There is an interesting commentary on reductionism versus emergence in today's issue of Nature:
Binder, P.-M. (2009). The Edge of Reductionism. Nature 459: 332-334.
It discusses the difference between irreducible and undecidable systems (in Wolfram's sense) and provides examples of cellular automata that illustrate undecidability despite knowledge of the simple local governing laws.
It's worth a glance.
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