Nonlinearity is natural ...

... and good and pure. Aside from nonlinear perceptual systems, to be discussed in a moment, lots of natural systems are nonlinear. When you toss a rock off a bridge, its altitude as it falls is related to the square of the time in transit - a square law, or in other words, a power law with an exponent of 2. Kepler's Third Law can be phrased, "The orbital period of a satellite is proportional to the three-halves power of the radius of its orbit." That's a three-halves power law.

A CRT has a five-halves power law response. A CRT whose black level is set correctly produces intensity proportional to the 2.5-power of applied voltage.

To read about nonlinearities inherent in other perceptual modes, consult S.S. Stevens, Psychophysics (New York: Wiley, 1975). Stevens talks about nonlinearities in the perception of loudness, saltiness, small, and heaviness.

Timo considers nonlinearity to be unnatural.