Full Idea
Every necessary proposition is demonstrable, at least by someone who understands it.
Gist of Idea
Every necessary proposition is demonstrable to someone who understands
Source
Gottfried Leibniz (De arcanus motus [1676], 203), quoted by Daniel Garber - Leibniz:Body,Substance,Monad 6
Book Reference
Garber,Daniel: 'Leibniz: Body, Substance, Monad' [OUP 2009], p.238
A Reaction
This kind of optimism leads to the crisis of the Hilbert Programme in the 1930s. Gödel seems to have conclusively proved that Leibniz was wrong. What would Leibniz have made of Gödel?