Note 3 — Chapter I

In the opinion of some modern writers the indestructibility of matter and the conservation of energy are alone sufficient to explain all the facts of natural causation. 'For,' it is urged, 'if in any case similar antecedents did not determine similar consequents, on one or other of these occasions some quantum of force, or of matter, or of both, must have disappeared—or, which is the same thing, the law of causation cannot have been constant.' In a future chapter I shall have to recur to this view. Meanwhile I have only to observe that whether or not the law of causation is nothing more than a re-statement of the fact that matter and energy are indestructible, it is equally true that this fact is at least a necessary condition to the operation of that law.