I am currently using the book Language, Proof and Logic

So far I have 1. a=c v b=c

subproof 2. a=c =intro 3 c=c =elim 1,2 4. c=a =elim 2,3

end subproof

new subproof 5. b=c 6. c=c =intro 7. c=b =elim 5,6

and when i end this subproof, and put down: 8. c=a v b=c velim 1, 2-4, 5-6

it is invalid? I am not sure how to approach this

  • 1
    Does the problem prohibit using symmetry of equality?
    – commando
    May 11 '14 at 18:39

It's certainly valid. Your derivation doesn't seem exactly right, but it can be modified to become correct.

enter image description here

  • Is this some kind of software package? Which is it? (Just curious.)
    – user3164
    May 11 '14 at 18:50
  • 1
    It's from the old edition of Language, Proof, and Logic. Barwise & Etchemendy have named it Fitch. (You have to admit, it looks much sexier than the new version that you can see in the modified question.) May 11 '14 at 18:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.