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.