PRESIDENTIAL ADDRESS SECTION A. 39 



all-important corollary to his second law — that forces are iiidc- 

 pendent, do not interfere with one another when acting simul- 

 taneously — Newton seems to utilize our preconceptions of the 

 word " force." 



The third law. which means, in plain language, that " the 

 intiuence experienced by A owing to the presence of B, is always 

 equal in amotmt, and opposite in direction to that ex])erienced 

 by B, owing to the presence of A," contains more material for 

 thought and even doubt than seems to be allowed for by the 

 best commentators. Mach says that this law is redundant, and 

 should be included tacitly in a definition of mass (which appar- 

 ently he would foiuid on an instinctive concejrtion of force). 

 This great assumjition is at the foundation of all Newtonian 

 reasoning, and is not, in the new schemes of Relativity and the 

 Quantum Theory, to be kept inviolate. It is not true of personal 

 emotions, as we know well. Is it inconceivable that the sun 

 should " attract "' the earth while the earth is passive towards 

 the sun ? 



A third fundamental idea in the laws (and this is the side 

 on which they are regarded as vulnera'ble by the priests of Rela- 

 tivity ) is the tacit assumption of an absolute space and time in 

 the background — an absolute framework of reference. The 

 great second law, on which is founded the mass of analysis which 

 has borne the fruit of all our mechanical knowledge — our astrono- 

 mical basis of time and sjjace, our engineering, oiu- means of 

 transport, our electricity of all kinds, all our machinery, and 

 also our high explosives and half unsuspected powers of de- 

 struction — depends only on relative velocities ; for most of these 

 results the motion of the earth is of no account ; and for all, the 

 motion of the " universe " of heavenly bodies round us is negli- 

 gible : but in the more intimate probing into nature — into the 

 properties of a:ther, hght, gravitation — men have come right uj) 

 against this tacit assumj:)tion of an absolute frame of reference, 

 a conceivable origin and axes of space and time ; and some deci- 

 sion has to be made. 



But to complete (uu- analysis of the history of the Newtonian 

 Ijhilosophy which, seldom though we realize it, represents much 

 of our present-day intellectual environment: is the ccther. we may 

 >;ay. of our accepted facts — 



Newton, by what to-day seems the simplest possible sequence 

 of a few propositions, deduced from his laws a complete ex- 

 ])lanation of the motion of the solar system. The riddle of the 

 ages, which Kei^ler had focussed so neatly, Newton was able 

 to answer in a few words— the sun attracts all the planets with 

 a force varying as the mass of each planet, and inversely as the 

 square of its distance : and the same is true of the planets and 

 their moons. 



Hence, without nuich difficulty, the generalized Law of 

 ("iravitation : " E\ery material particle in the universe attracts 

 every other material particle with a force varying as their two 

 masses and inversely as the s(|uare of their distance." 



