It is evident that all these results have for their original basis the Theory of Probabilities. Consequently, because these conclusions are thus based, they must be interpreted according to the general method underlying this Theory. This method essentially is the determination of average (mean) values and calling them the probable ones. We therefore conclude that each state is characterized by the mean number of complexions belonging to that state, that is, by this mean number which changes always in a one-sided way, ever in the same sense, inasmuch as it inevitably and invariably grows till the normal, settled condition is reached.
For the sake of clarity we must keep in mind that the motions of the individual atoms are reversible and that in this sense the irreversible processes are reduced to reversible ones. But the process as a whole is not reversible because, by the very act of complete reversal, we would suspend the general, chaotic character of the elementary motions and give them to this extent a special, prearranged feature which would be more or less hostile to the original definition of "elementary chaos." The irreversibility is not in the elementary events themselves, but solely in their irregular arrangement. It is this which guarantees the one-sided change of the mean value characteristic of each one of the successive states of the process.
Now remembering that the kernel of the Second Law is that all processes in Nature are irreversible, or, that all changes in Nature vary in one direction only, we can, in the light of what of has just preceded, repeat the following precise, scientific statement:
"The Second Law, in its objective-physical form (freed from all anthropomorphism) refers to certain mean values which are found from a great number of like and 'chaotic' elements."
If we now go back to what constitutes the kernel of the Second Law, we will see the relevance and force of PLANCK'S enunciation of this law:
"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 the same may be said of the earlier statement of this law, "all changes in Nature vary in one direction only." The character of this proof is, moreover, exactly like that for the First Law, the Conservation of Energy, and has the same sort of validity.
When we compared and interpreted the current statements of the Second Law (pp. [44]-[47]) we enunciated and made use of three helpful propositions that will now be repeated:
(a) All cases of irreversibility stand or fall together; if any one can be reversed all can be reversed.
(b) Any general consequence of any one correct statement of the Second Law may be regarded as itself a valid and complete statement of the Second Law.