“Herbert Simon contrasted systems that are non-decomposable with systems that are (or are nearly) decomposable. Brooks’s conception of the 360 operating system created a non-decomposable system: every part communicates with virtually every other part.8 By contrast, a decomposable system is one in which all (or most) communication takes place within the subsystems and little or none among the subsystems. As Simon put it more recently, a decomposable system “can be thought of as a boxes-within-boxes hierarchy with an arbitrary number of levels. Its special characteristic is that equilibrating interactions between boxes at any level take place much more rapidly than the interactions between boxes at that same level, and similarly all the way to the top of the hierarchy.” (Simon 2002, p. 589). (http://web.uconn.edu/ciom/Open1C.pdf)
Simon, Herbert A. 1998. “The Architecture of Complexity: Hierarchic Systems,” in Idem, The Sciences of the Artificial, 3rd edition, second printing. Cambridge, Mass.: MIT Press: 183-216. Originally published in 1962, Proceedings of the American Philosophical Society 106(6): 467-82 (December).
Simon, Herbet A. 2002. “Near Decomposability and the Speed of Evolution,” Industrial and Corporate Change 11(3): 587-99 (June).