284 THE PRINCIPLES OF SCIENCE. 



chord, the Sages of Thebes, and the Gates of Thebes, but 

 it does not extend beyond the fact that they were all 

 seven in number. Between the most discrete notions, as, 

 for instance, those of time and space, analogy may exist, 

 arising from the fact that the mathematical conditions of 

 the lapse of time and of motion along a line are similar. 

 There is no identity of nature between a word and the 

 thing it signifies ; the substance iron is a heavy solid, 

 the word iron is either a momentary disturbance of the 

 air, or a film of black pigment on white paper ; but there 

 is analogy between words and their significates. The 

 substance iron is to the substance iron-carbonate, as the 

 name iron is to the name iron-carbonate, when these names 

 are used according to their correct scientific definitions. 

 The whole structure of language and the whole utility of 

 signs, marks, symbols, pictures, and representations of 

 various kinds, rest upon analogy. I may, perhaps, hope 

 to enter fully upon this important subject at some future 

 time, and to attempt to show how the invention of signs 

 enables us to express, guide, and register our thoughts. 

 It will be sufficient to observe here that the use of words 

 constantly involves analogies of a subtle kind ; we should 

 often be at a loss how to describe a notion, were we not 

 at liberty to employ in a metaphorical sense the name of 

 anything sufficiently resembling it. There would be no 

 expression for the sweetness of a melody, or the brilliance 

 of an harangue, unless it were furnished by the taste of 

 honey and the brightness of a torch. 



A very cursory examination of the cases in which we 

 popularly use the word analogy, shows that it includes all 

 degrees of resemblance or similarity. The analogy may 

 consist only in similarity of number or ratio ; or in like 

 relations of time or space. It may also consist in more 

 simple resemblance between physical properties. We 

 should not be using the word inconsistently with custom, 



