(1) J. W. GIBBS. "The impossibility of an uncompensated decrease in entropy seems to be reduced to an improbability." This of course considers all the participating bodies of the system.
(2) All changes in nature involve a net growth in entropy; when such a change is measured in reversible ways, the growth is indicated by the summation:
, when the
sign refers to processes which on the whole are completely
. Of course it is now thoroughly understood that the latter case is a purely ideal one, which is really never realized in nature and is only a convenient and fruitful fiction in theoretical demonstrations.
(3) M. PLANCK. "It is not possible to construct a periodically functioning motor which effects nothing more than the lifting of a load and the cooling of a heat reservoir."
The proof of this is purely experimental and cumulative and in this respect is exactly like that for the First Law, the Conservation of Energy, and has exactly the same sort of validity.