578 PHILOSOPHICAL THOUGHT. 



made of such vague conceptions as the " persistence 

 of force." 



26. In Germany the introductory lines of Kirchhoff's 



Kirchhoff. 



Lectures on " Mechanics " mark an era in scientific 

 thought : " Mechanics is the science of motion : we 

 define her task : to describe completely and in the 

 simplest manner the motions which take place in 

 nature." This definition implies a great deal more 

 than it actually states. In confining itself to descrip- 

 tion it discards explanation i.e., the search after 

 the causes, and, still more, after the ends of motion. 

 And as to the simplest manner of the description 

 Kirchhoff adds significantly : " It is quite imaginable 

 that doubts can exist whether one or the other descrip- 

 tion of certain phenomena is the simpler; it is also 

 thinkable that a description which to-day is the simplest 

 that can be given may in the further development of 

 science be replaced by one still more simple." Since 

 Kirchhoff wrote these words, they have been endlessly 

 repeated by men of science and philosophers alike, to all 

 of whom they have given much occasion for reflection. 



Kirchhoff's work appeared in 1876. Before that 

 time two thinkers of eminence had been led, through 

 purely scientific interests, to an analysis and discussion 

 of the axioms of physics and dynamics. They were : 



27. Wilhelm Wundt, who published in 1866 a tract "On 

 Mach. the physical axioms and their relation to the principle 



of causality," and Ernst Mach, who published in 1872 

 a tract " On the history and origin of the principle of 

 the conservation of energy." To these two writers we 

 owe, in their further publications, the most successful 



