[ ^°5 ] 



perceive, if they refledl for a moment, that effeds alone lie 

 within our cognizance. By our fenfes we difcover that a change 

 has happened in our own bodies, or in thofe by which they are 

 furrounded ; and by the faculty of perception we perceive that an 

 idea is prefent to the mind ; but in neither cafe does any cir- 

 cumftance fuggeft to us the manner in which the effeQ. has been 

 produced. We may in either cafe obferve a continued feries of 

 effeds happening in a regular order, which may induce us to 

 conjedure that they are conneded amongft themfelves as caufes 

 and effedls ; but however probable fuch a conjedure may be, 

 fmce it is founded merely on the obfervation of the order in 

 which one follows another, there is not any circumftance which 

 can guide us to any conjedure concerning the nature of that 

 connedion. 



Natural philofophers do indeed enquire into the laws ac- 

 cording to which forces ad, but the law of a force does not 

 point out its manner of operating. The law only tells us what 

 variety there would be in the effed in confequence of a certain 

 variety in the circumftances under which the caufe operated. 

 Thus the law of gravity is, that one body is attraded by another 

 v>'ith a force inverfely as the fquare of the diftance. This only 

 tells us that the quantities of the effeds produced by the un- 

 known caufe called gravity, at different diftances, are inverfely 

 as the fquares of thofe diftances. 



Sir Ifaac Newton has fuggefted that the gravitation of bodies 

 is probably the effed of the repulfion of a very fubtil elaftic 



fluid. 



