First of all, please don't close this question cause I don't get the explanation given in: Use the Fitch system to prove the tautology (p ∨ ¬p)

I have been trying to solve this exercise for days now. I'm stuck in step 9: enter image description here

Any thoughts? Thank you.

  • Why do you think that is not a duplicate ? In the linked post there is a Fitch proof: instead of retrying with a new post, why not checking it ? Apr 20 '20 at 15:13
  • You step 4 is the wrong one: use the contradiction with 1 and 3 to derive not-p. Apr 20 '20 at 15:14
  • The explanation Virmaior gives is very unclear and it is 3 years old. I haven't been able to solve it based on his explanation. That's why I need a new input.
    – Luen
    Apr 20 '20 at 15:15
  • 1
    Having derived not-p, you discharge 1 to get : not-(p or not-p) to not-p. Having done so, repeat all the derivation from 1 with the new assumption not-p in 2. In this case you get not-(not-p) and discharge again the assumption to get: not-(p or not-p) to not-(not-p). Now you can apply negation-intro rule (as above) to conclude with not-( not-(p or not-p) ). Apr 20 '20 at 16:08
  • 1
    Step 12 is same as 3 and 4 will be not-p to (p or not-p). And so on. Apr 20 '20 at 16:31

In step 10, you need to assume ~(p | ~p) again, in order to use Negation Introduction.

In full:

enter image description here

  • Nevermind, I just fixed it and was able to solve it. MANY THANKS F. Zer! You saved my life today
    – Luen
    Apr 20 '20 at 17:39
  • You are welcome, @Nagma. Glad it helped.
    – F. Zer
    Apr 20 '20 at 18:09

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.