  The literature devoted to analysing the second law of thermodynamics in the context of statistical mechanics starts with the development of statistical mechanics in the late 19th century. Considerable confusion has arisen simply due to the fact that as the subject has developed, the meaning of key terms have changed or become ambiguous. When one paper speaks of the second law being violated or of entropy decreasing and another of it being saved or being non-decreasing, it is not necessarily the case that they are referring to the same things. While a review of the foundations of thermal physics is beyond the scope of this entry (though see related entries by  Sklar  and  Uffink),  some important distinctions need be borne in mind.