The author explains the Boltzmann–Planck interpretation that entropy equals the logarithm of a state's probability, identified with the number of its complexions, and thus measures the permutability or disorder of microscopic motions. He contrasts microscopic and macroscopic descriptions, introduces the hypothesis of elementary chaos, and shows how the calculus of probability applies to aggregates of microstates with many degrees of freedom. The discussion distinguishes settled and unsettled stages, formulates reversibility and irreversibility (including the H-theorem), and treats entropy as the universal criterion and quantitative measure of irreversibility, using these ideas to clarify thermodynamic statements and practical difficulties.