496 INDUCTION. 



is the sum of two motions in two other lines. Motion, how- 

 ever, is but change of place, and at every instant the body is 

 in the exact place it would have been in if the forces had 

 acted during alternate instants instead of acting in the same 

 instant ; (saving that if we suppose two forces to act succes- 

 sively which are in truth simultaneous, we must of course 

 allow them double the time.) It is evident, therefore, that 

 each force has had, during each instant, all the effect which 

 belonged to it ; and that the modifying influence which one of 

 two concurrent causes is said to exercise with respect to the 

 other, may be considered as exerted not over the action of the 

 cause itself, but over the effect after it is completed. For all 

 purposes of predicting, calculating, or explaining their joint 

 result, causes which compound their effects may be treated as 

 if they produced simultaneously each of them its own effect, 

 and all these effects coexisted visibly. 



Since the laws of causes are as really fulfilled when the 

 causes are said to be counteracted by opposing causes, as 

 when they are left to their own undisturbed action, we must 

 be cautious not to express the laws in such terms as would 

 render the assertion of their being fulfilled in those cases 

 a contradiction. If, for instance, it were stated as a law 

 of nature that a body to which a force is applied moves 

 in the direction of the force, with a velocity proportioned 

 to the force directly, and to its own mass inversely ; when 

 in point of fact some bodies to which a force is applied 

 do not move at all, and those which do move (at least 

 in the region of our earth) are, from the very first, 

 retarded by the action of gravity and other resisting forces, 

 and at last stopped altogether; it is clear that the general 

 proposition, though it would be true under a certain hypo- 

 thesis, would not express the facts as they actually occur. To 

 accommodate the expression of the law to the real pheno- 

 mena, we must say, not that the object moves, but that it 

 tends to move, in the direction and with the velocity specified. 

 We might, indeed, guard our expression in a different mode, 

 by saying that the body moves in that manner unless pre- 

 vented, or except in so far as prevented, by some counteracting 



