Full Idea
According to me there is nothing simple except true monads, which have no parts and no extensions.
Clarification
Monads are atomic ideas
Gist of Idea
The only simple things are monads, with no parts or extension
Source
Gottfried Leibniz (Letters to Samuel Clarke [1716], 5.24)
Book Reference
Leibniz,Gottfried: 'Philosophical Writings', ed/tr. Parkinson,G.H.R. [Dent 1973], p.225