90 ESSAYS AND OBSERVATIONS 



view of all the circnmftances foon difco- 

 vers, that body is equally indifferent to 

 cither ftate, of reft or motion. Body at 

 reft does not change its ftate of itfelf j no 

 more does body in motion. A body at 

 reft requires no caufe of its continuance ; 

 as little does a body in motion. This 

 property of body, by which it perfeveres 

 in its ftate of reft, or of uniform motion, 

 in a ftreight line, unlefs when forced to 

 change it by fome external influence, is 

 called the inertia of matter. 



We know not how motion is original- 

 ly communicated at all ; we know nothing 

 of the nature of motion or force, but by 

 experience. V/e can define neither, the 

 ideas being quite fimple. Natural philo- 

 fophy however takes it for granted, that 

 fuch a thing there is. Let a motion or 

 force begin any way you pleafe, we never 

 fee it ceafe till it be deftroyed. Why then 

 ihould w^e imagine a body ought to ftop 

 of itfelf ; and that, to preferve it in mo- 

 tion, a conftanc exertion is necclTary, like 

 that which produced it at firft ? What 

 argument can lead us to afcribe fuch an 



adivity 



