CHAPTER XXI. 



THEORY OF APPROXIMATION, 



In order that we may gain a true understanding of the 

 kind, degree, and value of the knowledge which we ac- 

 quire by experimental investigation, it is requisite that 

 we should be fully conscious of its approximate character. 

 We nmst learn to distinguish between what we can know 

 and cannot know — between the questions which admit of 

 solution, and those which only seem to be solved. Many 

 persons may be misled by the expression exact science, 

 and may think that the knowledge acquired by scientific 

 methods admits of our reaching absolutely true laws, 

 exact to the last degree. There is even a prevailing- 

 impression that when once mathematical formulai have 

 been successfully applied to a branch of science, this por- 

 tion of knowledge assumes a new nature, and admits of 

 reasoning of a higher character than those sciences whicli 

 are stdl unmatliematical. 



The very satisfactory degree of accuracy attained in the 

 science of astronomy gives a certain plausibility to erro- 

 neous notions of this kind. Some persons no doubt con- 

 sider it to be irrovcd that planets move in ellipses, in such 

 a manner that all Kepler's laws hold exactly true ; but 

 there is a double error in any such notions. In the first 

 place, Kepler's laws are not proved, if by proof we mean 

 certain demonstration of their exact truth. In the next 

 place, even assuming Kepler's laws to be exactly true in a 

 theoretical point of view, the planets never move according 

 to those laws. Even if we could oljserve the motions of a 

 planet, of a perfect globular form, free from all perturbing 



