THE PERSISTENCE OF FORCE. 199 



tion. From this, either of two inferences may be drawn. 

 Assuming the masses to be unchanged, their energies, ac- 

 tual and potential, may be proved to be undiminished; or 

 assuming their energies to be undiminished, the masses 

 may be proved unchanged. But the validity of one 

 or other inference, depends wholly on the truth of the as- 

 sumption that the unit of force is unchanged. Let it be sup- 

 posed that the gravitation of the two bodies towards each 

 other at the given distance, has varied, and the conclusions 

 drawn are no longer true. Nor is it only in their 



concrete data that the reasonings of terrestrial and celestial 

 physics assume the Persistence of Force. The equality of 

 action and reaction is taken for granted from beginning 

 to end of either argument ; and to assert that action and re- 

 action are equal and opposite, is to assert that Force is per- 

 sistent. The allegation really amounts to this, that there 

 cannot be an isolated force beginning and ending in noth- 

 ing; but that any force manifested, implies an equal ante- 

 cedent force from which it is derived, and against which it is 

 a reaction. 



We might indeed be certain, even in the absence of any 

 such analysis as the foregoing, that there must exist some 

 principle which, as being the basis of science, cannot be 

 established by science. All reasoned-out conclusions what- 

 ever must rest on some postulate. As before shown (§ 23), 

 we cannot go on merging derivative truths in those wider 

 and wider truths from which they are derived, without 

 reaching at last a widest truth which can be merged in no 

 other, or derived from no other. And whoever contem- 

 plates the relation in which it stands to the truths of science 

 in general, will see that this truth transcending demonstra- 

 tion is the Persistence of Force. 



§ 62. But now what is the force of which we predi- 

 cate persistence? It is not the force we are immediately 

 conscious of in our own muscular efforts; for this does 



