[2d Ed.] [The Axiom that _Reaction is equal and opposite to Action_, may appear to be at variance with a maxim concerning Cause which is commonly current; namely, that the 'Cause precedes Effect, and Effect follows Cause.' For it may be said, if _A_, the Action, and _R_, the Reaction, can be considered as mutually the cause of each other, _A_ must precede _R_, and yet must follow it, which is impossible. But to this I reply, that in those cases of direct Causation to which the maxim applies, the Cause and Effect are not successive, but simultaneous. If I press against some obstacle, the obstacle resists and returns the pressure at the instant it is exerted, not after any interval of time, however small. The common maxim, that the effect follows the cause, has arisen from the practice of considering, as examples of cause and effect, not instantaneous forces or causes, and the instantaneous changes which they produce; but taking, instead of this latter, the _cumulative_ effects produced in the course of time, and compared with like results occurring without the action of the cause. Thus, if we alter the length of a clock-pendulum, this change produces, as its effect, a subsequent change of rate in the clock: because the rate is measured by the accumulated effects of the pendulum's gravity, before and after the change. But the pendulum produces its mechanical effect upon the escapement, at the moment of its contact, and each wheel upon the next, at the moment of _its_ contact. As has {192} been said in a Review of this work, 'The time lost in cases of indirect physical causation is consumed in the movements which take place among the parts of the mechanism in action, by which the active forces so transformed into momentum are transported over intervals of space to new points of action, the motion of matter in such cases being regarded as a mere carrier of force.' (_Quarterly Rev._ No. cxxxv. p. 212.) This subject I have further treated in the _Memoirs of the Cambridge Philosophical Society_, vol. vii. part iii.] [In this Third Edition I add this discussion.] _Discussion of the Question:--Are Cause and Effect successive or simultaneous?_