Full Idea
Every idea is analysed perfectly only when it is demonstrated a priori that it is possible.
Gist of Idea
An idea is analysed perfectly when it is shown a priori that it is possible
Source
Gottfried Leibniz (Of Organum or Ars Magna of Thinking [1679], p.3)
Book Reference
Leibniz,Gottfried: 'Philosophical Writings', ed/tr. Parkinson,G.H.R. [Dent 1973], p.3
A Reaction
I take it he means metaphysical possibility, rather than natural, or we can't think about pigs flying. He probably has maths in mind. Seeing the possibility of something may well amount to understanding its truth conditions.