THE PRINCIPLES OF SCIENCE. [CHAP. 



three-sided rectilinear figure, with or without three equal 

 angles," the last alternatives really express a property of 

 triangles, namely, that some triangles have three equal 

 angles, and some do not have them. If we put P = 

 " Some," meaning by the indefinite adjective " Some," one 

 or more of the undefined properties of triangles with three 

 equal angles, arid take 

 A = triangle 



B = three-sided rectilinear figure, 

 C = with three equal angles, 



then the knowledge implied is expressed in the two 

 propositions 



PA = PEG 

 pA. = pBc. 



These may also be thrown into the form of one pro- 

 position, namely, 



A = PBC -I- pRc ; 



but these alternatives cannot be reduced, and the propo- 

 sition is quite different from 



A = BC -I- Be. 



Illustrations of the Indirect Method. 



A great variety of arguments and logical problems 

 might be introduced here to show the comprehensive 

 character and powers of the Indirect Method. We can 

 treat either a single premise or a series of premises. 



Take in the first place a simple definition, such as " a 

 triangle is a three-sided rectilinear figure." Let 

 A = triangle 

 B = three-sided 

 C = rectilinear figure ; 

 then the definition is of the form 

 A = BC. 



If we take the series of eight combinations of three 

 letters in the Logical Alphabet (p. 94) and strike out 

 those which are inconsistent with the definition, we have 

 the following result : ABC 

 dBc 

 abC 

 ale. 



