THE IDEAS ON CONTAGION PRIOR TO 1866 227 



tance of centuries. One finds the same trouble in assim- 

 ilating them that he would if they were some philo- 

 sophical work of the middle ages, and therein we see very 

 well what a chimera is the history of scientific ideas. 

 In order to understand the past state of a question, it 

 is necessary to assume an artificial state of mind, to pass 

 the sponge over certain ideas which we believe to be 

 true, putting in their place others which we know to be 

 false, in brief, to change the state of one's brain, and 

 that is impossible. 



I know very well that there remain in the books of 

 the period words which are supposed to be the clothing 

 of ideas, and through which one may try to see what 

 they covered. The partisans of the history of science 

 tell us, when it is a question of mathematics or physics 

 or of natural history, that these words have a more 

 precise meaning than when it is a question of philosophy, 

 and they are right. But if they conclude therefrom that 

 the history of science is easy to write, or even possible, 

 they are wrong, for, even in science homonyms are not 

 synonyms 30 years apart. The same tinsels cover very 

 different small rough models. We have just here a 

 striking example of this fact. 



For example, the words contagium vivum or animatum 

 have been current in science for a long time. They 

 have been found in Varro and Columella. Acknow- 

 ledging that they may still serve to express the ideas of 

 to-day, one has sometimes concluded that these ideas 

 are very old, that only knowledge of the mode of con- 

 tagion has been perfected with progress of time, and 

 that Pasteur is only the last one come, and the most 

 powerful, of a series of investigators who have labored 

 with the same directive idea. 



I have no need to go back very far to demonstrate the 

 inexactness of this point of view. I will confine myself 



