to another with sound, and so on. a We may think that we can account for the stimulus by saying that it comes from an object, but all the objects we have any acquaintance with are resolvable themselves into sensations and groups of sensations like the preliminary ones we set out to explain. If we say, for example, that green comes from a tree, we soon become aware that the tree itself (so far as it is separable from its greenness) is but a cluster of other, perhaps more elementary, sensations of the same general kind, such as hardness, resistance, pressure, or weight. If we abstract from all the sensations, no tree is left. As Nietzsche puts it, the known outer world is born after the effect, of which it is supposed to be the cause.[1] Our bodies themselves are, as we know them, groups of sensations like everything else—what they really are in their intimate nature we have not the slightest idea. b
Nor if we consider the more refined world of science, do we leave the subjective sphere. The world of atoms and their movements, which physicists conceive of as a true world in contrast with the ordinary world of sense-perception, is not essentially different from the ordinary world; its molecules or atoms are only what we should see or handle had we finer senses,—they and their movements are entirely of a sensational nature.[2] Moreover, the supposition that there are ultimate, indivisible, unalterable units like molecules or atoms is pure invention; it is convenient to have them for purposes of reckoning, and, as we do not find them, we proceed to create them—this is all we can say.[3] c Mechanics is purely a practical or regulative science.[4] (I may remark in passing that Nietzsche thinks that the Dalmatian Boscovitch put an end to materialistic atomism, as the Pole Copernicus had done to the notion of a fixed earth).[5] It is the same with "force" or "forces," in the purely mechanical sense. We know only effects—no one has ever got hold of a force, as mechanical philosophy pictures it. Force, in this sense, is really a piece of abstraction, a more or less arbitrary creation. We ourselves have a certain feeling of
