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The Mule is a fictional character from Isaac Asimov's Foundation series. One of the greatest conquerors the galaxy has ever seen, he is a mentalic who has the ability to reach into the minds of others and "adjust" their emotions, individually or en masse, using this capability to conscript individuals to his cause. Not direct mind-control per se, it is a subtle influence of the subconscious; individuals under the Mule's influence behave otherwise normally - logic, memories, and personality intact. This gives the Mule the capacity to disrupt Seldon's plan by invalidating Seldon's assumption that no single individual could have a measurable effect on galactic socio-historical trends on their own, due to the plan relying on the predictability of the actions of very large numbers of people.
From the book "Foundation's Edge" we learn that Mule is from planet Gaia. The Mule establishes his empire incrementally, using past conquests to aid new ones: first by mentally converting a pirate band to his allegiance, then a whole planet, then the militarily powerful kingdom of Kalgan, which he obtains by mentally converting the warlord of the planet Kalgan, and then the Foundation. The Mule sets up his own short-lived Galactic Empire, the "Union of Worlds", styling himself "First Citizen of the Union" and making Kalgan its capital planet. Leading up to, during, and for a good time after the Mule's conquest of the Foundation and its trade confederacy, almost no one ever actually sees the Mule or knows what he looks like.
The Foundation, after the death of the Empire, is the sole supplier of nuclear weaponry in the galaxy, and using this asset the Mule begins rapidly conquering surrounding territories, all of which lack nuclear power, sweeping aside the remnants of the Galactic Empire centered on Neotrantor.
The Mule's conquest is amazingly fast: he defeats the Foundation and establishes the Union of Worlds after only five years. Then, just as suddenly, and seemingly randomly, he stops his advance and settles into a five-year period of consolidation. This is actually because the Mule fears the mysterious Second Foundation, which is rumoured to be capable of defeating a mentalic individual like himself. According to a hint given by Hari Seldon, the Second Foundation was founded at "the opposite end of the galaxy" from the First Foundation, which was itself founded on the distant galactic rim planet Terminus. However, the true location of the Second Foundation is a total mystery and popular sentiment is that it does not exist. Nevertheless, the Mule launches repeated expeditions in search of the Second Foundation during this period of consolidation.
These expeditions, particularly the last, led by Han Pritcher and Bail Channis, are very nearly successful. Ultimately, however, the Second Foundation succeeds in defeating the Mule, transforming him into a relatively harmless individual lacking ambition, and ending his threat to the Seldon Plan.
The Mule's name is a reference to the sterility of mules, since he too is genetically sterile. Without a child available as a successor, the Mule's empire ends soon after his death. No reference is ever given about any other name for him besides "The Mule", a name he apparently gave himself, and 'Magnifico Giganticus', an alias he takes up to travel among Foundationers (in that role, he also claims his name used to be Bobo). In Second Foundation, it is revealed that the Mule dies in his thirties as a result of poor health.
His childhood is one of torment and alienation. The Mule becomes aware of his great mental powers in his twenties, and develops a desire to compensate for his earlier life by taking revenge on humanity.
Seldon Plan disturbed by the Mule
The Mule is a central character in Foundation and Empire and Second Foundation. Foundation's Edge reveals that he originally came from the planet Gaia, but was regarded as an aberration on a world where mental powers were being developed for benevolent ends.
According to his autobiography In Memory Yet Green, Asimov modeled the Mule's physical appearance on Leonard Meisel, a friend at the World War II-era Navy Yard in Philadelphia. In keeping with the Foundation series being based on the Roman Empire, the Mule has historical parallels with Attila the Hun, Tamerlane, and Charlemagne; he has also been compared to the Roman Emperor Augustus, Adolf Hitler, Stalin and several other absolute tyrants of recent history.