Thetans have been described in the Church of Scientology in a number of ways.
Hubbard once defined a thetan as: "...having no mass, no wave-length, no energy, no measurable qualities and no time or location in space except by consideration or postulate. The spirit is not a thing. It is the creator of things." Oddly, in a lecture series later published as a book ("The Phoenix Lectures"), he pointed to a study that implied a thetan manifests a small but measurable amount of mass:
Although Hubbard did not name the doctor concerned, there was indeed such an attempt, by a Dr. Duncan MacDougall, to measure the weight of dying patients to determine the weight of the soul. MacDougall's experiments took place about fifty years before Hubbard's lectures, not fifteen or twenty. They are generally not regarded as having any sort of scientific validity.
According to Hubbard's son Ronald DeWolf (born L. Ron Hubbard, Jr.), his father stated that thetans are immortal and perpetual, having willed themselves into existence at some point several trillion years ago. After they originated, thetans generated "points to view" or "dimension points", causing space to come into existence. They agreed that other thetans' dimension points existed, thus bringing into existence the entire universe. All matter, energy, space, and time exists solely because thetans agree that it exists.
Jon Atack, whose book A Piece of Blue Sky details how he reached Operating Thetan level V before leaving Scientology, describes Hubbard's doctrines about thetans: "Thetans are all-knowing beings, and became bored because there were no surprises. Hubbard asserted that the single most important desire in all beings is to have a 'game'. To have a 'game' it was necessary to 'not know' certain things, so certain perceptions were negated ('not-is-ed')." Since thetans knew everything, this required them to abandon or suppress perceptions and knowledge. Over time, the loss of perception accumulated and certain thetans began to cause harm to others. MEST (physical) beings also sought to "trap" thetans in order to control them. Thetans came to learn contrition, punishing themselves for their own "harmful" acts.
According to Hubbard, an essential part of the thetans' game was the "conquest" of matter, energy, space, and time by the life force, theta. This has produced multiple universes which have ended and begun in succession, each new one being more solid and entrapping than the last. The thetans have by now become so enmeshed in the physical universe that many have identified themselves totally with it, forgetting their quadrillions of years of existence and their original godly powers.
Nonetheless, according to Scientology, thetan powers are said to remain potent and restorable. One of the Church of Scientology's stated goals is "the rehabilitation of the human spirit", by which it means the restoration of the thetan's original abilities. Hubbard claims that thetans are able to change reality through "postulates" - decisions made by the individual about the nature of the reality around them. Some thetans are said to have (mis)used this ability to "implant" others with hypnotic suggestions, forcing other thetans to "cluster" around bodies (hence body thetans). This sort of directed control is referred to as "other-determinism". Scientology seeks to undo it and return the thetan to "self-determinism", where he can control himself and his environment. The eventual goal is to achieve "pan-determinism", where he acts for the good of all.
According to Scientology doctrine, a thetan exists whether operating a human body or not. Scientology advertises itself as being able to "rehabilitate" the thetan of a practitioner to a state where the individual can operate with or without a "meat body". The term "operating thetan" would then apply as it does when an individual is operating a body. The Operating Thetan (OT) levels are the upper level courses in Scientology.
The Church of Scientology states as a point of doctrine that an individual exists with or without a body. Scientology says that people with proper Scientology training can "exteriorize with full perceptics" (leave the body in spirit form and return) after completing OT levels, but this claim has yet to be publicly stated by any practitioner.
"A thetan who is completely rehabilitated and can do everything a thetan should do, such as move MEST and control others from a distance, or create his own universe; a person who is able to create his own universe or, living in the MEST universe is able to create illusions perceivable by others at will, to handle MEST universe objects without mechanical means and to have and feel no need of bodies or even the MEST universe to keep himself and his friends interested in existence".
A Body Thetan is an alien thetan who is 'stuck' in, on or near a human body, and all human bodies are said to be covered in these misplaced thetans, or clusters of them. This information is not divulged until a Scientologist reaches the third Operating Thetan level (or OT III). Most Body Thetans were said by Hubbard to be a result of a prehistoric "Incident" involving Xenu, as well as other similar alien occurrences on the whole track.