70S THE PRINCIPLES OF SCIENCE. [CHAP. 



a certain way recognising that combination as an object of 

 thought. 



The conclusion at which I arrive is in opposition to 

 that of Mr. Spencer. I think that whenever we abstract 

 a quality or circumstance we do generalise or widen the 

 notion from which we abstract. Whatever the terms A, 

 B, and C may be, I hold that in strict logic AB is mentally 

 a wider term than ABC, because AB includes the two 

 species ABC and A Be. The term A is wider still, for it 

 includes the four species ABC, ABc, AbC, Abe. The Logi- 

 cal Alpaabet, in short, is the only limit of the classes of 

 objects which we must contemplate in a purely logical 

 point of view. Whatever notions be brought before us, 

 we must mentally combine them in all the ways sanc- 

 tioned by the laws of thought and exhibited in the Logical 

 Alphabet, and it is a matter for after consideration to 

 determine how many of these combinations exist in out- 

 ward nature, or how many are actually forbidden by the 

 conditions of space. A classification is essentially a 

 mental, not a material thing. 



Discovery of Marks or Characteristics. 



Although the chief purpose of classification is to disclose 

 the deepest and most general resemblances of the objects 

 classified, yet the practical value of a system will depend 

 partly upon the ease with which we can refer an object to 

 its proper class, and thus infer concerning it all that is 

 known generally of that class. This operation of discover- 

 ing to which class of a system a certaiu'specimen or case be- 

 longs, is generally called Diagnosis, a technical term familiarly 

 used by physicians, who constantly require to diagnose or 

 determine the nature of the disease from which a patient is 

 suffering. Now every class is defined by certain specified 

 qualities or circumstances, the whole of which are present 

 in every object contained in the class, and not all present in 

 any object excluded from it. These defining circumstances 

 ought to consist of the deepest and most important circum- 

 ritances, by which we vaguely mean those probably forming 

 the conditions with which the minor circumstances are 

 correlated. But it will often happen that the so-called 

 important points of an object are not those which can 



