I'm reluctant to characterize the world as being composed of independent agents passing messages. I can't put my finger on it exactly, but it just feels like a very lossy oversimplification.
Can you elaborate at all? I'd be very interested to hear (since this is a topic I find quite intriguing).
For humans, we ourselves may work sequentially, but there are many of us working concurrently. Like many many sequential processing cores. Perhaps your comment was targeted at the "message passing" aspect. Its a bit of shared state mixed with message passing, really. Often we interact directly - I could tell you something (message passing) or I can shout something (multicast message passing). Of course, we also operate in a shared state model: multiple people taking food from a plate, we have to synchronise this so that we don't clash (physically, in this case).
If I were to conceive of the universe as a program, I would think of it as one giant state with a few fundamental particles and a simple functional program that resolves the state over time based on the previous state.
Passing messages to me implies a finite set of messages and pre-conceived interaction, whereas in reality there is no such thing as discrete objects or interactions, everything interacts with everything, and the world we know is emergent from the low level physical properties.
I suppose there could be an infinitely polymorphous OO program behind the universe, but that just doesn't strike me as a good fit with what we know about physics.
I never meant to imply OOP when I said message passing. I guess it was the wrong term to use, because its got too much of an OOP ring to it. What I mean is that events or packets of data or what have you are passed between independent agents. The communication is loosely coupled fropm the agents and the agents all function independently (they often work together, but the "function" independently).
If I say "Hello, Bob", this "message" or communicatable data is passed from me to Bob. But coupling is also low - Mary and Peter, who are standing nearby, can also hear this - unless I whisper.
Basically, the events or messages or data packets or whatever you want to call them are simply sent OUT for whatever to pick them up and do with them what they will. The messages can be directed though (say to Bob vs simply yell) and the listeners can be restricted (whisper). In this respect, I see the world as independent threads of execution which communicate through some (anonymous, if I hear someone yell "Fire", I don't care who) message dispatch system. Something like this: http://didntread.files.wordpress.com/2009/07/components.png
People are made up of sub-agents, which themselves are made up of smaller, independent agents and so on. Cells communicate with nearby cells through proteins and chemical reactions and electrical signals - these are messages being passed (anonymously) between agents.
Now, if each agent was a single thread of execution, then you have discrete points where communication and interaction occurs. You said that this is not how the world works, and you are right. Imagine instead a network of connected operations, where when one operation produces some result, it passes it to all the connected operations. When an operation has received all its inputs, it can execute - concurrently, in parallel with any other operations which also have their dependencies met. At some point, these operations too will complete their processing and their output is propogated to connected operations. In such a system, when a message is received or when an interaction occurs, the network of operations (the agent) can process it immediately. No longer does the agent have to wait for a certain discrete point in a sequential processing system to communicate or interact - it can do so as soon as some data is available. These entworks do contain state, just like you said the agents in the real world do, but instead of having some fixed memory buffer contain the state (well, technically, you could have that too..), the state is maintained in feedback loops between operations (think of a flip-flop in electronics: the output of a logic gate is fed back as its input, either directly or indirectly through other logic gates).
What I have just described is a dataflow system and THAT is how I think the world works, if it were a programming system.
Note that some things ARE inherently sequential - the operations are still ordered by their dependencies. Maybe the operations themselves process their data in a number of sequential steps. This is fine, because certain things require ordering to make sense or to work well.
