AUTHOR'S PREFACE 



On opening this volume someone will say: "How is it 

 possible to make the history of a mind? One could 

 write an exact history of a man: he has spoken, he has 

 written, he has done things; we know where to lay hold 

 of him, and can follow him and judge him. But a mind, 

 especially that of a scientific man, is a bird on the wing; 

 we see it only when it alights, or when it takes flight. 

 When it is the mind of a genius, like Pasteur, the diffi- 

 culty seems almost insoluble. We may, by watching 

 closely, keep it in view, and point out just where it 

 touches the earth. But why does it alight here and not 

 there? Why has it taken this direction and not that 

 in its flight toward new discoveries? If it were possible 

 for you to know this and tell us, Pasteur would no longer 

 be a genius, escaping analysis; and if you do not tell us, 

 you will merely draw up a report, not write a history." 



All this is true, and nevertheless I have written this 

 book. I have done so for two reasons: the first is that 

 Pasteur was not a savant like the others. His scientific 

 life had an admirable unity; it was the logical and har- 

 monious development of one and the same thought. Of 

 course he did not know when he made his first studies in 

 crystallography that he would end by discovering a 

 means of preventing rabies. But neither did Chris- 

 topher Columbus know when he set forth, that he would 

 discover America. He only divined that by going always 

 in the same direction he would find something new. So 

 with Pasteur. From the beginning of his studies he had 

 before him a problem of life, and, having found the road 

 to it, from that time he always traveled in the same di- 

 rection, consulting the same compass. Without doubt 



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