80 ESSAYS ann OBSERVATIONS 
different to either ftate, of reft or motion. 
Body at reft does not change its ftate of itfelf ; 
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 zmertia of 
matter. 
We know not how motion is originally 
communicated at all; we know nothing of 
the nature of motion or force, but by expe- 
rience. Wecan define neither; the ideas 
being quite fimple. Natural philofophy 
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 fhould we 
imagine a body ought to ftop of itfelf; and 
that, to preferve it in motion, a conftant exer- 
tion is neceflary, like that which produced it 
at firft? What argument can lead us to a- 
{cribe fuch an activity to body? What fhould 
determine the body to ftop, if there be no- 
thing to oppofe its motion? Is there any 
experiment painting that way? yes, ‘* we 
(a3 may 
