422 THE PRINCIPLES OF SCIENCE. 



observed that classification is the opposite process to dis- 

 crimination. If we feel that two tastes differ, for instance, 

 the tastes of two specimens of wine, the mere fact of 

 difference existing prevents inference. The detection of 

 the difference saves us, indeed, from false inference, be- 

 cause so far as difference exists, all inference is impossible. 

 But classification consists in detecting resemblances of all 

 degrees of generality, and ascertaining exactly how far 

 such resemblances extend, while assigning precisely at the 

 same time the points at which difference begins. It 

 enables us, then, at once to generalize and make inferences 

 where it is possible, and it saves us at the same time from 

 going too far. Full classifications constitute a complete 

 record of all our knowledge of the objects or events 

 classified, and the limits of exact or scientific knowledge 

 are identical with the limits of classification. 



It must by no means be supposed that every group 

 of natural objects will be found capable of rigorous 

 classification. There may be substances which vary by 

 insensible degrees, consisting, for instance, in varying 

 mixtures of simpler substances. Granite is a mixture 

 of quartz, felspar, and rnica, but there are hardly 

 two specimens in which the proportions of these three 

 constituents are alike, and it would be impossible to lay 

 down definitions of distinct species of granite without 

 finding an infinite variety of intermediate species. The 

 only true classification of granites, then, would be founded 

 on the proportions of the constituents present, and a 

 chemical or microscopic analysis would be requisite, in 

 order that we should assign any specimen to its true 

 position in the series. Granites vary, again, by insensible 

 degrees, as regards the magnitude of the crystals of fel- 

 spar and mica. Precisely similar remarks might be made 

 concerning the classification of other plutonic rocks, such 

 as syenite, basalt, pumice-stone, lava, tuff, &c. 



