OjST THOUGHT IN MEDICINE. 207 



been handed down through several generations. We 

 must admit it all upon good faith; we cannot arrive 

 at the source ; and when many generations have con- 

 tented themselves with such knowledge, have brought 

 no criticism to bear upon it ; have, indeed, gradually 

 added all kinds of small alterations, which ultimately 

 grew up to large ones after all this, strange things are 

 often reported and believed under the authority of 

 primeval wisdom. A curious case of this kind is the 

 history of the circulation of the blood, of which we 

 shall still have to speak. 



But another kind of tradition by speech, which 

 long remained undetected, is even still more confusing 

 for one who reflects upon the origin of knowledge. 

 Speech cannot readily develop names for classes of 

 objects or for classes of processes, if we have not been ac- 

 customed very often to mention together the correspond- 

 ing individuals, things, and separate cases, and to assert 

 what there is in common about them. They must, 

 therefore, possess many points in common. Or if we, 

 reflecting scientifically upon this, select some of these 

 characteristics, and collate them to form a definition, 

 the common possession of these selected characteristics 

 must necessitate that in the given cases a great num- 

 ber of other characteristics are to be regularly met 

 with ; there must be a natural connection between the 

 (kst and the last-named characteristics. If, for instance, 



