|Geach, Peter T.
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|Practical Inference||I 286
Practical Inference/Kenny/Geach: indicative inference: here the addition of a premise from a valid inference cannot make an invalid (and also not by the absence of one of several premises). - On the other hand, imperative inference: here, added premises may very well make a valid inference an invalid! Commands must be consistent with each other.
Practical Inference/Kenny/Geach: Surprising result: in practical concluding, the command FKpq is not deductively equivalent to the pair Ep, Eq. - This is not really paradox: the equivalence would lead to an absurd result because for the same reason, the set Fp, Fq, Fr ... would be deductively equivalent to FKpKqKr ... but this latter command could only be fulfilled if it was guaranteed that all our wishes could be fulfilled at the same time - so we need further inference rules for practical inference.
Logic Matters Oxford 1972