The philosophy of thermal and statistical physics is that part of the philosophy of physics whose subject matter is an amalgam of classical thermodynamics, statistical mechanics, and related theories. Its central questions include: What is entropy, and what does the second law of thermodynamics say about it? Does either thermodynamics or statistical mechanics contain an element of time-irreversibility? If so, what does statistical mechanics tell us about the arrow of time? What is the nature of the probabilities that appear in statistical mechanics?