Full Idea
If there are things which are possible but inconceivable, we must abandon the view, which has had a considerable following since Descartes, that the conceivable is a test of the possible.
Clarification
Warning against deducing possibility and necessity from what we can imagine
Gist of Idea
The conceivable can't be a test of the possible, if there are things which are possible but inconceivable
Source
Roger Scruton (Modern Philosophy:introduction and survey [1994], 25)
Book Reference
Scruton,Roger: 'Modern Philosophy: introduction and survey' [Sinclair-Stevenson 1994], p.355