second law of thermodynamics
Boltzmann worked with it in terms of statistics:
Boltzmann suggested that the entropy was really counting the number of ways we could arrange the components of a system (atoms or whatever) so that it really didn’t matter. That is, the number of different microscopic states that were macroscopically indistinguishable. ... There are far fewer ways for the molecules of air in a box to arrange themselves exclusively on one side than there are for the molecules to spread out throughout the entire volume; the entropy is therefore much higher in the latter case than the former. With this understanding, Boltzmann was able to “derive” the Second Law in a statistical sense — roughly, there are simply far more ways to be high-entropy than to be low-entropy...
from Boltzmann’s Anthropic Brain, at Cosmic Variance. ("The Second Law of Thermodynamics — the entropy of a closed system will not spontaneously decrease,
from the same article)
also: The fact that entropy increases defines the arrow of time. It is a statistical phenomenon, valid for large systems...
quote from a post on the "arrow of time" at Preposterous Universe. interesting to think of time as defined by the changing state of the universe... ie, as a function of events, relating time to events rather than placing events in time. got this sense from the article on boltzmann's stuff, but the arrow of time article had the clearer quote.
