In chemistry, the spontaneity criterion for a basic chemical reaction, at constant temperature and pressure, is that the calculated measurement of the Gibbs free energy change ΔG for the reaction process, over the extent of the reaction, is a negative value. [1]

Overview
The spontaneity criterion state, in chemical reaction language translation, that for any generic reaction between reactants and products:

Reactants → Products

such as in a combination reaction:

A + B → AB

the "spontaneity" criterion equates to the following set of three rules:

Condition
Description

Reaction or process is spontaneous in the forward direction

$dG > 0 \,$ Reaction or process is nonspontaneous (reaction is favored in the opposite direction)
$dG = 0 \,$ System is at equilibrium (there is no net change)

Condition	Description
	Reaction or process is spontaneous in the forward direction
$dG > 0 \,$	Reaction or process is nonspontaneous (reaction is favored in the opposite direction)
$dG = 0 \,$	System is at equilibrium (there is no net change)

In this sense, the Gibbs free energy G or simply “free energy” represents, in the words of American chemist Raymond Chang, “the energy available to do work”. [2] Here, G equates to instanteous measurements of:

G = H – TS

of each component species, A, B, AB, etc., per unit time, where H is the enthalpy component, T is the absolute temperature of the system, and S is the entropy component. Thus, if a particular reaction is accompanied by the release of usable energy (i.e. if ΔG is negative), this fact alone guarantees that it will be spontaneous and will go naturally on its own without an external energy input.


The three HT principles as listed on the HumanThermodynamics.com "principles" page in 2005. [2]

The three HT principles as listed on the HumanThermodynamics.com "principles" page in 2005. [2]

Hmolscience

See main: HT principles

In 2005, Libb Thims posted the adjacent set of general rules, classified as HT principles, something he has used as a philosophical guide since circa 2000, extrapolation of the above outlined spontaneity criterion into the language of crude or general rules to crouch one's choices upon, as reaction guides in times of confusion, so to say. [2] The following Pinterest poster seems to capture principle one: (Ѻ)

In 2012, Brazilian mechanical engineer, thermal scientist, and philosopher Mahesh Deva gave an in depth critique of the three HT principles. [3]

Thims has pointed these out to young students looking for a basic rule book set of guidance, e.g. to Inderjit Singh (2014), to the six kids in the "Zerotheism for Kids" (2015) lecture, to name two examples, of many, that come to mind.

References
1. Chang, Raymond. (1998). Chemistry, 6th ed. (ch. 18: “Entropy, Free Energy, and Equilibrium”, pgs. 725-55; pg. 738). New York: McGraw-Hill.
2. Principles – HumanThermodynamics.com.
3. (a) Deva, Mahesh. (2012). “Email to Libb Thims” [to HT.com questions email], read Oct 25.
(b) Mahesh Deva (query emailed to HT.com) – Hmolpedia threads.