Great post Menno!  
  
My only quibble is with the discussion of emergence  
as the justification for reasoning in terms of aggregates.  
Unless we draw the kinds of distinctions you have so  
concisely laid out elsewhere in your post, this seems  
again to pose the danger of needlessly conflating   
ontological and epistemological issues.  
There may be scant profit in debating whether some   
exploitable aggregate regularity is "truly" emergent.  
I would argue that the core justification is pragmatic.  
If I want to buy a car, I do not find it useful to  
ponder the molecular constitution of the salesman.  
If I want to predict the stock price of a large firm,  
I do not seek information about the lives of the employees.  
If I want to think about the Sunni insurgency, I do not  
use the fact that Iraq is full of individuals as an  
excuse to ignore that fact that Iraq is full of tribes  
and clans, which have proved to be important *actors*  
in the conflict.  
  
Cheers,  
Alan Isaac