  Williams (1962/3) judged (B) to be invalid, on the ground that its conclusion carried a “permissive presupposition” that the hearer may do X or do Y. Hare defended it on the ground that the permission is a conversational implicature that is cancelled in the context defined by the premise “Do X”. However, even (A) may be less plausible than it looks. Take this example (which comes from Bob Hale):