“Pyscho-history dealt not with man, but with man-masses. It was the science of mobs; mobs in their billions. It could forecast reactions to stimuli with something of the accuracy that a lesser science could bring to the forecast of a rebound of a billiard ball. The reaction of one man could be forecast by no known mathematics; the reaction of a billion is something else again.”

“The Three Theorems of Psychohistorical Quantitivity:The population under scrutiny is oblivious to the existence of the science of Psychohistory.The time periods dealt with are in the region of 3 generations.The population must be in the billions (±75 billions) for a statistical probability to have a psychohistorical validity.”

“You are a military man and should know better. If there is one science into which man has probed continuously and successfully, it is that of military technology. No potential weapon would remain unrealized for ten thousand years.”

“all the suffering that humanity ever knew can be traced to the one fact that no man in the history of the Galaxy, until Hari Seldon, and very few men thereafter, could really understand one another. Every human being lived behind an impenetrable wall of choking mist within which no other but he existed. Occasionally there were the dim signals from deep within the cavern in which another man was located - so that each might grope toward the other. Yet because they did not know one another, and could not understand one another, and dared not trust one another, and felt from infancy the terrors and insecurity of that ultimate isolation - there was the hunted fear of man for man, the savage rapacity of man toward man.”

“A "simple aspect of science" may be defined as one which, through good fortune, I happen to understand.”

“The most exciting phrase to hear in science, the one that heralds the most discoveries, is not "Eureka!" (I found it!) but 'That's funny...”

“It is the chief characteristic of the religion of science that it works.”