Full Idea
If there were no divisions of matter in nature, there would be no things that are different; just the mere possibility of things. It is the actual division into masses that really produces things that appear distinct, which presupposes simple substances.
Gist of Idea
The division of nature into matter makes distinct appearances, and that presupposes substances
Source
Gottfried Leibniz (Letters to Burcher De Volder [1706], 1704 or 1705)
Book Reference
Leibniz,Gottfried: 'Philosophical Essays', ed/tr. Arlew,R /Garber,D [Hackett 1989], p.182
A Reaction
This shows Leibniz to be a straightforward realist about the physical world, and certainly not an 'idealist', despite the mind-like character of monads. I take this to be an argument for reality from best explanation, which is all that's available.