The original non-statistical formulation of thermodynamics was asymmetrical in time : it claimed that the entropy in a closed system can only increase with time. This was derived from the Second Law (any of the two, Clausius ' or Lord Kelvin 's statement can be used since they are equivalent) and using the Clausius' Theorem (see Kerson Huang ISBN 978-0471815181). The later theory of statistical mechanics, however, is symmetric in time. Although it states that a system significantly below maximum entropy is very likely to evolve towards higher entropy, it also states that such a system is very likely to have evolved from higher entropy.