In his capacity as mathematics professor at Streeling University on the planet Trantor, Seldon develops psychohistory, an algorithmic science that allows him to predict the future in probabilistic terms.

On the basis of his psychohistory he is able to predict the eventual fall of the Galactic Empire and to develop a means to shorten the millennia of chaos to follow.

Hari Seldon is first mentioned in the short story "Foundation", published in Astounding Science Fiction in May 1942, as the creator of psychohistory, and appears as a prerecorded hologram near the end.

"The Psychohistorians" depicts Seldon put on trial by the Commission of Public Safety for his dire predictions about the eventual fall of the Galactic Empire, after which he is exiled to the distant planet Terminus.

[1]: 23–28 Describing "The Psychohistorians" as "28 pages of nonstop world-building", Josh Wimmer and Alasdair Wilkins of Gizmodo wrote that in the story, "Hari Seldon isn't so much a character as he is the living embodiment of psychohistory, an ethereal presence who's about as relatable as Gandalf.

Born to middle-class parents on Helicon, Arcturus sector (where his father, in a legend of doubtful authenticity, was a tobacco grower in the hydroponic plants of the planet), he showed amazing abilities in mathematics from an early age.

Apparently Seldon had been working up to his last moments on psychohistorical equations; his activated Prime Radiant was discovered clutched in hand.

Attempts by the Commission of Public Safety to locate Demerzel in the days following the Seldon memorial proved to be unsuccessful.

The planet—or so Seldon claimed—was originally occupied to create the Encyclopedia Galactica, a vast compilation of the knowledge of a dying galactic empire.

He is able to show that Galactic society can be represented in a simulation simpler than itself (in a finite number of iterations before the onset of chaotic noise smears discerning sets of events).[5]: p.

The rest of the novel tells of his flight, which lasts for approximately a year and which takes him through the complex and variegated world of Trantor.

They take place at intervals a decade or more apart, and tell the story of Hari's life, starting about ten years after Prelude and ending with his death.

Seldon becomes involved in politics when Eto Demerzel becomes a target for a smear campaign conducted by Laskin Joranum.

His career comes to an end when Cleon I is assassinated by his gardener (a random event Seldon could not have predicted) and the seizure of power by a military junta.

Raych eventually decides to move his family to the planet Santanni, but Wanda chooses to remain with her elderly grandfather.

[17] In 2019, the term Seldonian algorithm was chosen to honor the character in new artificial intelligence techniques intended to avoid undesirable behaviors in decision-making systems.

[18] People who credit to Hari Seldon for the career choices that they made include economist and New York Times columnist Paul Krugman[19] and US politician Newt Gingrich.