In chemistry, the spontaneity of a reaction, namely whether it will go naturally on its own or will require an energy input, is determined by the calculation of the change in the Gibbs free energy ΔG of the reaction.

Spontaneity rule
According to the "spontaneity criterion", if ΔG is positive a reaction will only occur if energy is supplied to force it away from the equilibrium position (i.e. when ΔG = 0). If ΔG is negative the reaction will proceed spontaneously on its own to equilibrium. [1]

Human chemistry
In human chemistry, with reference to human chemical reactions, the definition is the same, however, the application of this logic is compounded due to factors such as coupling effects, external forces, the differences of human molecules involved, particle count, system migrations, among other factors. In general, as a first approximation, for earth-bound system, being isothermal-isobaric systems, changes in the Gibbs free energy are useful in indicating the conditions under which a chemical reaction will occur.

References
1. Daintith, John. (2004). Oxford Dictionary of Chemistry. New York: Oxford University Press.