Complex systems (quote)

...Herbert Simon's argument that modular design in computers and minds is a special case of modular, heirarchical design in all complex systems. Bodies contain tissues made of cells containing organelles; armed forces comprise armies which contain divisions broken into battalions and eventually platoons; books contain chapters divided into sections, subsections, paragraphs, and sentences; empires are assembled out of countries, provinces, and territories. These "nearly decomposable" systems are defined by rich interactions among the elements belonging to the same components and few interactions among elements belonging to different components. Complex systems are hierarchies of modules because only elements that hang together in modules can remain stable long enough to be assembled into larger and larger modules. Simon gives the analogy of two watchmakers, Hora and Tempus:

The watches the men made consisted of about 1,000 parts each. Tempus had so constructed his that if he had one partly assembled and had to put it down - to answer the phone - it immediately fell to pieces and had to be reassembled from the elements ...

The watches that Hora made were no less complex than those of Tempus. But he had designed them so that he could put together sub-assemblies of about ten elements each. Ten of these subassemblies, again, could be put together in a larger subassembly; and a system of ten of the latter subassemblies constituted the whole watch. Hence, when Hora had to put down a partly assembled watch in order to answer the phone, he lost only a small part of his work, and he assembled his watches in only a fraction of the man-hours it took Tempus.