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# First order logic (is gay) watch

1. I'm doing exercise 6.12 in Language Proof and Logic, Barwise and Etchemendy and basically, I dunno how to do it. I have to prove "Dodec(f)" from the premises at the top. My problem is where I've used "taut con" - basically that's cheating, but I duno what other rule to use.

Basically, I should be able to prove any statement once I have a contradiction, but I don't know what rule to use for this. I need Dodec(f) at the end of my three subproofs, as that seems to be the only way to eliminate a disjunction.
2. (Original post by Hathlan)
.
Your attachment is broken (at least for me).
3. Does this work?

4. Apologies for crashing into this thread .. but could you please tell me what this interesting-looking software is, and why 'Fitch'?
5. (Original post by Hathlan)
Does this work?
Yep.

If you want to avoid using 'Taut con', you can use negation_introduction instead. i.e. in your subproof which starts "¬Dodec(e)", introduce another subproof with the premise "¬Dodec(f)", do the same contradiction_introduction as before, and then exit the subproof and use negation introduction to give you "¬¬Dodec(f)".

That process is what "Taut Con" packs up. If you have a contradiction and want to derive anything, say P, just stick the contradiction in a subproof with the premise ¬P and s'all good.
6. (Original post by Kolya)
Yep.

If you want to avoid using 'Taut con', you can use negation_introduction instead. i.e. in your subproof which starts "¬Dodec(e)", introduce another subproof with the premise "¬Dodec(f)", do the same contradiction_introduction as before, and then exit the subproof and use negation introduction to give you "¬¬Dodec(f)".

That process is what "Taut Con" packs up. If you have a contradiction and want to derive anything, say P, just stick the contradiction in a subproof with the premise ¬P and s'all good.
Thanks, I'll give that a go when the not-linux computer room is free.
7. (Original post by ian.slater)
Apologies for crashing into this thread .. but could you please tell me what this interesting-looking software is, and why 'Fitch'?
http://en.wikipedia.org/wiki/Fitch-style_calculus

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