166 THE PRINCIPLES OF SCIENCE. 



are certainly in favour of a discovery being occasionally 

 made in this manner. But the greater the tact and 

 industry with which a physicist applies himself to the 

 study of nature, the greater is the probability that he 

 will meet with fortunate accidents, and will turn them 

 to good account. Thus it comes to pass that, in the 

 refined investigations of the present day, genius united to 

 extensive knowledge, cultivated powers and indomitable 

 industry, constitute the characteristics of the great dis- 

 coverer. 



Empirical Observations subsequently Explained. 



The second great portion of scientific knowledge consists 

 of facts which have been first learnt in a purely empirical 

 manner, but have afterwards been shown to follow from 

 some law of nature, that is, from some highly probable 

 hypothesis. Facts are said to be explained when they are 

 thus brought into harmony with other facts, or bodies of 

 general knowledge. There are few words more familiarly 

 used in scientific phraseology than this word explanation, 

 and it is necessary to decide exactly what we mean by it, 

 since the question touches the very deepest points con- 

 cerning the nature of science. Like most terms referring 

 to mental actions, the verbs to explain, or to explicate, 

 involve material similes. The action is ex plicis plana 

 reddere, to take out the folds, and render a thing plain or 

 even. Explanation thus renders a thing clearly compre- 

 hensible in all its points, so that there is nothing left 

 outstanding or obscure. 



Every act of explanation consists in detecting and 

 pointing out a resemblance between facts, or in showing 

 that a greater or less degree of identity exists between 

 apparently diverse phenomena. This resemblance may be 

 of any extent and depth ; it may be a general law of 



