CHAPTER XX. 



DOCTRINE OF CAUSE. 



THE very conception of motion necessarily implies 

 initial impulse. This is the principle underlying the 

 laws of motion. And the impossibility of motion or 

 change of motion taking place otherwise than through an 

 initial impulse, is itself but a special case of that wider 

 law that no event can take place without a cause. 



Considered as involved in time-relations, the event 

 must follow the cause. Such is the ordinary view. A 

 bullet is fired from a gun. At the end of a second it 

 strikes a bird in the air. The bird falls dead to the 

 ground. The elastic force of the explosive is the cause of 

 the velocity of the bullet. The velocity of the bullet, com- 

 bined with its mass, is the cause of the bird's death. The 

 action of gravity is the cause of the bird's fall. The 

 explosion seems to occur before the velocity of the ball; 

 the accumulation of velocity by the ball occurs before the 

 death of the bird. But a moment's reflection shows that 

 the action of gravity was not only prior to, but was also 

 simultaneous with, the bird's fall. A little further reflec- 

 tion will bring to light the fact that the expansive force of 

 the explosive is nothing else than the energy of molecular 

 motion, which, when transferred to the ball, becomes the 

 energy of molar motion. That is, the momentum of the 

 ball is really the same force as that of the explosive whose 



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