PHIL 2020 Lecture Notes - Lecture 5: Reductio Ad Absurdum, Conditional Proof, Propositional Calculus
17 May 2017
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Unit 6: we have two or more rules of inference to introduce to our system. These rules will involve creating sub-proofs, proofs within a proof, so their citation and application will need to work differently. Indirect proof (ip: formal equivalent: reduction ad absurdum, begins by assuming opposite of what it tries to prove, assume what you want to disprove. A (asm) (asm) (asm for ip) (3. 1 and 1, ds) (2 and 3. 3, conj) (3. 1-3. 3, ip) Support only works one way: you can use propositions shown earlier in your proof if you move them into a sub-proof, but not if you move them out of your sub-proof. ~p): the inferences cites the whole range of steps involved in the sub-proof. C: 3. (c b) (asm) (asm for cp) (1, simp) (2. 1-2. 2, cp) Unit 6: the biggest difference between cp and ip is that there is no requirement as to when you stop a.
