THAT ALL MATTER IS HEAVY. 205 



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 sup- 

 positions, even in the extreme one, we must suppose the rule to be universal. 

 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 law of continuity ; and the same is true of the intensity 

 of the Aveight. For if this were not so, the rest and motion, the velocity 

 and direction, the permanence and change of bodies, as to their mechanical 

 condition, would be arbitrary and incoherent: 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 determined by general conditions; which cannot be, 

 except the communication of motion be regulated by an inertia propor- 

 tional 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 ? 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 

 our convictions do not rest upon experiment. We learn by observation 



