The human activity system is a notional purposive system
which expresses some purposeful human activity, activity which could in
principle be found in the real world. Such systems are
notional in the sense that they are not descriptions of
actual real-world activity (which is an exceptionally complex phenomenon) but
are intellectual constructs; they are ideal types for use in
a debate about possible changes which might be introduced into
a real-world problem situation (Checkland, 1993: 314).