Could you explain a bit?

This is how I understood the argument: Parts can’t form machines which perceive (I couldn’t find a justification for this assumption – that’s the sticking point), therefore perception is not created by parts but by a substance.

The meat comment targets the first part of the argument. Leibniz seems to make an assumption (parts can’t from perceiving machines ≈ meat can’t think) which he doesn’t really justify.

I thought he was using the mill as an example to show that perceiving machines made from parts are self-evidently absurd, to ridicule the idea of perceiving (or conscious) machines. What am I missing?

Yet another dead/hidden comment that shouldn't be: http://news.ycombinator.com/item?id=2598559 by brudgers. Here's the text:

Keep in mind that the analogy of the mill is only one element in support of Leibnitz's central argument in the Monadology. A crude outline:

    Perception exits. 
    Perception cannot be found in the parts. 
    Therefore there must be something else. 
    To avoid the same problem the something else
    must not be composed of parts.
    Therefore a simple substance [the monad] which 
    perceives must exist.

What the passage from Leibnitz suggests is that the step from drawing an analogy using most sophisticated technology of the day as representative of the brain to the belief that the brain is the technology is not unique to the invention of computers.