Wednesday, 11 January 2012

Logic and Philosophy, A. Hausman, H. Kahane, P. Tidman, Wadsworth, 11th ed., 2010, 13-1(6)

Prove the validity:
  1. (∃x)(y){[ ¬ Fxy ⊃ x=y ] Gx}
  2. (x){ ¬ Gx ⊃(∃y)[ y x • Fyx]}
  3. * ¬ Gx ......... ACP
  4. * (y){[ ¬ Fay ⊃ a=y ] Ga} ......... 1 EI x/a
  5. * ¬ Fax ⊃ a=x Ga ......... 4 UI y/x
  6. * Ga ......... 5 Simp.
  7. * a ≠ x ......... 3,6 Id
  8. * ¬ Fax ⊃ a=x ......... 5 Simp.
  9. * Fax ......... 7,8 MT
  10. * a ≠ x • Fax ......... 7,9 Conj.
  11. * (∃y)( y ≠ x • Fyx) ......... 10 EG
  12. ¬ Gx ⊃(∃y)[ y x • Fyx] ......... 3-11 CP
  13. (x){ ¬ Gx ⊃(∃y)[ y x • Fyx]} ......... 12 UG

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