There would arise further difficulties as to whether the magnitude of this lapse is a function of A, or whether it is the same in all cases of causal sequence. But until some one can be found to defend such a general theory of causal sequence it is premature to discuss difficulties of detail. Footnote 107: For the English reader the best sources of information as to the “descriptive” theory of science are probably volume i. of Professor Ward’s _Naturalism and Agnosticism_; and Mach, the _Science of Mechanics_ (Eng. trans.). Students who read Gennan may advantageously add Avenarius, _Philosophie als Denken der Welt gemäss dem Princip des kleinsten Kraftmasses_. Professor J. A. Stewart is surely mistaken (_Mind_, July 1902) in treating the doctrine as a discovery of “idealist” metaphysicians. Whatever may be thought of some of the uses to which “idealists” put the theory, they cannot claim the credit of its invention. Footnote 108: Cf. Mach, _op. cit._, p. 483 ff.; Pearson, _Grammar of Science_, chap. 4. Footnote 109: _E.g._, eclipses can be calculated equally well for the future or the past. Footnote 110: _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. Footnote 111: Neither can have a first term, because each has two opposite _senses_, positive and negative in the one case, before and after in the other. Footnote 112: