He is transported to that curious world of decibels and negative frequencies where filter experts live. - Arnold Tustin

Consideration of a further possibility, namely that of constructing physical systems that are analogues of the economic system, and of observing and recording their behaviour.

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

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.