On the difference between tractable and intractable systems... #readingToday
_In order for a system to be controllable, it is necessary that what goes on 'inside' it is known and that a sufficiently clear description or specification of the system and its functions can be provided. The same requirements must be met in order for a system to be analysed, so that its risks can be assessed. That this must be so is obvious if we consider the opposite. If we do not have a clear description or specification of a system, and/or if we do not know what goes on 'inside' it, then it is clearly impossible to control it effectively as well as to make a risk assessment. We can capture these qualities by making a distinction between tractable and intractable systems, cf. Table 3.4. A system is tractable if the principles of functioning are known, if descriptions are simple and with few details and, most importantly, if it does not change while it is being described. An example could be an assembly line or a suburban railway. Conversely, a system is intractable if the principles of functioning are only partly known (or in extreme cases, completely unknown), if descriptions are elaborate with many details, and if systems change before descriptions can be completed. An example could be emergency management after a natural disaster or, sans comparison, financial markets.
The ETTO Principle, Erik Hollnagel