able physically. As for the motion of the projected bullet, so also for the thinking of the brain, a physical explanation must ultimately be in itself possible, which would make the latter just as comprehensible as is the former. But even the former, which we imagine we understand so perfectly, is at bottom as obscure to us as the latter; for what the inner nature of expansion in space may be – of impenetrability, mobility, hardness, elasticity, and gravity remains, after all physical explanations, a mystery, just as much as thought. But because in the case of thought the inexplicable appears most immediately, a spring was at once made here from physics to metaphysics, and a substance of quite a different kind from all corporeal substances was hypostatised – a soul was set up in the brain. But if one had not been so dull as only to be capable of being struck by the most remarkable of phenomena, one would have had to explain digestion by a soul in the stomach, vegetation by a soul in the plant, affinity by a soul in the reagents, nay, the falling of a stone by a soul in the stone. For the quality of every unorganised body is just as mysterious as the life in the living body. In the same way, therefore, the physical explanation strikes everywhere upon what is metaphysical, by which it is annihilated, i.e., it ceases to be explanation. Strictly speaking, it may be asserted that no natural science really achieves anything more than what is also achieved by Botany: the bringing together of similars, classification. A physical system which asserted that its explanations of things – in the particular from causes, and in general from forces – were really sufficient, and thus exhausted the nature of the world, would be the true Naturalism. From Leucippus, Democritus, and Epicurus down to the Système de la Nature, and further, to Delamark, Cabanis, and to the materialism that has again been warmed up in the last few years, we can trace the persistent attempt to set up a system of physics without metaphysics, that is, a system which would make the phenomenon the thing in itself. But all their explana-
