Any processEarth-bound process
Natural process (1933)natural process (criterion)
English chemical thermodynamicist Edward Guggenheim's 1933 definition of what constitutes the differences between a natural process and an unnatural process, based on the Clausius inequality (that which governs the known universe), for all systems (left) and for standard earth-bound systems ( (right), for the latter of which: differential Gibbs free energy decrease (dG < 0) quantifies what is "natural" whereas differential Gibbs free energy increase (dG > 0) quantifies what is "unnatural". [2]
In science, nature is a difficult to define word.

Overview
In 1821, French philosopher Stendhal gave the following definition of nature in the context of an action: [1]

“We call an action natural when it does not differ from the habitual mode of action.”

The definition of what constitutes a "nature" or a so-called "natural process" was seeded with the 1856 introduction of the Clausius inequality, which led to the Gibbs inequalities (1876), to the Lewis inequality (1923), and to the Guggenheim inequalities (1933), the latter of which are shown adjacent.

In 1950, American chemical thermodynamicist Frederick Rossini defined natural and unnatural processes, in terms of the Lewis inequality, as such: [3]

Rossini notation (1950)
Modern notation
Description

δu < 0ΔG < 0If the value of δu is negative, useful energy is obtainable from the system and we know the change is a naturally occurring one in the direction toward the state of equilibrium.
δu > 0ΔG > 0If the value of δu is positive, useful energy is required to be supplied to the given system to bring about the desired change and we know that the change is an unnatural one in the direction away from equilibrium.

Hence, for earth-bound natural processes, measurements of Gibbs free energy change can determine what is natural (dG < 0) and unnatural (dG > 0) in the corpus of human affairs.

Morality
The idea that the simple inequality sign, < or >, could differentiate between natural and unnatural, was an addition to German polymath Johann Goethe's 1809 definition of the moral symbols of physical chemistry applied to human interactions.

See also
‚óŹ Nature abhors a vacuum

References
1. Stendhal. (1821). On Love. Publisher.
2. Guggenheim, Eduard, A. (1933). Modern Thermodynamics by the Methods of Willard Gibbs (pgs. 5, 17). London: Methuen & Co.
3. Rossini, Frederick D. (1950). Chemical Thermodynamics (pg. 122). John Wiley & Sons, Inc.

TDics icon ns