Full Idea
The manifestation of the belief that a mode of inference is logically necessarily truth-preserving is the preparedness to employ that mode of inference in reasoning from any set of suppositions whatsoever.
Gist of Idea
The mark of logical necessity is deduction from any suppositions whatever
Source
Ian McFetridge (Logical Necessity: Some Issues [1986], §4)
Book Reference
-: 'Aristotelian Society' [], p.153
A Reaction
He rests this on the idea of 'cotenability' of the two sides of a counterfactual (in Mill, Goodman and Lewis). There seems, at first blush, to be a problem of the relevance of the presuppositions.