LAW OF CAUSATION. 381 



effeetus, has been a dogma of the schools : the necessity for 

 the continued existence of the cause in order to the continu 

 ance of the effect, seems to have heen once a generally received 

 doctrine. Kepler s numerous attempts to account for the 

 motions of the heavenly bodies on mechanical principles, were 

 rendered abortive by his always supposing that the agency 

 which set those bodies in motion must continue to operate in 

 order to keep up the motion which it at first produced. Yet 

 there were at all times many familiar instances of the continu 

 ance of effects, long after their causes had ceased. A coup de 

 soleil gives a person a brain fever : will the fever go off as soon 

 as he is moved out of the sunshine ? A sword is run through 

 his body : must the sword remain in his body in order that he 

 may continue dead? A ploughshare once made, remains a 

 ploughshare, without any continuance of heating and ham 

 mering, and even after the man who heated and hammered it 

 has been gathered to his fathers. On the other hand, the 

 pressure which forces up the mercury in an exhausted tube 

 must be continued in order to sustain it in the tube. This 

 (it may be replied) is because another force is acting without 

 intermission, the force of gravity, which would restore it to 

 its level, unless counterpoised by a force equally constant. 

 But again ; a tight bandage causes pain, which pain will some 

 times go off as soon as the bandage is removed. The illumina 

 tion which the sun diffuses over the earth ceases when the sun 

 goes down. 



There is, therefore, a distinction to be drawn. The con 

 ditions which are necessary for the first production of a phe 

 nomenon, are occasionally also necessary for its continuance ; 

 though more commonly its continuance requires no condition 

 except negative ones. Most things, once produced, continue 

 as they are, until something changes or destroys them; but 

 some require the permanent presence of the agencies which 

 produced them at first. These may, if we*please, be considered 

 as instantaneous phenomena, requiring to be renewed at each 

 instant by the cause by which they were at first generated. 

 Accordingly, the illumination of any given point of space 

 has always been looked upon as an instantaneous fact, which 



