The Second Law of Thermodynamics has multiple equivalent statements that highlight fundamental limits:

1. Clausius Statement: Heat cannot spontaneously flow from a colder body to a hotter body without external work.
2. Kelvin-Planck Statement: It is impossible to construct a heat engine that operates in a cycle and converts all the heat absorbed from a single reservoir into work.
3. Entropy Statement: The total entropy of an isolated system can never decrease over time; it can only stay constant (reversible) or increase (irreversible). Violation of any of these would violate the Second Law.