778 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



foundation of his classification ; the value of characters being determined, on the 

 one hand by their constancy, and on the other by their degree of coincidence 

 with important features of general organization or of physiological history. 1 In 

 the determination of Physical laws, the process is somewhat of the same kind ; 

 but the similarities with which we have here to do, are not, as in the preceding 

 case, objective resemblances, but exist only among our subjective ideas of the 

 nature and causes of the phenomena brought under our consideration. Thus, 

 there is no obvious relation between the fall of a stone to the Earth, and the 

 motion of the Moon in an elliptical orbit around it ; but the penetrating mind 

 of Newton detected a relation of common causation between these two pheno- 

 mena, which enabled him to express them both under one law. It was by a 

 like intellectual perception of similarity, that Franklin was led to determine the 

 identity of lightning with the spark from an electrical machine. And it would 

 be easy to show that it has been in their extraordinary development of this 

 power of recognizing causative similarity, leading to a kind of intuitive percep- 

 tion of its existence where no adequate ground can be assigned by the reason 

 for such a relationship, that those men have been eminent, who have done the 

 most to advance science by the process of inductive generalization. The same 

 kind of mental action is also employed in the contrary direction ; namely, in 

 that extension of generic definitions to new objects, which takes place upon every 

 discovery of a new species ; and in that application of general laws to particular 

 instances, which constitutes deductive reasoning. We may trace it, again, even 

 in the extension of the meaning of words so as to become applicable to new 

 orders of ideas, in consequence of the resemblance which the latter are felt to 

 bear to those of which the words were previously the symbols : as in the ap- 

 plication of the word " head," which primarily designated the most elevated 

 part of the human body, in such phrases as the " head of a house," the " head 

 of a state," the " head of an army," the " head of a mob," in each of which the 

 idea of superiority and command is involved; or in the phrases the " heads of a 

 discourse," or the " heads of an argument," in which we still trace the idea of 

 authority or direction ; or in the phrases the " head of a table," the " head of 

 a river," in which the idea of superiority or origin comes to be locally applied; 

 or in the " head of a bed," or " head of a coffin," in which we have the more 

 distinct local association with the position of the head of man. Of the foregoing 

 applications, those first cited belong to the nature of a metaphor, which has been 

 described to be "a simile comprised in a word;" and the judicious use of meta- 

 phors, which frequently adds force as well as ornamental variety to the diction, 

 is most seen amongst men who possess a great power of bringing together the 

 "like" in the midst of the " unlike." Every effort, in fact, to trace out unity, 

 consistency, and harmony, in the midst of the wonderful and (at first sight) 

 perplexing variety of objects and phenomena amidst which we are placed, is a 

 manifestation of this tendency of the Human Mind ; and, when conducted in 

 accordance with the highest teachings of the Intellect, or guided by that Intui- 

 tion which in some minds supersedes and anticipates all reasoning, it enables us 

 to rise towards the comprehension of that great Idea of the Universe, which we 

 believe to exist in the Divine Mind in a majestic simplicity of which we can 

 here but faintly conceive, and of which all the phenomena of Nature are but the 

 manifestations to our consciousness. It may be remarked that this mode of 

 action of the mind is in some degree opposed to the preceding ; for whilst con- 

 tiguity leads to the arranging of ideas as they happen to present themselves in 



1 Thus, for example, it is now generally admitted, amongst Zoologists, that the Impl it- 

 cental Mammalia should constitute a separate sub-class, in virtue of the peculiar con- 

 formation of their generative apparatus, instead of being distributed among other Orders, 

 as they were left by Cuvier. 



