PREFACE. 7 



eighteenth century conception of " laws of nature." 

 Even the Newtonian dynamics, which for over two 

 hundred years had seemed to embody a definite con- 

 quest, must now be regarded as doubtful, and as 

 probably only a first rough sketch of the ways of 

 matter. And thus, in virtue of the very rapidity of 

 our progress, a new theory of knowledge has to be 

 sought, more tentative and more modest than that of 

 more confident but less successful generations. Of 

 this necessity Poincar^ was acutely conscious, and it 

 gave to his writings a tone of doubt which was hailed 

 with joy by sceptics and pragmatists. But he was in 

 truth no sceptic : however conscious of the difficulty 

 of attaining knowledge, he never admitted its impos- 

 sibility. " It is a mistake to believe," he said, " that the 

 love of truth is indistinguishable from the love of cer- 

 tainty ;" and again: "To doubt everything or to believe 

 everything are two equally convenient solutions ; both 

 dispense with the necessity of reflection," His was the 

 active, eager doubt that inspires a new scrutiny, not 

 the idle doubt that acquiesces contentedly in nescience- 

 Two opposite and conflicting qualities are required 

 for the successful practice of philosophy — comprehen- 

 siveness of outlook, and minute, patient analysis. Both 

 exist in the highest degree in Descartes and Leibniz ; 

 but in their day comprehensiveness was less difficult 

 than it is now. Since Leibniz, I do not know of any 

 philosopher who has possessed both : broadly speaking, 

 British philosophers have excelled in analysis, while 

 those of the Continent have excelled in breadth and 

 scope. In this respect, Poincare is no exception : in 

 philosophy, his mind was intuitive and synthetic ; 

 wonderfully skilful, it is true, in analysing a science 



