The arrow of time is the direction, apparently inviolable, of the "flow" of time that distinguishes the past from the future. There has been much debate as to the physical origin of of the arrow of time. Most of physics does not distinguish between a forward and a reverse direction of time. The one exception is the second law of thermodynamics which states that entropy must increase with time. One possibility is that the overall arrow of time in the universe is due to its beginning in a state of very low entropy.