"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 with the kinetic theory can predict the mass action of the gas to a high level of accuracy. Asimov applied this concept to the population of his fictional Galactic Empire, which numbered one quintillion. The character responsible for the science's creation, Hari Seldon, established two axioms:
that the population whose behavior was modeled should be sufficiently large
that the population should remain in ignorance of the results of the application of psychohistorical analyses because if it's aware, the group changes its behaviour.
Ebling Mis added these axioms
That there would be no fundamental change in the society
That human reactions to stimuli would remain constant.
Golan Trevize in Foundation and Earth added this axiom
that humans are the only sentient intelligence in the galaxy."
https://en.wikipedia.org/wiki/Psychohistory_%28fictional%29