258' INDUCTION. 



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 

 compoimd their effects may be treated as if they produced simultane- 

 ously 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 pi'oportioned to the force directly, and to 

 its o^vn 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 are, fi-om 

 the very first, retarded by the action of gravity and other resisting 

 forces, and at last stopped altogether ; it is clear that the general propo- 

 sition, although it would be true imder a certain hypothesis, would not 

 express the facts as they actually occur. To accommodate the expres- 

 sion of the law to the real phenomena, 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 prevented, or except 

 in so far as prevented by some counteracting cause. , But the body 

 does not only move in that manner unless counteracted; it tends to 

 move in that manner even when counteracted ; it still exerts, in the 

 original direction, the same energy of movement as if its first impulse 

 had been undisturbed, and produces, by that energy, an exactly equiva- 

 lent quantity of effect. This is true even when ' the force leaves the 

 body as it found it, in a state of absolute rest ; as when we attempt to 

 raise a body of three tons weight with a force equal to. one ton. For 

 if, while we are applying this force, the wind or water or any other 

 agent supplies an additional force just exceeding two tons, the body 

 will be raised ; thus proving that the force we applied exerted its full 

 effect, by neutralizing an equivalent portion of the weight which it was 

 insufficient altogether to overcome. And if) while we are exerting 

 this force of one ton upon the object in a direction contrary to that of 

 gravity, it be put into a'scale and weighed, it will be fbund to have 

 lost a ton of its weight, or, in other words, ' to press downwards with 

 a force only equal to the difference of the two forces. 



These facts are coiTectly indicated by the expression tentlcncy. All 

 laws of causation,, in consequence of their liability to be counteracted, 

 require to be stated in words affirmative of tendencies only, and not of 

 actual results. In those sciences of causation which have an accurate 

 nomenclature, there are special w'or'ds which signify a tendency to the 

 , particular effect with which the science is 'conversant ; thus j)resst/rc, in 

 mechanics, is synonymous with tendency to motion, and forces are not 

 reasoned upon as causing actual motion, but as exerting pressure. A 

 similar improvement in terminology would be very salutary in many 

 other branches of science. 



Tlie habit of neglecting this necessary element in the precise ex- 

 pression of the laws of nature, has given birth to the popular prejudice 



