Full Idea
Non-black non-ravens have no role to play in assessing the plausibility of 'All ravens are black' because their existence is not among the existential presuppositions of that statement.
Gist of Idea
Non-black non-ravens just aren't part of the presuppositions of 'all ravens are black'
Source
Rom Harré (Laws of Nature [1993], 5)
Book Reference
Harré,Rom: 'Laws of Nature' [Duckworth 1993], p.111
A Reaction
[He cites Strawson for the 'presupposition' approach]