52 PASTEUR: THE HISTORY OF A MIND 



phenomena too curious not to have attracted, from 

 the beginning, the attention of philosophers, who con- 

 tented themselves with borrowing from them comparisons 

 and figures, and the curiosity of searchers for the philos- 

 opher's stone, who were less disinterested. Might 

 not a base metal be transformed into a precious one 

 by means analogous to that which derived a savory 

 bread from an indigestible paste? Is there not some 

 powder of transmutation acting like a ferment? Here 

 we have the question which the alchemists asked 

 themselves and which naturally they did not solve, 

 first because it is insoluble, second because though they 

 were experimenters, they were still more logicians, 

 believing in the power of the idea, and inclined to sub- 

 ordinate experiment to it. 



It is not that there do not exist in their writings 

 phrases in which, if one is so inclined, it is possible to 

 see, like the break of day, the forecast of recent discov- 

 eries. But in reading these ancient authors we must 

 always bear in mind that the word with them has often 

 preceded the idea because of the general mode of 

 education of the middle ages, and that in the sciences 

 the idea has almost always preceded the fact. The 

 word has no value of its own; an idea, so long as it 

 remains a view of the mind, is always balanced by an 

 opposing idea; the fact alone is convincing and brings 

 certainty. But facts are what the alchemists scarcely ever 

 found on the question of fermentation. The defini- 

 tions of it which they have given are only obscure or 

 pretentious paraphrases of the phenomena observed in the 

 manufacture of wine or of bread. They make allusions 

 sometimes to the setting free of gas (exaltatio), sometimes 

 to the fact that the fermented bread can, in its turn, 

 act as a yeast (immutatio) . But as they knew nothing 

 of the nature of the substance which ferments, nor of 



