Not even the vast majority of Scientologists can fully answer the question. In the Church of Scientology, there is no one book that comprehensively sets forth the religion's beliefs in the fashion of, say, the Bible or the Koran.
Piece by piece, his teachings are revealed to church members through a progression of sometimes secret courses that take years to complete and cost tens of thousands of dollars. Out of a membership estimated by the church to be 6.5 million, only a tiny fraction have climbed to the upper reaches. In fact, according to a Scientology publication earlier this year, fewer than 900 members have completed the church's highest course, nicknamed "Truth Revealed."
While Hubbard's "Dianetics: The Modern Science of Mental Health" typically is one of the first books read by church members, its relationship to Scientology is like that of a grade school to a university.
What Scientologists learn in their courses is never publicly discussed by the church, which is trying to shake its cultish image and establish itself as a mainstream religion. For to the uninitiated, Hubbard's theology would resemble pure science fiction, complete with galactic battles, interplanetary civilizations and tyrants who roam the universe.
Here, based on court records, church documents and Hubbard lectures that span the past four decades, is a rare look at portions of Scientology's theology and the cosmological musings of the man who wrote it.
Central to Scientology is a belief in an immortal soul, or "thetan," that passes from one body to the next through countless reincarnations spanning trillions of years.
Collectively, thetans created the universe -- all the stars and planets, every plant and animal. To function within their creation, thetans built bodies for themselves of wildly varying appearances, the human form being just one.
But each thetan is vulnerable to painful experiences that can diminish its powers and create emotional and physical problems in the individual it inhabits. The goal of Scientology is to purge these experiences from the thetan, making it again omnipotent and returning spiritual and bodily health to its host.
The painful experiences are called "engrams." Hubbard said some happen by accident -- from ancient planetary wars, for example -- while others are intentionally inflicted by other thetans who have gone bad and want power. In Scientology, these engrams are called "implants."
According to Hubbard, the bad thetans through the eons have electronically implanted other thetans with information intended to confuse them and make them forget the powers they inherently possess -- kind of a brainwashing procedure.
While Hubbard was not always precise about the origins of the implants, he was very clear about the impact.
"Implants," Hubbard said, "result in all varieties of illness, apathy, degradation, neurosis and insanity and are the principal cause of these in man."
Hubbard identified numerous implants that he said have occurred through the ages and that are addressed during Scientology courses aimed at neutralizing their harmful effects.
Hubbard maintained, for example, that the concept of a Christian heaven is the product of two implants dating back more than 43 trillion years. Heaven, he said, is a "false dream" and a "very painful lie" intended to direct thetans toward a non-existent goal and convince them they have only one life.
In reality, Hubbard said, there is no heaven and there was no Christ.
"The (implanted) symbol of a crucified Christ is very apt indeed," Hubbard said. "It's the symbol of a thetan betrayed."
Hubbard said that one of the worst implants happens after a person dies. While Hubbard's story of this implant may seem outlandish to some, he advanced it as a factual account of reincarnation.
"Of all the nasty, mean and vicious implants that have ever been invented, this one is it," he declared during a lecture in the 1950s. "And it's been going on for thousands of years."
Hubbard said that when a person dies, his or her thetan goes to a "landing station" on Venus, where it is programmed with lies about its past life and its next life. The lies include a promise that it will be returned to Earth by being lovingly shunted into the body of a newborn baby.
Not so, said Hubbard, who described the thetan's re-entry this way:
"What actually happens to you, you're simply capsuled and dumped in the gulf of lower California. Splash. The hell with ya. And you're on your own, man. If you can get out of that, and through that, and wander around through the cities and find some girl who looks like she is going to get married or have a baby or something like that, you're all set. And if you can find the maternity ward to a hospital or something, you're OK.
"And you just eventually just pick up a baby."
But Hubbard offered his followers an easy way to outwit the implant: Scientologists should simply select a location other than Venus to go "when they kick the bucket."
Another notorious implant led Hubbard to construct an entire course for Scientologists who want to be rid of it.
Shrouded in mystery and kept in locked cabinets at select church locations, the course is called Operating Thetan III, billed by the church as "the final secret of the catastrophe which laid waste to this sector of the galaxy." It is taught only to the most advanced church members, at fees ranging to $6,000.
Hubbard told his followers that while unlocking the secret, he "became very ill, almost lost this body and somehow or another brought it off and obtained the material and was able to live through it."
Here's what he said he learned:
Seventy-five million years ago a tyrant named Xenu (pronounced Zee-new) ruled the Galactic Confederation, an alliance of 76 planets, including Earth, then called Teegeeack.
To control overpopulation and solidify his power, Xenu instructed his loyal officers to capture beings of all shapes and sizes from the various planets, freeze them in a compound of alcohol and glycol and fly them by the billions to Earth in planes resembling DC-8s. Some of the beings were captured after they were duped into showing up for a phony tax investigation.
The beings were deposited or chained near 10 volcanoes scattered around the planet. After hydrogen bombs were dropped on them, their thetans were captured by Xenu's forces and implanted with sexual perversion, religion and other notions to obscure their memory of what Xenu had done.
Soon after, a revolt erupted. Xenu was imprisoned in a wire cage within a mountain, where he remains today.
But the damage was done.
During the last 75 million years, these implanted thetans have affixed themselves by the thousands to people on Earth. Called "body thetans," they overwhelm the main thetan who resides within a person, causing confusion and internal conflict.
In the Operating Thetan III course, Scientologists are taught to scan their bodies for "pressure points," indicating the presence of these bad thetans. Using techniques prescribed by Hubbard, church members make telepathic contact with these thetans and remind them of Xenu's treachery. With that, Hubbard said, the thetans detach themselves.
Hubbard first unveiled his Scientology theories during a series of often breathless lectures he delivered in Wichita, Kan., Phoenix and Philadelphia in 1952.
His talks were sprinkled with tales of interplanetary adventures he said he had experienced during earlier lives.
There was the time, for instance, that Hubbard said he was resting in a peaceful valley on a barren planet in some remote galaxy, and decided to spruce up the place. He said he "fixed up a lake" and "managed to coax into existence a few vines."
Then, "all of a sudden -- zoop boom -- and there was a spaceship," Hubbard recalled, saying "I got pretty mad about the whole thing."
"I remember bringing a thunderstorm," Hubbard said. "Moved it over the ship. ... And then (I) let them have it."
Hubbard told associates that he had been many people before being born as Lafayette Ronald Hubbard on March 13, 1911, in Tilden, Neb. One of them was Cecil Rhodes, the British-born diamond king of southern Africa. Another, according to a former aide, was a marshal to Joan of Arc.
After Hubbard's death in 1986, a Scientology publication described him as "the original musician," who 3 million years ago invented music while going by the name "Arpen Polo." The publication noted that "he wrote his first song a bit after the first tick of time."
Hubbard realized that his accounts of past lives, implants and extraterrestrial creatures might sound suspect to outsiders. So he counseled his disciples to keep mum.
"Don't start walking around and telling people about space opera because they're not going to believe you," he said, "and they're going to say, 'Well, that's just Hubbard.' "