How to Test Validity using Reductio ad Absurdum

  • Thread starter StevieTNZ
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  • #1
StevieTNZ
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Hi there,

I'm quite interested in propositional logic. Can anyone point me to a document out there (preferably from a university website, e.g. a lecturer/Professor etc. of Philosophy's personal webpage) that explains, in a clear manner, how to test for validity using the Reductio ad Absurdum method.

Many thanks,
Stevie
 

Answers and Replies

  • #2
ImaLooser
487
3
Hi there,

I'm quite interested in propositional logic. Can anyone point me to a document out there (preferably from a university website, e.g. a lecturer/Professor etc. of Philosophy's personal webpage) that explains, in a clear manner, how to test for validity using the Reductio ad Absurdum method.

Many thanks,
Stevie

Start with a self-consistent system. This means that (A and not A) is false for all A.

Let's say that the truth value of X is unknown. Assume X is true. If you can deduce for some A that (A and not A) is true, then you have done the impossible. Since it is not possible to do the impossible, something is wrong. You've reduced the system to an absurdity. The only possible thing that could be wrong is your assumption that X is true. So X must be false.
 
  • #3
Akshay_Anti
62
0
a very elementary example could be proving that √2 is irrational
 

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