The Gricean natural deduction system 's "most counterintuitive feature" (Grice, p. 128)

Grice has enough charm (why shouldn't he) to go on to state what he finds the 'most counter-intutuitive feature' of his natural deduction system. It is --

the admissiblity of an appropriate reading for two inference patterns:

first counterintuitive inference pattern:

a does not exist
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Therefore, a is not F.


second counterintuitive inference pattern:


a is F; alternatively a is not F
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Therefore, something is F; alternatively, something is NOT F.

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