The purpose and real value of systems engineering is... to keep going around the loop; find inadequacies and make improvements. - Robert E. Machol

The lack of formal definition does not prevent us from noting the characteristics which are frequently present in large scale systems. Each such system has a certain integrity. It may or may not be rigidly controlled from some central point, but in every case, all the parts of the system have some common purpose; in some sense, they all contribute to the production of a single set of optimum outputs from the given set of inputs, with respect to some appropriate measure of effectiveness.

Sometimes, where a complex problem can be illuminated by many tools, one can be forgiven for applying the one he knows best.

If the assumptions are wrong, the conclusions aren't likely to be very good.