What’s the difference between a complex system and a complicated one?
It’s all in the number of connections.
A complicated mechanism is just that – a mechanism. Like a machine. It may be built up from lots of different components and parts (think of modern cars with their onboard computer systems), but each part interacts with very few other parts. So if you want to understand it you can take it to pieces and understand each part, then build it back up if you know how each part connects to other parts.
A complex system is not like a machine. It may also have many different components and parts (think of a brain with all its neurones) but each part has multiple connections with other parts, and each part can in turn be influenced by multiple other parts. You cannot understand this by taking it to pieces. Its characteristics are dependant on the whole interactive system. A single part, or even set of parts, will not necessarily behave the same way when considered in the entirety of the whole organ.
All life forms are complex systems, not complicated systems. Treating human beings as machines is a failure of understanding.
Heroes Not Zombies 
http://edge.org/response-detail/2837/what-is-your-favorite-deep-elegant-or-beautiful-explanation
Deric Bownds has a shortened version of this …seems like you are on the same wavelength as Nesse …
“But, can an ecological metaphor replace the metaphor of body as machine? Not likely. Perhaps someday understanding how natural selection shapes organic complexity will be so widely and deeply understood that scientists will be able to say “A body is like…a living body,” and everyone will know exactly what that means”.
I read his book on evolutionary medicine a while back –fascinating …….http://www.amazon.com/Why-We-Get-Sick-Darwinian/dp/0679746749