The
philosophy of thermal and statistical physics is one of the major subdisciplines of the
philosophy of physics. Its subject matter is classical thermodynamics, statistical mechanics and related theories. Some of its central questions are: 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 is its connection with the arrow of time? Is thermodynamics reducible to statistical physics?