10 COSMOS. 



ditions under which physical changes regularly and period- 

 ically manifest themselves ; and must conduct to the thought- 

 ful consideration of the results yielded by empirical observa- 

 tion, but not to " a contemplation of the universe based on 

 speculative 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 upward of a century before Lord Bacon's time, by 

 Leonardo da Vinci, in these lew words : " Cominciare dall' 

 esperienza e per mezzo di questa scoprirne la ragione."* 

 " Commence by experience, and by means 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 in- 

 quiry must ever be the discovery of their causal connection.^ 

 The most satisfactory and distinct evidence will always ap- 

 pear where the laws of phenomena admit of being referred 

 to mathematical principles of explanation. Physical cosmog- 

 raphy constitutes merely in some of its parts a cosmology. 

 The two expressions can not yet be regarded as identical. 

 The great and solemn spirit that pervades the intellectual 



* Op. cit., vol. ii. p. 283. 



*t In the Introductory Observations, in Cosmos, vol. i., p. 50, it should 

 not have been generally stated that " the ultimate object of the experi- 

 mental sciences is to discover laws, and to trace their progressive gen- 

 eralization." The clause "in many kinds of phenomena" should have 

 been added. The caution with which I have expressed myself in the 

 second volume of this work (p. 313), on the relation borne by Newton 

 to Kepler, can not, I think, leave a doubt that I clearly distinguish be- 

 tween the discovery and interpretation of natural laws, i.e., the explana- 

 tion of phenomena. I there said of Kepler: " The rich abundance of 

 accurate observations furnished by Tycho Brahe, the zealous opponent 

 of the Copernican system, laid the foundation for the discovery of those 

 eternal laws of the planetary movements which prepared imperishable 

 renown for the name of Kepler, and which, interpreted by Newton, 

 and proved to be theoretically and necessarily true, have been transferred 

 into the bright and glorious domain of thought, as the intellectual rec- 

 ognition of nature." Of Newton I said (p. 351): "We close it [the 

 great epoch of Galileo, Kepler, Newton, and Leibnitz] with the figure 

 of the earth as it was then recognized from theoretical conclusions. New- 

 ton was enabled to give an explanation of the system of the universe, 

 because he succeeded in discovering the force from whose action the 

 laws of Kepler necessarily result." Compare on this subject ("On Laws 

 and Causes") the admirable remarks in Sir John Herschel's address at 

 the fifteenth meeting of the British Association at Cambridge, 1845, p. 

 xlii. ; and Edinb. Rev., vol. 87, 1848, p. 180-183, 



