AS A MENTAL OPERATION 5 



indeed, we have to be content with mere analogy. 

 Sometimes we can use induction, and sometimes deduc- 

 tion. There is no one rule. For, if man shows to 

 advantage in the possession of reason, he is equally 

 at a disadvantage in his scientific investigation of Nature. 

 Man was, as it were, born too late. He came into the 

 world long after the beginning; in a very small world, . 

 in a planet of the sun which is not a star of the first 

 magnitude ; when this planet had gone through much 

 evolution before it became adapted to him ; and within 

 the limits of a puny organism, with imperfect senses, 

 which are however his only direct avenues to the ex- 

 ternal universe, and therefore the necessary conditions 

 of his reason. In these circumstances he has to fasten on 

 Nature as best he can. He knows more about present^ 

 facts than about past causes ; and when he knows causes, 

 it is sometimes the efficient cause, as in the mechanics 

 of the forces by whichTodies act on one another ; some- 

 times the material cause, as when the chemist knows 

 the elements out of which a body is composed without 

 having discovered the force of chemical affinity so-called 

 by which they compose it ; sometimes again the final 

 cause, as in morals, where we know for what end we 

 act without fully understanding the voluntary, nervous, 

 and muscular forces by which we perform our actions. 



Aristotle perceived this defect of human science when 

 he enunciated the fundamental principle of method 

 We must proceed from the better known to us. As what 

 is best known to us is various, the starting-point is 

 various, the processes of inference are various, and 

 therefore scientific methods are various. Aristotle long 

 ago said that methods vary with the subject-matter. 

 Mill, the best logician of our times, may be said to have 

 written the commentary on this text. He distinguishes 



