The total change of entropy of a system and its environment is always positive, and tends to zero for the transformation towards reversibility.

The thermodynamics is based on two essential principles:

the first-principle, which establishes the equivalence of different forms of energy, including heat and work. It is a principle of conservation, it implies that the sum of the energies associated with a system even if this energy can be transformed into each other according to their equivalence.

-the second principle, which introduces in addition to the energy of a physical system a magnitude characterizing the system and is called entropy. This is a principle of evolution, because it determines how far and in what sense the various transformations of energy in the world are possible. For example, some chemical transformations are possible and others not. Likewise all the warmth of a body, eg the ocean, can not be transformed completely into work.

The second principle has a long history where there are names as famous as Carnot, Clausius, Kelvin, Helmholtz, Gibbs and Boltzmann especially.

Applied to an isolated system, trade or work or heat with the outside world, the second principle states that the system state called entropy can only increase to a maximum value where the system remains in balance. This is a translation of the complicated simple observation that any physical system left to itself, as a living being, tends to disrupt. This implies that in any transformation of energy in these systems, the ability to use energy to produce work and organization is lost, is often a deterioration in the quality of energy. That's why it introduces a non-equivalence between the past and the future and irreversibility of the transformations, an arrow and a sense of time then.

This raises many questions.

Applied to the universe conceived as an isolated system, it would mean that it is moving slowly but inexorably towards the "dead heat" where disorder is maximal.

Thermodynamics can be deduced from the mechanical basis of certain assumptions of order statistics, or the laws of mechanics, they are reversible! The arrow of time and the second principle would be illusions and simple approximations practices but fundamentally false.

These issues are still not resolved today and are the subject of incessant debate.