ISAAC NEWTON 87 



inertia is only a special case of the general second law of 

 motion, but one which, on account of its great importance, 

 appears to merit a special statement. Force, according to the 

 definition of the second law, is decisive for all change of 

 motion. But here we have the peculiar fact that force can be 

 detected and measured without change of motion, namely in 

 cases, investigated by Archimedes, of equilibrium between 

 forces; for example the force of gravity, or weight, in the case 

 of a balance. This is taken into account by Newton in a 

 special analysis of the concept of force. 



The third law of motion is one introduced by himself; 

 it is the law of the equality between action and reaction. It 

 deals with a peculiarity of all forces in nature, namely their 

 occurrence in pairs, so that to every force acting in any 

 direction, there belongs another force acting in the same 

 straight line, and of the same magnitude, but opposite in 

 direction. Newton derives this law expressly from experi- 

 ence - very simple and everyday experience, of which he 

 gives examples - just as Galileo derived the two first laws 

 from experience. Indeed, every person of sense must 

 regard it as self-evident, that everything relating to natural 

 science can only be derived from experience. To experience 

 then belong all observations made to test the correctness 

 of conclusions drawn from laws which, perhaps, were 

 originally only founded upon a small degree of experience. 



These three laws of motion occupy a further peculiar 

 position in Newton's case; they are given by him a meaning 

 hitherto unthought of, even by Huygens, namely that they 

 are simple and self-evidently valid for all matter, including 

 that in cosmic space. This makes Newton the founder of 

 the mechanics of the heavens. The fact that he could devise 

 this mechanics successfully - and he did so to a point which 

 has left us nothing of importance to add up to to-day - using 

 laws of motion derived only from terrestrial matter, the fact 

 in other words, that there are laws of motion valid for all 



