 Science includes many principles at least once thought to be laws of nature: Newton’s law of gravitation, his three laws of motion, the ideal gas laws, Mendel’s laws, the laws of supply and demand, and so on. Other regularities important to science were not thought to have this status. These include regularities that, unlike laws, were (or still are) thought by scientists to stand in need of stronger ground. These include the regularity of the ocean tides, the perihelion of Mercury’s orbit, the photoelectric effect, that the universe is expanding, and so on. Scientists also use laws but not other regularities to sort out what is possible: It is based on their consistency with Einstein’s laws of gravity that cosmologists recognize the possibility that our universe is closed and the possibility that it is open (Maudlin 2007, 7–8). In statistical mechanics, the laws of an underlying physical theory are used to determine the dynamically possible trajectories through the state space of the system (Roberts 2008, 12–16).