Not only do the elements united by apperceptive synthesis gain, in the aggregate idea that results from their combination, a new significance which they did not have in their isolated state, but what is of still greater importance, the aggregate idea itself is a new psychical content that was made possible, to be sure, by these elements, but was by no means contained in them. The law of psychical resultants which expresses a principle which we may designate, in view of its results, as a principle of creative synthesis. This has long been recognized in the case of higher mental creations, but generally not applied to the other psychical processes. In fact, through an unjustifiable confusion with the laws of physical causality, it has even been completely reversed. A similar confusion is responsible for the notion that there is a contradiction between the principle of creative synthesis in the mental world and the general laws of the natural world, especially that of the conservation of energy.
Physical measurements have to do with objective masses, forces, and energies.
Psychical measurements, which are concerned with the comparison of psychical components and their resultants, have to do with subjective values and ends. The muscular movements of an external volitional act, the physical processes that accompany sense-perception, association, and apperception, will follow invariably the principle of the conservation of energy. But the mental values and ends that these energies represent may be very different in quantity even while the quantity of these energies remains the same. The differences pointed out show that physical measurement deals with quantitative values, that is, with quantities that admit of a variation in value only in the one relation of the quantity of the phenomena measured. Psychical measurement on the other hand, deals in the last instance in every case with qualitative values, that is, values that vary in degree only in respect to their qualitative character.