16 THE PRINCIPLES OF SCIENCE. 



language convenient, but it is essential to the expression 

 of those general truths which are the very soul of science. 

 To apprehend the truth of special cases of inference does 

 not constitute logic ; we must apprehend them as cases of 

 more general truths. The object of all science is the 

 separation of what is common and general from what is 

 accidental and different. In a system of logic, if anywhere, 

 we should esteem this generality, and strive to exhibit 

 clearly what is similar in very diverse cases. Hence the 

 great value of general symbols by which we can represent 

 the form and character of a reasoning process, disentangled 

 from any consideration of the special subject to which it is 

 applied. 



The signs required in logic are of a very simple kind. 

 As every sameness or difference must exist between two 

 things or notions, we need signs or terms to indicate the 

 things or notions compared, and other signs to denote the 

 relation between them. We shall need, then, (i) symbols 

 for terms, (2) a symbol for sameness, (3) a symbol for differ- 

 ence, and (4) one or two symbols to take the place of 

 conjunctions. 



Ordinary nouns substantive, such as Iron, Metal, Elec- 

 tricity, Undulation, might serve as terms, but for the 

 reasons explained above it is better to adopt blank letters, 

 devoid of special signification, such as A, B, C, D, E, &c. 

 Each letter must be understood to represent a noun, and, 

 so far as the conditions of the argument allow, any noun. 

 Just as in Algebra, x, y, z, p, q, r, &c. are used for any 

 quantities, undetermiAed or unknown, except when the 

 special conditions of the problem are taken into account, 

 so will our letters stand for undetermined or unknown 

 things. 



These letter-terms will be used indifferently for nouns 

 substantive and adjective. Between these two kinds of 

 nouns there may be important differences in a metaphysical 



