Conditional Proof Assumption & Reductio Ad Absurdum part 1

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Intermediate Logic by James Nance; exercises 18 & 19, explained.

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Your videos have helped me to make it through directing Ch B for 2 years. It makes it so much easier to understand. I saw that Jim Nance addressed numbers 7 and 8 in a question posted on his blog. He used an example from Esther for
'U, therefore If W then W' -- "I will go to the king; If I perish, I perish." While this example does show that the assumption used for the conditional proof doesn't have to relate to anything in the previous propositions, I think you could assume that a proposition is implied but missing here..."If I go to the king, ...he might kill me...." That would connect to "If I perish, then I perish.", but changes the propositions.

michelew

I'm using my daughter's laptop, but still wanted to say THANK YOU (!) for your diligence in recording and posting these videos. They have helped me navigate 2nd semester when otherwise I would have thrown in the towel! I am so grateful!

delanagrace

Thank you for taking the time to explaining Logic. It has helped me a lot in my first year of tutoring.

tomiward

SO HELPFUL! YUO NEED TO DO ALL THE LESSONS

gracelibman

In the last few examples of the conditional proof she is NOT using the rule correctly. You are NOT allowed to derive anything like that. Why? There are intermediate steps left out. At the end of a conditional proof you must have used other INFERENCE RULES to move forward --she just reasserts the same lines which is a no no. You are not allowed to just reassert any premises at will. The rule tautology is the closest thing you have to do so AND notice she did not mention it at all. So there were steps missed. You can not just use cp like that! Every line has to actually be a rule we can look up directly.

roychess

U therefore If W then W is not circular reasoning. It is just bad logic.
If the conclusion FOLLOWS from the premises, it is valid.
The conclusion does not follow, therefore this is not a valid argument.
If the premises were true AND the conclusion followed, it would also be sound.

gettingdialectic

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