CHAPTEK IX. 



THE DIRECTION OF MOTION. 



§ 74. The Absolute Cause of changes, no matter what 

 may be their special natures, is not less incomprehensible in 

 respect of the unity or duality of its action, than in all other 

 respects. We cannot decide between the alternative supposi- 

 tions, that phenomena are due to the variously-conditioned 

 workings of a single force, and that they are due to the con- 

 flict of two forces. Whether, as some contend, everything is 

 explicable on the hypothesis of universal pressure, whence 

 what we call tension results differentially from inequalities 

 of pressure in opposite directions; or whether, as might be 

 with equal propriety contended, things are to be explained 

 on the hypothesis of universal tension, from which pressure 

 is a differential result; or whether, as most physicists hold, 

 pressure and tension everywhere co-exist; are questions 

 which it is impossible to settle. Each of these three suppo- 

 sitions makes the facts comprehensible, only by postulating 

 an inconceivability. To assume a universal pressure, con- 

 fessedly requires us to assume an infinite plenum — an un- 

 limited space full of something which is everywhere pressed 

 by something beyond; and this assumption cannot be men- 

 tally realized. That universal tension is the immediate 

 agency to which phenomena are due, is an idea open to a 

 parallel and equally fatal objection. And however verbally 

 intelligible may be the proposition that pressure and tension 

 everywhere co-exist, yet we cannot truly represent to our- 



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