Page:Philosophical Review Volume 2.djvu/341

No. 3.] of time seems shorter when the mind is occupied than when idle, and to the old than to the young, and the quantitive relations can be determined. The accuracy and limits of the perception of intervals of time is an important factor in the composition and appreciation of music and poetry. The appreciation of intervals of time may depend on a rhythm of attention. The attention may be concentrated by an effort, but it will soon relax, and must be again concentrated only to relax again. These waves of attention seem to follow each other at constant intervals, and are an important factor in our mental life.

This was urged by Kant, who claimed that mental processes are extended in time only. If this be the case, it is evident that a mental mechanics is not possible. Astronomy could not be treated as "celestial mechanics," did it not measure the positions and masses of the heavenly bodies as well as the times of their movements. But Locke claimed that mental processes are not extended even in time, and later writers may be mistaken in holding that they have no other dimension; Indeed it would be commonly admitted that sensations and feelings vary in intensity, but it is claimed that this intensity is not measurable. The measurement of mental intensity is certainly beset with greater theoretical and practical difficulties than the measurement of mental time. A starting point is found, however, in the fact that the intensity of a sensation is a function of the intensity of the stimulus, otherwise it would never have been discovered that one light is brighter, one sound louder, or one weight heavier than another. Even in physical science the brightness of lights and the loudness of sounds are only measured by the comparison of sensations.