Leibniz also called monads the 'true unities' and thus the only substances in the universe. At the same time, he refers to them as mental entities that are capable of perception and appetites, while remaining self sufficient, and capable of (indeed going on ) developing independently of each other. That's what the 'windowless' term in the wu above means.

It's hard for us to imagine a set of entirely independent atomic units that are capable of looking around and indeed feeling hunger, but are nevertheless entirely separate and independent of each other in their own development.

This is hard to swallow.

Nevertheless what was harder to swallow, logically, at the time was the notion of inductive reasoning, which was shattered by Hume and his empiricism.

We should try to see Leibniz's attempt at a new world view in this light, his ability to perceive the essential nature of things (which Kant declares impossible) can be thought of as a sincere attempt to address the issues of idealism vs empiricism, and for this reason alone perhaps he should be granted respite from the sarcasm of Voltaire.

By removing the relational aspect of the universe, Leibniz was showing that the universe could still be modelled cogently, and while his model may seem odd to us (hammer and mirror example above) it nevertheless is considerably more difficult to defeat using Hume's logic, as simply doesn't allow for the same holds.

Cletus_the_Fetus:It might also be noted that Leibniz's monad theory is, in part at least, an attempt to formulate a Catholic response to Spinoza's theory of modes.

(thanks to Cletus for the insight)