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252 CRITICAL NOTICES : certainly appear to be irreconcilable. If the doctrine of Conser- vation is entirely a quantitative law, it is hard to see how it can be a " real " and not a " mathematical " principle. If it is not a law of change, can it be a form of the principle of causality ? At the same time, it seems also evident that the second of these two accounts is the most consistent with the facts of the case as well as with the general spirit of Prof. Ward's philosophy. You can never get from the law of causality to the law of the quantitative identity of energy under its various transformations and transferences without making an illegitimate saltus. Though we should admit that the total energy of a system were capable of varying from time to time in dependence upon conditions of a non-physical kind, as Dr. Ward is inclined to believe of the total energy of the physical world, no violence would be done by such a view to the law of causality ; and if the doctrine of conservation is to be got out of such a formula as causa (equal effectum a formula, by the way, to which many of us would hesitate to subscribe, it can only be done by understanding the word cequat in a very narrow and peculiar sense. We may in fact turn a favourite and perfectly sound argument of Prof. Ward's own against himself on this point. No relation which can be adequately expressed by an equation, he tells us, is a relation of cause and effect. For the sides of an equation are interchangeable, but the order of causation is not reversible. 1 Now the law of " Conservation " is pre-eminently one which can be expressed in the form of an equation ; ergo the law of Conservation is not, strictly speaking, a causal law. A second difficulty of detail arises in connexion with the doctrine of Energy when we come to Dr. Ward's treatment of the second law of thermodynamics, the law of the dissipation of Energy. It is of course this law of dissipation more than any other single feature of the mechanical scheme which appears most violently in conflict with our desire for our kind and our sense of the worth of human existence. Hence it is natural that a philosopher should be glad to show, if he can, that the evidence for the law is inconclusive. 1 Prof. Karl Pearson indeed asserts (Grammar of Science, ed. 2, p. 540) that " irreversibility of natural processes is a purely relative conception. History goes forward or backward according to the relative motion of the events and their observer." He illustrates this principle by the imaginary case of a "colleague of Clark Maxwell's demon," who is supposed to travel away from the earth with a velocity greater than that of light, and con- sequently to see the panorama of human history in what is, relative to ourselves, a backward order. The Professor apparently overlooks an im- portant exception to this reversibility. Even for his imaginary " demon " one process, the transition from volition to act in his own case, would always proceed in one direction, from present to future, and would thus be strictly irreversible. The visible events of his own past might unroll themselves once more before his eyes in a reverse order, but his inward mental life would still be proceeding in the inevitable order from idea or wish to act ; your own life, at anyrate, cannot be lived backwards except in Wonderland.