212 ON THOUGHT IN MEDICINE. 



serration of the operation of blood-letting must have 

 taught that, in the veins, it comes from the periphery, 

 and flows towards the heart. But this false theory 

 had become so mixed up with the explanation of fever 

 and of inflammation, that it acquired the authority of 

 a dogma, which it was dangerous to attack. 



Yet the essential and fundamental error of this 

 system was, and still continued to be, the false kind of 

 logical conclusion to which it was supposed to lead ; 

 the conception that it must be possible to build a 

 complete system which would embrace all forms of dis- 

 ease, and their cure, upon any one such simple explana- 

 tion. Complete knowledge of the causal connection of 

 one class of phenomena gives finally a logical coherent 

 system. There is no prouder edifice of the most exact 

 thought than modern astronomy, deduced even to the 

 minutest of its small disturbances, from Newton's law of 

 gravitation. But Newton had been preceded by Kepler, 

 who had by induction collated all the facts; and the 

 astronomers have never believed that Newton's force 

 excluded the simultaneous action of other forces. They 

 have been continually on the watch to see whether 

 friction, resisting media, and swarms of meteors have 

 not also some influence. The older philosophers and 

 physicians believed they could deduce, before they had 

 settled their general principles by induction. They 

 forgot that a deduction can have no more certainty than 



