Among the very many puzzling aspects of the physical world is this: how do we explain the fact that the laws of thermodynamics are time-asymmetric while those of statistical mechanics are time-symmetric? If the fundamental physical laws do not require events to occur in any particular temporal direction, why do we observe a world in which, for example, we will always see milk dispersing in tea but never coming together in tea – at least not unless we film the dispersal and then run the film backwards? In The Road to Maxwell’s Demon: Conceptual Foundations of Statistical Mechanics (Cambridge University Press, 2012), Meir Hemmo of the University of Haifa and Orly Shenker of the Hebrew University of Jerusalem provide a fascinating and accessible defense of the position that the laws of thermodynamics are observer-relative, that the evolutions of physical microstates in classical mechanics have a direction of time but no determinate direction, and that the relation between observers and the dynamics determines the direction of time that we observe and capture in our thermodynamical laws. In consequence, they argue, it’s just a contingent fact that we remember the past rather than the future, and Maxwellian Demons – perpetual motion machines that can exploit more and more energy while putting in less and less work – are possible.