LAWS OF MOTION. 411 



of rest, or of motion in a right line, is to 

 suppose that which never has, and never can 

 happen; it is necessary to suppose, that there 

 are situations in which no external agents are 

 present ; or, if present, that they exert on them 

 no influence whatever; either that space ex- 

 ists without matter, or matter without resist- 

 ance: a supposition such as this, is not only 

 without proof, but contrary to proof; a mere 

 petitio principii; so far, however, from acced- 

 ing to the truth of it, I reject, as inadmissi- 

 ble the principium petitionis ; I reject altoge- 

 ther the proposition, which requires for its 

 accomplishment the annihilation of all the 

 gaseous, the liquid, as well as all the solid 

 matter of which the world is composed ; for 

 so long as matter, such as this, continues to 

 exist, not only the direction in a right line, 

 which a moving body would describe, will be 

 perpetually changed, and the motion itself be 

 ultimately arrested; but the moving body there- 

 by " is from necessity," and by a sensible and 

 obvious force, " compelled to change its state." 

 And the law of nature is that as obstacles con- 

 stantly intervene, a body once put in motion 

 cannot move for ever in a right line. 



The necessity which a moving body is 

 compelled of changing its line of motion, in 

 consequence of the nature of the medium 

 through which it is impelled, proves that the 

 .second Newtonian proposition, or law, is not 



