VI.] THE INDIRECT METHOD OF INFERENCE. Ill 



or class so far as furnished by tliat proposition in accordance 

 with tlie Laws of Thought. The machine is thus the em- 

 bodiment of a true logical system. The combinations are 

 classified, selected or rejected, just as they should be by a 

 reasoning mind, so that at each step in a problem, the 

 Logical Alp]ial:)et represents the proper condition of a mind 

 exempt from mistake. It cannot be asserted indeed tliat 

 the machine entirely supersedes the agency of conscious 

 thouglit; mental labour is required in interpreting the 

 meaning of grammatical expressions, and in correctly im- 

 pressing that meaning on the machine ; it is further required 

 in gathering the conclusion from the remaining combina- 

 tions. Nevertheless the true process of logical inference 

 is really accomplished in a purely meclianical manner. 



It is worthy of remark that the machine can detect any 

 self-contradiction existing between the premises presented 

 to it ; should the premises be self-contradictory it will be 

 found that one or more of the letter-terms disappears 

 entirely from the Logical Alphabet. Thus if we work the 

 two propositions, A is B, and A is not-B, and then inquire 

 for a description of A, the machine will refuse to give it 

 by exhibiting no combination at all containing A. This 

 result is in agreement with the law, which I have ex- 

 plained, that every term must have its negative (p. 74). 

 Accordingly, whenever any one of the letters A, B, C, D, a, 

 b, c, d, wholly disaj^pears from tlie aljjhabet, it may be 

 safely inferred tliat some act of self-contradiction has been 

 committed. 



It ought to be carefully observed that the logical 

 machine cannot receive a simple identity of the ibrni 

 A = B except in the double form of A = B and B = A. 

 To work the proposition A = B, it is therefore necessary to 

 press the keys — 



A (left), copula, B (right), full stop ; 

 B (left), copula, A (right), full stop. 

 The same double o[)ei'ation will Ije necessary whenever tho 

 proposition is not of the kind called a partial identity 

 (p. 40). Thus AB = CD, AB = AC, A = B -I- C, A I- B 

 = C -I- D, all require to Ije read from botli ends separately. 



The proper rule for using the machine may in fact be 

 given in tlu; Ibl lowing way: — (i) Ikad cadi propcsition as 

 it stands, and play the corresponding keys : (2) Convert the 



