THE INDIRECT METHOD OF INFERENCE. 1 1 1 



indirect process of inference becomes reduced to the repe- 

 tition of a few uniform operations of classification, selection, 

 and elimination of contradictories. Logical deduction even 

 in the most complicated questions becomes a matter of 

 mere routine, and the amount of labour required is the 

 only impediment when once the meaning of the premises 

 is rendered clear. But the amount of labour is often 

 found to be considerable. The mere writing down of 

 sixty-four combinations of six letters each is no small 

 task, and, if we had a problem of five premises, each of 

 the sixty-four combinations would have to be examined 

 in connection with each premise. The requisite com- 

 parison is often of a very tedious character and consider- 

 able chance of errors thus arises. 



I have given much attention therefore to reducing both 

 the manual and mental labour of the process, and I shall 

 describe several devices which may be adapted for saving 

 trouble and risk of mistake. 



In the first place, as the same sets of combinations 

 occur over and over again in different problems, we may 

 avoid the labour of writing them out by having the sets 

 of letters ready printed upon small sheets of writing paper. 

 It has also been suggested by a correspondent that, if any 

 one series of combinations were marked upon the margin 

 of a sheet of paper, and a slit cut between each pair of 

 combinations, it would be easy to fold down any particular 

 combination, and thus strike it out of view. The combi- 

 nations consistent with the premises would then remain 

 in a broken series. This method answers sufficiently well 

 for occasional use. 



A more convenient mode, however, is to have the series 

 of letters shown on p. 109, engraved upon a common 

 school writing slate, of such a size, that the letters may 

 occupy only about a third of the space on the left hand 

 side of the slate. The conditions of the problem can then 



