Full Idea
One mark of a perfect idea is that it shows conclusively that the object is possible.
Gist of Idea
A perfect idea of an object shows that the object is possible
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 2.31)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.268
A Reaction
Subtle but nice. My favourite example would be that the perfect idea of a bonfire on the Moon shows that it is not possible. Essence reveals necessity, as Aristotle and Kit Fine claim. A perfect idea has a single definition.