"

 Clausius 

""There are many statements of the second law which use different terms, but are all equivalent. (Fermi, 1936) Another statement by Clausius is:  Heat cannot of itself pass from a colder to a hotter body.""   I think this statement needs to be qualified, as heat most certainly can and does pass from colder to hotter bodies. To take a macro example - if you place two stars of greatly different surface temperatures in the same neighbourhood, they will both be radiating energy and each will be subjected to incoming radiation from the other body. The radiation emitted by each body is proportional to the 4th power of its absolute temperature and is not dependent on the temperature of the other body. What we are usually interested in is the net gain or loss of energy from a body, but we shouldn't lose sight of the two-way traffic of radiant energy.  "