natural vs unnatural
The chemical thermodynamic definition of a "natural" vs "unnatural" (and reversible) process, according to Edward Guggenheim (1933), based on the Clausius inequality (1862), for "freely-running" earth-bound reacting systems. [1]
In science, natural or ‘what is natural’, as contrasted with unnatural, is defined by processes or reactions, for standard earth-bound systems, that meet the following criterion: [1]

dG lt 0

This is called the Lewis inequality that are ‘thermodynamically possible’, defined in 1923 by American physical chemist Gilbert Lewis, based on the various Gibbs inequalities, which he termed as a “universal rule” for freely reacting isothermal-isobaric processes (e.g. ones that occur between reacting humans). [2] In generalized form, what is natural for any system or body of the universe is defined or quantified those processes or reactions that meet the Clausius inequality (1856).

Quotes
The following are related quotes:

Natural things are exactly those things which do move continuously, in virtue of the principle inherent in themselves, towards a determined goal; and the final development which results from any one such principle is not identical for any two species, nor yet is it any random result, but in each there is always a tendency towards an identical result if nothing interferes.”
Aristotle (322BC), Physics (2:8) [3]

See also
‚óŹ Natural science

References
1. Guggenheim, Eduard, A. (1933). Modern Thermodynamics by the Methods of Willard Gibbs (pgs. 5, 17). London: Methuen & Co.
2. Lewis, Gilbert N. and Randall, Merle. (1923). Thermodynamics and the Free Energy of Chemical Substances (pg. 160). McGraw-Hill Book Co., Inc.
3. Brown, Richard H. (1977). A Poetic for Sociology: Toward a Logic of Discovery for the Human Sciences (pg. 130). University of Chicago Press, 1989.

TDics icon ns