ch. v.J THE TWO METHODS. 127 



causation were metaphysically explained "by the supposition 

 of a specific hidden power in the cause, which constrains 

 the effect to follow. Hume denied the existence of any such 

 specific hidden power, and his denial was also metaphysical, 

 because neither the presence nor the absence of such a 

 specific power is a necessary inference from the phenomena. 

 If we would keep clear of metaphysics, we must in such a 

 case neither affirm nor deny concerning a subject which lies 

 utterly beyond our reach. Physics knows nothing of causa- 

 tion except that it is the invariable and unconditional sequence 

 of one event upon another: whether the one event, in a 

 metaphysical sense, constrains the other to follow it or not 

 we cannot tell. Physics knows nothing of such constraint 

 — neither that it exists, nor that it does not exist. 



For the moment I have, somewhat too freely, used the 

 word "physics" as synonymous with "science " ; for I have 

 aimed at bringing out the fundamental distinction between 

 metaphysics and science, — which is this : — A scientific ex- 

 planation is a hypothesis which admits of verification, — it can he 

 either proved or disproved; while a metaphysical explanation is 

 a hypothesis which docs not admit of verification, — it can neither 

 he proved nor disproved. Newton's hypothesis of gravitation, 

 to account for the planetary motions, was strictly scientific ; 

 and so was Descartes' hypothesis of vortices, to account for 

 the same phenomena. The former admitted of proof, and 

 the latter admitted of disproof. But Stahl'a hypothesis of a 

 Vital Principle, to account for the phenomena of life, was 

 strictly metaphysical. Whether it is true or not, we can 

 never know. Push our researches as far as we may, we can 

 know life only as the assemblage of certain phenomena, 

 displaying the activity of certain forces. Whether in addition 

 to this there is a Vital Principle or not, no amount of research 

 can ever tell us. Science has simply nothing to do with it. 



Thus we see that the fundamental difference between 

 metaphysics and science is the difference between the 



