"The Second Law [of thermodynamics] tells us that nature works inexorably to disperse, to eliminate distinctions, to bring all things up or down to the same level. Sometimes it’s called 'time’s arrow'; it explains (among other things) why time can run only one way. For time to run backwards, nature would have to bust open the locked door of the Second Law. Now look at halakha, a system that is large and wide-ranging and alive. From outside it seems shapeless, but it has a recurring theme: separation, in space and time. Keeping kosher means separating meat from dairy, kosher from unkosher food. Keeping the Sabbath means separating the day of rest from the forward-tumbling chaos of ordinary time."