Full Idea
A thing is shown to be impossible when a repugnancy is demonstrated between the ideas comprehended in its definition.
Gist of Idea
A thing is shown to be impossible if a contradiction is demonstrated within its definition
Source
George Berkeley (Three Dialogues of Hylas and Philonous [1713], II p.214)
Book Reference
Berkeley,George: 'The Principles of Human Knowledge etc.', ed/tr. Warnock,G.J. [Fontana 1962], p.214
A Reaction
The problem is always that imagination is needed to see the 'repugnancy', and that is relative and limited.