6 PREFACE. 



describing. There results a certain richness and 

 resonance in his words: the sound emitted is not 

 hollow, but comes from a great mass of which only 

 the polished surface appears. His wit, his easy mas- 

 tery, and his artistic love of concealing the labour of 

 thought, may hide from the non-mathematical reader 

 the background of solid knowledge from which his 

 apparent paradoxes emerge : often, behind what may 

 seem a light remark, there lies a whole region of 

 mathematics which he himself has helped to explore. 



A philosophy of science is growing increasingly 

 necessary at the present time, for a variety of reasons. 

 Owing to increasing specialization, and to the con- 

 stantly accelerated accumulation of new facts, the 

 general bearings of scientific systems become more 

 and more lost to view, and the synthesis that depends 

 on coexistence of multifarious knowledge in a single 

 mind becomes increasingly difficult. In order to over- 

 come this difficulty, it is necessary that, from time to 

 time, a specialist capable of detachment from details 

 should set forth the main lines and essential structure 

 of his science as it exists at the moment. But it is 

 not results, which are what mainly interests the man 

 in the street, that are what is essential in a science: 

 what is essential is its method, and it is with method 

 that Poincar^'s philosophical writings are concerned. 



Another reason which makes a philosophy of science 

 specially useful at the present time is the revolutionary 

 progress, the sweeping away of what had seemed fixed 

 landmarks, which has so far characterized this century, 

 especially in physics. The conception of the " working 

 hypothesis," provisional, approximate, and merely use- 

 ful, has more and more pushed aside the comfortable 



