66 INTRODUCTION TO PHILOSOPHY OF SCIENCE 



but when one examines the locality thus called to his atten- 

 tion he may find nothing. Symbols enable one to talk about 

 absent objects, but, unfortunately, by virtue of this same 

 capacity, permit him to talk about objects which are not 

 even known to exist at all. Hence it is an important feature 

 of symbols that one may use them without any prejudice to 

 the existence or non-existence of their referents. 



It may be well at this point to clarify a distinction in the 

 referential character of symbols, which will be of basic 

 importance in later chapters. As was pointed out in the 

 preceding paragraph, symbols have the power not only to 

 point to events but also to call to mind other symbols with 

 which they apparently have significant relations. This 

 suggests that every symbol has two very important types of 

 relationship: (1) reference to events which are not themselves 

 normally symbols; x and (2) reference to other symbols. 

 These two types of reference are approximately what are 

 called in the traditional logic, as applied to concepts, exten- 

 sion or denotation, and intension or connotation. The term 

 "intension" is here used in a slightly broader sense than it 

 has been historically, since it applies to all of the relations 

 of a concept to other concepts and not merely to its defining 

 relations. Furthermore, the notions of extension and inten- 

 sion are generalized so as to be applicable to all symbols 

 rather than merely to concepts. In this general sense, then, 

 one may say roughly that extension of symbols is that prop- 

 erty by virtue of which they assume relations with the 

 realm of events, and intension of symbols is that property 

 by virtue of which they assume relations with one another. 

 The intensional relations, as applied to symbols of a verbal 

 character, are usually called formal relations, and are illus- 

 trated by implication, equivalence, and incompatibility; they 

 are the relations through which symbolic schemes are 

 formed. The intensional relations of images, diagrams, 

 models, and other pictorial symbols cannot be of this kind 



1 The events may, of course, be themselves symbols, but then the symbols which 

 refer to them are symbols of symbols. 



