i.] INTRODUCTION. 



into prisms, so that the power of refracting light could 

 be directly observed ; but he fell upon the ingenious device 

 of compounding a liquid possessing the same refractive 

 power as the transparent fragment under examination. 

 The moment when this equality was attained could be 

 known by the fragments ceasing to reflect or refract light 

 when immersed in the liquid, so that they became almost 

 invisible in it. The refractive power of the liquid being 

 then measured gave that of the solid. A more beautiful 

 instance of representative measurement, depending im- 

 mediately upon the principle of inference, could not be 

 found. 1 



Throughout the various logical processes which we are 

 about to consider Deduction, Induction, Generalisation, 

 Analogy, Classification, Quantitative Reasoning we shall 

 find the one same principle operating in a more or less 

 disguised form. 



Deduction and Induction. 



The processes of inference always depend on the one 

 same principle of substitution ; but they may nevertheless 

 be distinguished according as the results are inductive or 

 deductive. As generally stated, deduction consists in 

 passing from more general to less general truths ; induc- 

 tion is the contrary process from less to more genera- 

 truths. We may however describe the difference in 

 another manner. In deduction we are engaged in develop- 

 ing the consequences of a law. We learn the meaning, 

 contents, results or inferences, "which attach to any given 

 proposition. Induction is the exactly inverse process. 

 Given certain results or consequences, we are required to 

 discover the general law from which they flow. 



In a certain sense all knowledge is inductive. We can 

 only learn the laws and relations of things in nature by 

 observing those things. But the knowledge gained from 

 the senses is knowledge only of particular facts, and we 

 require some process of reasoning by which we may 

 collect out of the facts the laws obeyed by them. 



1 Brewster, Treatise on New . Philosophical Instruments, p. 273. 

 Concerning this method see also Whewell, Philosophy of the Inductive 

 Sciences, vol. ii. p. 355 ; Tomlinson, Philosophical Magazine, Fourth 

 Series, voL xl. p. 328 ; Tyndall, in Youmans' Modern Culture, p. 16. 



