INTRODUCTION. 7 



enunciated by me more than twenty years ago, in the French 

 and German lectures I gave at Paris and Berlin, compre- 

 hended the endeavour to combine all cosmical phenomena 

 in one sole picture of nature ; to show in what manner the 

 common conditions, that is to say, the great laws, by which 

 individual groups of these phenomena are governed, have 

 been recognized ; and what course has been pursued in ascend- 

 ing from these laws to the discovery of their causal con- 

 nexion. Such an attempt to comprehend the plan of the 

 universe the order of nature must begin with a genera- 

 lization of particular facts, and a knowledge of the con- 

 ditions under which physical changes regularly and periodi- 

 cally manifest themselves ; and must conduct to the thoughtful 

 consideration of the results yielded by empirical observation, 

 but not to " a contemplation of the universe based on specu- 

 lative deductions and development of thought alone, or to a 

 theory of absolute unity independent of experience." We are, 

 I here repeat, far distant from the period when it was thought 

 possible to concentrate all sensuous perceptions into the 

 unity of one sole idea of nature. The true path was indicated 

 upwards of a century before Lord Bacon's time, by Leonardo 

 da Vinci, in these few words: " Cominciare dalT esperienza 

 e per mezzo di questa scoprirne la ragione.'' 8 " Commence 

 by experience, and by mains of this discover the reason." In 

 many groups of phenomena we must still content ourselves 

 with the recognition of empirical laws ; but the highest and 

 more rarely attained aim of all natural inquiry must ever be 

 the discovery of their causal connexion. 9 The most satisfactory 



8 Op. cit. vol. ii. p. 661. 



9 In the Introductory Observations, in Cosmos, v. i. p. 30, 

 it should not have been generally stated that " the ultimate 

 object of the experimental sciences is to discover laws, and to 

 trace their progressive generalization." The clause " in 

 many kinds of phenomena," should have been added. The 

 caution with which I have expressed myself in the 2nd 



