Sir Isaac Newton 13 



always be a definite answer to a definite question, and, 

 unless the mathematical difficulties v are insuperable, the 

 consequences of any assumption may be obtained in a form 

 in which they can be tested, not only as to their general 

 nature but also as to their numerical values. The result 

 may not be far-reaching, but within its limited field it is 

 definite. We may not have penetrated to the foundation 

 of the building, but we shall have mapped out one of its 

 apartments and perhaps reached a fresh starting point. 



Two centuries and a quarter have now passed since the 

 publication of Newton's " Principia," and during that time 

 our astronomical measurements have become more and 

 more accurate. Though the mathematical analysis has 

 sometimes found it difficult to keep pace with the improved 

 methods of observation, Newton's simple law of the inverse 

 square has hitherto always been found sufficient to explain 

 apparent irregularities in the motion of the celestial bodies, 

 with perhaps the solitary exception of an irregularity in 

 the motion of Mercury, which may ultimately be cleared up 

 without calling in some other agency or perhaps is destined 

 to open out an entirely new aspect of gravitation. 



The most precious heritage bequeathed to us by Newton 

 is this : He has given us the confidence that, complicated 

 as the problems of Nature may be, they are soluble if we 

 confine ourselves to a limited and definite range, and follow 

 up by irrefragable logical or mathematical reasoning the 

 consequences of clearly-defined premises. 



By his laws of motion Newton laid the foundation of 

 modern dynamics. The next great advance relates to 

 the constitution of "matter." Common experience shows 

 that . each .piece of matter may change in shape or volume ; 

 it even seemingly vanishes, as when water evaporates, or 

 is freshly formed, as when dew is deposited on. a blade of 

 grass. If this be kept in mind, we are forced to concede, 

 in opposition to the school which, professes to. reject all 

 theories, that an introspective philosophy entirely detached 

 from observation may lead to a truth hidden from the pure 

 experimentalist. To perceive that matter in spite of all 

 appearances is indestructible goes beyond the limits of 

 our direct observation; and a science without imagination 



