In classical statistical mechanics, the H -theorem, introduced by Ludwig Boltzmann in 1872, describes the tendency to decrease in the quantity H (defined below) in a nearly- ideal gas of molecules. As this quantity H was meant to represent the entropy of thermodynamics, the H -theorem was an early demonstration of the power of statistical mechanics as it claimed to derive the second law of thermodynamics —a statement about fundamentally irreversible processes —from reversible microscopic mechanics. It is thought to prove the second law of thermodynamics, albeit under the assumption of low-entropy initial conditions.