16 CORRELATION OF PHYSICAL FORCES. 



been a source of great confusion in physical theories, and 

 philosophers are even now by no means agreed as to theii 

 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. Many instances of in 

 variable sequence might however be selected, which do not 

 present the relation of cause and effect : thus as Reed observes, 

 and Brown does not satisfactorily answer, day invariably 

 precedes night and yet day is not the cause of night. The seed , 

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

 we study physical phenomena it becomes difficult to separate the 

 idea of causation from that of force, and these have been regarded 

 as identical by some philosophers. To take an example which 

 will contrast these 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 causes it to flow. But 

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

 causes the water to flow, we cannot in truth abstract the pro 

 position, and say, generally, that gravitation is the cause of 

 water flowing, as water may flow from other causes, gaseous 

 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- 



