PREFACE 



Henri Poincare was, by general agreement, the 

 most eminent scientific man of his generation — more 

 eminent, one is tempted to think, than any man of 

 science now living. From the mere variety of the 

 subjects which he illuminated, there is certainly no 

 one who can appreciate critically the whole of his 

 work. Some conception of his amazing comprehen- 

 siveness may be derived from the obituary number of 

 the Revue de Metaphysique et de Morale (September 

 191 3), where, in the course of 130 pages, four eminent 

 men — a philosopher, a mathematician, an astronomer, 

 and a physicist — tell in outline the contributions which 

 he made to their several subjects. In all we find the 

 same characteristics — swiftness, comprehensiveness, 

 unexampled lucidity, and the perception of recondite 

 but fertile analogies. 



Poincare's philosophical writings, of which the pres- 

 ent volume is a good example, are not those of a 

 professional philosopher : they are the untrammelled 

 reflections of a broad and cultivated mind upon the 

 procedure and the postulates of scientific discovery. 

 The writing of professional philosophers on such sub- 

 jects has too often the deadness of merely external 

 description ; Poincare's writing, on the contrary, as 

 the reader of this book may see in his account of 

 mathematical invention, has the freshness of actual 

 experience, of vivid, intimate contact with what he is 



