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# CONSTRUCT A FORMAL PROOF FOR THE ARGUMENT USING THE INDIRECT METHOD

Mon 14 Dec, 2015 08:58 pm
a V ( c → ~p )
~a → ( ~p V c )
_____________
~p V a
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fresco

1
Mon 1 Feb, 2016 12:57 pm
@ArianaMaria,
For the conclusion to be F, a is F and p is T

this would make both the first and second premises F

therefore the argument is valid by 'the method of backward fell swoop' .
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fresco

1
Fri 5 Feb, 2016 11:57 am
@ArianaMaria,
Correction
Putting a=F and p=T makes the conjunction of the premises F, hence the argument is valid by backward fell,swoop.
bobhh

1
Fri 24 Mar, 2017 05:24 pm
@fresco,
What's a backward fell swoop, please ?
fresco

1
Sat 25 Mar, 2017 09:42 am
@bobhh,
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hibbitus

1
Sat 16 Sep, 2017 01:49 pm
This argument form leaves out the possibility ~c. That is, indirect proof is not valid under the following conditions: if a number of pre-conditions are necessary for a result to be true, and the result is not true, any of the pre-conditions can be false. It is only legitemate if you assume that all c -> p has been established. Normally, the method is used to point out, you either have to give up a or you have to give up a causal relationship that you are unlikely to be willing to abandoned. This is not to attack the method itself, it is an extremely valuable tool. And it can be valid when arguing carefully from a set of explicitly stated axioms. However, this is not the way it is normally used and the presumed validity should be recognized.
fresco

1
Sat 16 Sep, 2017 03:33 pm
@hibbitus,
Not sure what you mean here. The truth value of c is irrelevent to the proof. If in doubt construct the truth table.
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