Re: Derivations (SD)

Torkel Franzen <torkel@xxxxxxxxxx> writes:

>"1st Semester Logic Student" <jzarwel@xxxxxxxxx> writes:
>
>> d. {A -> B, B -> C, ~C} |- ~A
>>
>> Couldn't A be derived from A -> B with horseshoe elimination and then
>> just slap the tilde on the A with negation introduction? I think I'm
>> missing something here.
>
> Let's see, you're assuming a rule called "horseshoe elimination" by
>which A follows from A->B, and a rule called "negation introduction"
>by which ~A follows from A. That must be an interesting text you're
>using.

Well, it is and it isn't.

Lee Rudolph
.

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