344 PERSONAL PREPARATION FOR RESEARCH. 



correct reasoning in matters of science ; and in scientific 

 argument and inference we must reject all ideas which con- 

 tradict them or any of the other great truths of nature. 1 



All the materials of our reasoning in science are pri- 

 marily obtained by means of comparison from the results 

 of our experience ; thus we compare facts and draw 

 general truths and conclusions from them ; we compare 

 those truths, and draw still wider conclusions ; we ex- 

 clude circumstances and conditions, then compare the 

 effects, and infer causes, coincidences, and explanations, 

 and so on ; and, having found the causes and explanations, 

 we deductively infer from them the existence of new facts. 

 When we cannot compare, we cannot obtain the primary 

 means of reasoning. 



The fundamental basis of all reasoning in science is a 

 perception of identity, and the most simple cases of infer- 

 ence are those in which identities alone are concerned. 

 The simplest rule of inference is, that so far as there 

 exists real sameness or equality in two different objects, 

 that which is true of one thing may be safely affirmed of 

 the other ; and this rule applies not only to sameness of 

 quality or kind, but also to sameness of quantity, and to 

 all identities whatever. As soon as we are able to infer, 

 we are able to predict, and in this way reason enables us 

 to argue from the seen to the unseen, from the known to 

 the unknown ; to judge before an act is performed what 

 the effect of that act will be. Scientific inference is 

 largely prophetic ; we often infer what we cannot observe. 

 Before we even see a thing, we may safely predict that it 

 cannot possess contradictory properties or attributes, or, 

 under the same conditions, produce contradictory effects ; 

 and that all its properties, attributes, and effects agree 

 with the laws of nature ; and the more perfectly we know 



1 See Chapter XIV. 



