58 THE PRINCIPLES OF SCIENCE. 



exactly corresponds with the circumstance of being iden 

 tical in magnitude. Similarly in 



Opacity = Incapability of transmitting light, 

 the quality of being incapable of transmitting light is 

 declared 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 exo- 

 gens are the same as those which belong to all dicotyle 

 dons, 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 involving concrete plural terms might 

 be converted into one involving only abstract singular 

 terms, and vice versa. But there are many reasons for 

 believing that the intensive or qualitative form of reason 

 ing is the primary and fundamental one. It is sufficient 

 to point out that we may use abstract terms which contain 

 no reference to an extensive meaning ; and when there 

 is a mode which we must sometimes and may always 

 adopt, it is higher in importance than a mode which we 

 never need adopt necessarily. 



