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The soft systems methodology was developed in the 1960s by Peter Checkland at Lancaster University. This methodology arose out of attempts to apply systems engineering principles ("hard" systems theory) to business problems. Systems engineering emphasises measurable system objectives and the top down decomposition of systems into subsytems. Advanced views of systems engineering (such as VSM) show how systems exhibit emergent (unexpected, counterintuitive) behaviour because of complex feedback loops among system components.
When applying systems engineering to what he came to call "human activity systems" (people working together to achieve something) Checkland found a number of problems. Organisation goals (I use "goals" and "objectives" more or less interchangeably) were matters of controversy; in particular most investigators assumed that all members of the organisation accepted goals set by top management, but this is usually not the case. Formal methods usually begin with a problem statement; Checkland found that fixing the problem too early made investigators unlikely to see different, possibly more basic,
problems. And the method itself restricted what could be found out; if we expect the organisation to be describable by the interaction among a number of clearly bounded subsystems then that will happen - we will see in the organisation a reflection of our methods.
To overcome these problems Checkland eventually proposed the following methodology. As with most methodologies, SSM was derived by summarising experiences from apparently successful projects done over a number of years. These projects usually involved external consultants advising on fairly high level problems (eg a publisher losing market share, a government agency wondering how it can benefit from new technology).
A key feature of SSM is the advice to keep the project vague and wide ranging for as long as possible - don't jump to conclusions, and don't ignore the current situation by concentrating on some utopian future. At times Checkland would probably say that the process is more important than the outcome - just going through SSM will change the organisation, and this will probably include a changed opinion about what problem we were really trying to solve. (SSM is not for high achievers, because a goal is never reached!)
It is important to realise that in principle an SSM project is done by the people in the organisation, with the consultant acting as a facilitator.