is a true conclusion is seemingly supported by the fact that when we wish to make our estimate correspond as closely as possible with external measurements, we exclude from the content, to the best of our ability, the general complex of external sensations, which vary with extreme irregularity; and confine the attention to the more uniformly varying bodily sensations.  We perhaps go even further, and inhibit certain bodily sensations, corresponding to activity of the more peripherally located muscles, that the attention may be confined to certain others.  But attention to a dermal stimulation is precisely the condition which would tend to some extent to prevent this inhibition.  For this reason we might well expect to find the error in estimation more variable, the ‘constant error’ in general greater, and the specific effects of variations which would affect the peripheral muscles, more marked in ‘tactual’ time than in either ‘auditory’ or ‘optical’ time.  Certainly all these factors appear surprisingly large in these experiments.

It is not possible to ascertain to how great an extent subject Sh inhibited the more external sensations, but certainly if he succeeded to an unusual degree in so doing, that fact would explain the absence of effect of stimulation difference in his case.

Explanation has still to be offered for the variable effect of intensity difference upon the second interval.  According to all subjects except Sn, there is a radical difference in attitude in the two intervals.  In the first interval the subject is merely observant, but in the second he is more or less reproductive.  That is, he measures off a length which seems equal to the standard, and if the stimulation does not come at that point he is prepared to judge the interval as ‘longer,’ even before the third stimulation is given.  In cases, then, where the judgment with equal intensities would be ‘longer,’ we might expect that the actual strengthening or weakening of the final tap would make no difference, and that it would make very little difference in other cases.  But even here the expectation of the intensity is an important factor in determining tension changes, although naturally much less so than in the first interval.  So we should still expect the lengthening of the second interval.