Actual economic systems are constantly subjected to change and disturbances, which would result in irregularity. - Arnold Tustin

An economic system is not a linear system, and... this fact stands in the way of the determination of the parameters of the system by methods that presume linearity, and... it introduces great difficulties in the extrapolation from past behaviour for purposes of prediction.

The ‘theory of control systems’ in engineering is now a well-developed subject, making use of some remarkably powerful concepts and methods of analysis, especially in relation to problems of stabilization and the prevention of unwanted oscillations.

The separate excitation of the dynamo corresponds with the independently determined investment in the economic model, and the total excitation with income. Perhaps in this electrical age, the conventional metaphor of ‘priming the pump’ might be dropped in favour of ‘exciting the dynamo’.