Managers are not confronted with problems that are independent of each other, but with dynamic situations that consist of complex systems of changing problems that interact with each other. I call such situations *messes*.

(...)

Problems are abstractions extracted from messes by analysis; they are to messes as atoms are to tables and chairs. We experience messes, tables, and chairs; not problems and atoms.

(...)  

Because messes are systems of problems, the sum of the optimal solutions to each component problem taken separately is *not* an optimal solution to the mess. The behaviour of a mess depends more on how the solutions to its parts interact......than on how they act independently of each other. But the unit in OR [Operational Research] is a problem, not a mess. Managers do not solve problems; they manage messes.

Russel Ackoff