Full Idea
All simple substances will always have a harmony among themselves, because they always represent the same universe.
Gist of Idea
All simply substances are in harmony, because they all represent the one universe
Source
Gottfried Leibniz (Letters to Samuel Clarke [1716], V §91), quoted by Richard T.W. Arthur - Leibniz
Book Reference
Arthur, Richard T.W.: 'Leibniz' [Polity 2014], p.162
A Reaction
We can accept that the universe itself does not contain contradictions (how could it), but it is a leap of faith to say that all monads represent the universe well enough to avoid contradictions. Maps can contradict one another.