In defence of Plathegel

 The licence, in circumstances comparable with these, to shift from 'ought*' to 'must*' is relevant to a celebrated complaint about Kant's ethical theory. Expressed in my terms, I think that Kant believed that imperfect or 'meritorious' obligations, such as the obligation to develop one's talents or to help others,

could be allowed to fall under generalizations ascribing one or other form of defeasible practical acceptability; we could (in his terms) allow here conflicting grounds of obligation, though not conflicting obligations. But with respect to perfect or strict obligations, like obligations to tell the truth or to keep promises, this treatment is not available; such obligations have to be thought of as matters of practical law, as falling (that is) under generalizations which invoke full (unqualified) practical acceptability. I suspect that he took this position partly from certain theoretical considerations and partly because he felt that, if he allowed the possibility of exceptions in such cases, allowed the 'must' to become an 'ought' (in the vernacular sense), he would be failing to capture the stringency which he felt to attach end p.86 to particular cases of perfect obligation. His 'hard line' in this matter has brought down on his head a modicum of ridicule, in respect of his well-known contention that one should tell the truth even to a would-be murderer searching for his intended victim. It seems to me that one could honour Kant's non-theoretical motivation, and at the same time save him from ridicule, by an application of the licence which I have sketched. I have not yet attempted to characterize the form of 'moral' acceptabilities, but let us suppose that, in the first instance, they differ from the practical acceptabilities which I have distinguished in that the generalizations associated with them omit, from their antecedents, any 'volitive' sub-clause; they are of the form Acc (⊢ Fx; ! Gx). Now it would be quite open to us to maintain that even the generalizations connected with 'perfect obligation' are of the ceteris paribus variety, and so to be expressed in terms of 'ought*'; but that, at the same time, it very often happens that, with respect to a particular case, we know that none of the sometimes defeating features applies; and so that, with respect to such cases, one is authorized to shift from 'ought*' to 'must*'. This seems to me to be not only a position which would both preserve Kant's intuition and save him from ridicule, but to be also a position of considerable plausibility.