that all Matter is Heavy. 271 



fallacy in this reasoning, for it proves a state of things to be necessary 

 when we can so easily conceive a contrary state of things. Is it denied, 

 the opponent may ask, that we can readily imagine a state of things in 

 which bodies have no weight ? Is not the uniform tendency of all bodies 

 in the same direction not only not necessary, but not even true ? For 

 they do in reality tend, not with equal forces in parallel lines, but to 

 a centre with unequal forces, according to their position : and we can 

 conceive these differences of intensity and direction in the force to be 

 greater than they really are ; and can with equal ease suppose the force 

 to disappear altogether. 



" To this I reply, that certainly we may conceive the weight of bodies 

 to vary in intensity and direction, and by an additional effort of imagi- 

 nation may conceive the weight to vanish : but that in all these suppo- 

 sitions, even in the extreme one, we must suppose the rule to be uni- 

 versal. If any bodies have weight, all bodies must have weight. If 

 the direction of weight be different in different points, this direction 

 must still vary according to the lav) of continuity ; and the same is true 

 of the intensity of the weight. For if this were not so, the rest and 

 motion, the velocity and direction, the permanence and change of bod- 

 ies, as to their mechanical condition, would be arbitrary and incohe- 

 rent : they would not be subject to mechanical ideas ; that is, not to 

 ideas at all ; and hence these conditions of objects would in fact be 

 inconceivable. ' In order that the universe may be possible, that is, may 

 fall under the conditions of intelligible conceptions, we must be able to 

 conceive a body at rest. But the rest of bodies (except in the absolute 

 negation of all force) implies the equilibrium of opposite forces. And 

 one of these opposite forces must be a general force, as weight, in order 

 that the universe may be governed by general conditions. And this 

 general force, by the conception of force, may produce motion, as well 

 as equilibrium ; and this motion again must be determined, and deter- 

 mined by general conditions ; which cannot be, except the communi- 

 cation of motion be regulated by an inertia proportional to the weight. 



" But it will be asked,' Is it then pretended that Newton's experiment, 

 by which it was intended to prove inertia proportional to weight, does 

 really prove nothing but what may be demonstrated a priori 1 Could 

 we know, without experiment, that all bodies, — gold, iron, wood, cork, — 

 have inertia proportional to their weight } And to this we reply, that 

 experiment holds the same place in the establishment of this, as of the 

 other fundamental doctrines of mechanics. Intercourse with the external 

 world is requisite for developing our ideas ; measurement of phenomena 

 is needed to fix our conceptions and to render them precise ; but the 

 result of our experimental studies is, that we reach a position in which 



