to diffuse from hot to cold bodies, and not the reverse. Another formulation of the second law is that: "In an isolated system, entropy as a measure of disorder or randomness can only increase." Let us have a closer look at this entropy notion. In Appendix A, we have seen that a system of particles that can be randomly distributed in W arrangement possibilities into a set of distinct boxes, called "microstates," is characterized by an entropy H. This entropy represents the infinite limit, normalized to the number of particles, of the quantity log W. Hence, the greater W or H, the more randomly distributed the system, which justifies the notion of entropy as a physical measure of disorder. By separating and ordering the gas molecules into two distinct families (slow and fast), the demon decreases the global randomness of the system, since there is a smaller number of microstate arrangements W for each of the initial molecules, the system's entropy log W is, thus, decreased, which is indeed in contradiction to the second law because the demon apparently did not perform any work. At the time, the conclusion reached by Maxwell to solve this paradox was that the second principle only applies to large numbers of particles as a statistical law, i.e., to be used with certain confidence at a macroscopic, rather than at a microscopic level. In the first place, Maxwell's paradox was never intended to challenge the second law. Yet, it kept puzzling scientists for generations with many unsolved issues. In 1929, for instance, L. Szilárd observed that in order to measure molecular velocities, the demon should spend some kind of energy, such as that of photons from a flashlight, to spot the slow or fast molecules, and then decide from the observation whether or not to open the trapdoor. Each of such measurements and the generation of photons would then increase the system's entropy, which should balance out the other effect of entropy decrease. In 1951, L. Brillouin observed that in order to distinguish molecules by use of light