16 CORRELATION OF PHYSICAL FORCES. 



been a source of great confusion in physical theories, and 

 philosophers arc even now by no means agreed as to their 

 conception of causation. The most generally received view 

 of causation, that of Hume, refers it to invariable antecedence 

 i. e., we call that a cause which invariably precedes, that 

 an effect which invariably succeeds. Mam* instances of in- 

 variable sequence might however be selected, which do not 

 present the relation of cause and etVect : thus as Reed obs> 

 and Brown does not satisfactorily answer, day invariably 

 precedes night and yet day is not the cause of night. Tin 

 again, precedes the plant, but is not the cause of it ; so that 

 we study physical phenomena it heroines ditlicult to separate the 

 idea of causal ion from that of force, and those have been regarded 

 as identical by some philosophers. To take an example which 

 will contrast tlu-se two views : if a floodgate be raised, the water 

 flows out ; in ordinary parlance, the water is said to flow be- 

 cause the floodgate is raised : the sequence is invariable ; no 

 floodgate, properly so called, can be raised without the water 

 flowing out, and yet in another, and perhaps more strict, sense, 

 it is the gravitation of the water which <-HM x it to flow. But 

 though we may truly say that, in this instance, gravitation 

 cause'- the water to ll>v . not in truth abstract the pro- 



position, and say, gem-rally, that gravitation is the cause of 

 water flowing, as water may flow from other < 

 elasticity, for instance, which will cause water to flow from a. 

 receiver full of air into one that is exhausted : gravitation may 

 also, under certain circumstances, arrest instead of cause the 

 flow of water. 



Upon neither view, however, can we get at anything like 

 abstract causation. If we regard causation as invariable se- 

 quence, we can find no case in which a given antecedent is 

 the only antecedent to a given sequent : thus if water could 

 flow from no other cause than the withdrawal of a floodgate, 

 we might say abstractedly that this was the cause of water 

 flowing. If, again, adopting the view which looks to causa- 



