PEEFACE, 1ST 



he left it. My own personal acquaintance with some of 

 the great men whose history I ventured to write, enabled 

 me to throw additional light upon it ; and respecting one, 

 whom of course I could not have known, Mr. Hume, I ob- 

 tained information from good sources through the kindness 

 of friends. The materials of his life are, however, chiefly 

 to be sought in his writings, arid especially in his letters. 

 The same remark is applicable to the Life of Voltaire. 

 Those who have written it, like the Marquis de Condorcet, 

 without ever referring to the fourteen large volumes (con- 

 taining nine thousand closely-printed pages) of his Corres- 

 pondence, might just as well have undertaken to give a life 

 of Rousseau without consulting his ' Confessions,' or of 

 Hume, without reading his ' Autobiography.' I have, 

 besides, had access to valuable original documents both of 

 Voltaire, Robertson, and Cavendish ; to some respecting 

 Watt and Simson. 



Scientific and literary history, the record of the progress 

 of science and of letters, and which is most usefully given 

 in the lives of their cultivators, serves two purposes ; the 

 one historical and critical, the other didactic. It is of 

 great importance to trace the progress of mankind in the 

 advancement of knowledge, and its diffusion ; to show by 

 what steps improvements have been made and applied ; to 

 estimate the relative merits of those whose claims upon 

 our gratitude are the most unquestionable ; and to ascertain 

 the position in which their labours have left the subjects of 

 those labours, with the aspect and extent of the region that 

 yet remains unexplored. But, it is hardly a less valuable 

 service of such works that they promote the knowledge of 

 the subject matter, both by exciting the desire of it, and 

 by facilitating its acquisition. The history of a philoso- 

 pher's life, that is, of his labours, the tracing of those steps 

 by which he advanced beyond his predecessors, the com- 



