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Deductive Logic

by George William Joseph Stock, M.A.

Part 3: Chapter 27

Additional Information
  • Year Published: 1888
  • Language: English
  • Country of Origin: England
  • Source: Stock, G. W. J. (1888). Deductive Logic. Oxford, England; Pembroke College.
  • Readability:
    • Flesch–Kincaid Level: 11.0
  • Word Count: 1,434
  • Genre: Informational
  • Keywords: math, math history
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PART III.—OF INFERENCES

CHAPTER XXVII.

Of the Reduction of the Dilemma.

796. As the dilemma is only a peculiar variety of the partly conjunctive syllogism, we should naturally expect to find it reducible in the same way to the form of a simple syllogism. And such is in fact the case. The constructive dilemma conforms to the first figure and the destructive to the second.

(1) Simple Constructive Dilemma.

                                      Barbara.
  If A is B or if E is F, C is D.   All cases of either A being B or E
                                       being F are cases of C being D.
  Either A is B or E is F.          All actual cases are cases of either
                                       A being B OP E being F.
  .'. C is D.                       .'. All actual cases are cases of C
                                       being D.
(2) Simple Destructive.

                                       Camstres.
  If A is B, C is D and E is F.      All cases of A being B are cases of
                                        C being D and E being F.
  Either C is not D or E is not F.   No actual cases are cases of C being
                                        D and E being F.
  .'. A is not B.                    .'. No actual cases are cases of A
                                        being B.
(3) Complex Constructive.

                                          Barbara.
  If A is B, C is D; and if E is F,     All cases of either A being B or
     G is H.                               being F are cases of either C being
                                           D or G being H.
  Either A is B or E is F.              All actual cases are cases of either A
                                           being B or E being F.
  .'. Either C is D or G is H.          .'. All actual cases are cases of either C
                                           being D or G being H.
(4) Complex Destructive.

  If A is B, C is D; and if E is F,   All cases of A being B and E being F
  G is H.                                are cases of C being D and G
                                         being H.
  Either C is not D Or G is           No actual cases are cases of C being
     not H                               D and G being H.
  Either A is not B or E is           No actual cases are cases of A being
  not F.                                 B and E being F.
797. There is nothing to prevent our having Darii, instead of Barbara, in the constructive form, and Baroko, instead of Camestres, in the destructive. As in the case of the partly conjunctive syllogism the remaining moods of the first and second figure are obtained by taking a negative proposition as the consequent of the major premiss, e.g.—

  Simple Constructive.                Celarent or Ferio.
  If A is B or if E is F, C is not D  No cases of either A being B or E
                                         being F are cases of C being D.
  Either A is B or E is F.            All (or some) actual cases are cases of
                                         either A being B or E being F
  .'. C is not D.                     .'. All (or some) actual cases are not
                                         cases of C being D.