Psychohistory is the name of a fictional science in Isaac Asimov's Foundation universe, which combined history, psychology and mathematical statistics to create a (nearly) exact science of the behavior of very large populations of people, such as the Galactic Empire. Asimov used the analogy of a gas: in a gas, the motion of a single molecule is very difficult to predict, but the mass action of the gas can be predicted to a high level of accuracy. Asimov applied this concept to the population of the fictional Galactic Empire, which numbered in the quadrillions.