48 THE PRINCIPLES OF SCIENCE. [CHAP. in. 



the assertion means that the circumstance of being" equal 

 exactly corresponds with the circumstance of being 

 identical in magnitude. Similarly in 



Opacity = Incapability of transmitting light, 

 the quality of being incapable of transmitting light is de- 

 clared to be the same as the intended meaning of the word 

 opacity. 



When general names form the terms of a proposition we 

 may apply a double interpretation. Thus 

 Exogens = Dicotyledons 



means either that the qualities which belong to all exogens 

 are the same as those which belong to all dicotyledons, or else 

 that every individual falling under one name falls equally 

 under the other. Hence it may be said that there are two 

 distinct fields of logical thought. We may argue either by 

 the qualitative meaning of names or by the quantitative, 

 that is, the extensive meaning. Every argument in- 

 volving concrete plural terms might be converted into 

 one involving only abstract singular terms, and vice 

 versd. But there are reasons for believing that the 

 intensive or qualitative form of reasoning is the primary 

 and fundamental one. It is sufficient to point out that the 

 extensive meaning of a name is a changeable and fleeting 

 thing, while the intensive meaning may nevertheless remain 

 fixed. Very numerous additions have been lately made 

 to the extensive meanings both of planet and element. 

 Every iron steam-ship which is made or destroyed adds to 

 or subtracts from the extensive meaning of the name 

 steam-ship, without necessarily affecting the intensive 

 meaning. Stage coach means as much as ever in one way, 

 but in extension the class is nearly extinct. Chinese 

 railway, on the other hand, is a term represented only by a 

 single instance ; in twenty years it may be the name of a 

 large class. 



