Non-linearity

1. Input-output relations of a system, in which there occur variable parameters and explicit time dependence, are said to be non-linear.

2. Non-linearity is a characteristic of biological and social systems which distinguishes them from physical systems (although there exist some non-living physical systems which are also non-linear). It is for this reason that biological systems cannot be satisfactorily reduced to and explained by physics, since the basic equations of quantum mechanics are linear. The relative infrequency of non-linearity in technological systems merely reflects the purpose of the designers, in that there is at least a fairly coherent linear system theory, which is not the case for the non-linear domain.