Psychohistory depends on the idea that, while one cannot foresee the actions of a particular individual, the laws of statistics as applied to large groups of people could predict the general flow of future events. Asimov used the analogy of a gas: an observer has great difficulty in predicting the motion of a single molecule in a gas, but can predict the mass action of the gas to a high level of accuracy. (Physicists know this as the Kinetic theory). Asimov applied this concept to the population of his fictional Galactic Empire, which numbered a quintillion. The character responsible for the science's creation, Hari Seldon, established two axioms:
that the population whose behaviour was modeled should be sufficiently large
that the population should remain in ignorance of the results of the application of psychohistorical analyses
There is a third underlying axiom of Psychohistory, which is trivial and thus not stated by Seldon in his Plan:
that Human Beings are the only sentient intelligence in the Galaxy.