Note 110 — Chapter V · THE WORLD OF THINGS—(2) CHANGE AND CAUSALITY

Infra, Bk. III. chap. 4. It will be enough to refer in passing to the curious blunder which is committed when the principle of Causality is confounded with the doctrines of the Conservation of Mass and Energy. That the principle of Causality has nothing to do with these special physical theories is manifest from the considerations: (1) That it is at least not self-evident that all causal relation is physical. Philosophers have indeed denied that one mental state directly causes another, but no one has based his denial on the assertion that there can be no causality without mass and energy. (2) The principle of Causality, as we have seen, is a postulate. If we are ever to intervene successfully in the course of events, it must be possible with at least approximate accuracy to regard events as determined by their antecedents. The doctrines of conservation of mass and energy are, on the contrary, empirical generalisations from the observed behaviour of material systems. Neither science nor practical life in the least requires them as an indispensable condition of success. In practical life they are never appealed to, and the ablest exponents of science are most ready to admit that we have no proof of their validity except so far as it can be established by actual observation. In short, they are largely a posteriori, while the principle of Causality is, as already explained, a priori. See infra, Bk. III. chap. 6, § 6.