  A definition of the thermodynamic entropy based on the time-dependent
probability distribution of the macroscopic variables is developed. When a
constraint in a composite system is released, the probability distribution for
the new equilibrium values goes to a narrow peak. Defining the entropy by the
logarithm of the probability distribution automatically makes it a maximum at
the equilibrium values, so it satisfies the Second Law. It is also satisfies
the postulates of thermodynamics. Objections to this definition by Dieks and
Peters are discussed and resolved.
