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… any program is a model of a model within a theory of a model of an abstraction of some portion of the world or of some universe of discourse. –Manny Lehman Programs, Life Cycles, and Laws of Evolution. 1980

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Lehman's categories of software system S-type formally defined by and derivable from a specification P-type solves a real-world problem but does not affect the world it models E-type embedded in the world it models; its operation changes that world

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E-type Systems

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Deploying the system changes what it must do

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The system is only useful if you can change what it does

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The way you change the system is part of the system

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The tests of the system are part of the system

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The people who change the system are part of the system

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The system is part of the organisational system that directs its evolution

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Changing how the organisation delivers the system is a delivery of the system

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Norbert Wiener, "father of cybernetics"