ON THE INDUCTIVE LOGICAL PROBLEM. 119 



XIV. On the Inverse, or Inductive, Logical Problem. By 

 W. Stanley Jevons, M.A., F.R.S., Professor of 

 Logic in The Owens College, Manchester. 



Read December 26th, 1871. 



Logical deduction consists in ascertaining from a law or 

 laws the combinations of qualities which may exist under 

 those conditions. The natural law that all metals are 

 conductors of electricity really means that in nature we 

 find three classes of objects, namely : — (i) metals, con- 

 ductors ; (2) not metals, conductors ; (3) not metals, not 

 conductors. It comes to the same thing if we say that it 

 excludes the existence of the class metals not conductors. 

 But every scientific process has its inverse process. As 

 addition is undone by subtraction, multiplication by divi- 

 sion, involution by evolution, differentiation by integration, 

 so logical induction is the inverse process of deduction. 

 Given certain, classes of objects, we endeavour by induction 

 to pass back to the laws embodied in those classes ; given 

 combinations, we have to learn the laws obeyed by them. 

 There does not exist, indeed, any distinct method of in- 

 duction, except such as consists in inverting the processes 

 of deduction, by noting and remembering the laws from 

 which certain effects necessarily follow. The difficulties 

 of induction are thus exactly analogous to those of inte- 

 gration, which can only be performed by trial, assisted by 

 a full knowledge of the effects of differentiation and a cer- 

 tain happy knack of arranging the formulae so as to bring 

 them into connexion with some previous result of the 

 direct process. 



