14 THE PRINCIPLES OF SCIENCE. [CHAP 



These letter-terms will be used indifferently for nouns 

 substantive and adjective. Between these two kinds of 

 nouns there may perhaps be differences in a metaphysical 

 or grammatical point of view. But grammatical usage 

 sanctions the conversion of adjectives into substantives, and 

 vice versd ; we may avail ourselves of this latitude without 

 in any way prejudging the metaphysical difficulties which 

 may be involved. Here, as throughout this work, I shall 

 devote my attention to truths which I can exhibit in a 

 clear and formal manner, believing that in the present 

 condition of logical science, this course will lead to greater 

 advantage than discussion upon the metaphysical questions 

 which may underlie any part of the subject. 



Every noun or term denotes an object, and usually 

 implies the possession by that object of certain qualities 

 or circumstances common to all the objects denoted. There 

 are certain terms, however, which imply the absence of 

 qualities or circumstances attaching to other objects. It 

 will be convenient to employ a special mode of indicating 

 these negative terms, as they are called. If the general 

 name A denotes an object or class of objects possessing 

 certain denned qualities, then the term Not A will denote 

 any object which does not possess the whole of those 

 qualities ; in short, Not A is the sign for anything which 

 differs from A in regard to any one or more of the assigned 

 qualities. If A denote " transparent object," Not A will 

 denote " not transparent object." Brevity and facility of 

 expression are of no slight importance in a system of 

 notation, and it will therefore be desirable to substitute 

 for the negative term Not A a briefer symbol. De Morgan 

 represented negative terms by small lioman letters, or 

 sometimes by small italic letters ; l as the latter seem to 

 be highly convenient, I shall use a, b, c, . . . p, g, &c., as 

 the negative terms corresponding to A, B, 0, . . . P, Q, &c. 

 Thus if A means " fluid," a will mean " not fluid." 



Expression of Identity and Difference. 



To denote the relation of samauess or identity I unhesi- 

 tatingly adopt the sign =, so long used by mathematicians 

 to denote equality. This symbol was originally appropriated 

 1 Formal Logic, p. 38. 



