According to the doctrines of Scientology:
The usual residence of the thetan is in the skull or near the body. A thetan can be in one of four conditions. The first would be entirely separate from a body or bodies, or even from this universe. The second would be near a body and knowingly controlling the body. The third would be in the body (the skull) and the fourth would be an inverted condition whereby he is compulsively away from the body and cannot approach it. There are degrees of each of these states. The most optimum of these conditions, from the standpoint of man, is the second. (Scientology: The Fundamentals of Thought, pp. 64-65)
Scientology maintains that a thetan is subject to deterioration but that he can at any moment return to an entirety of his ability. One of the goals of "processing" or "auditing" in Scientology is to place the individual in the second condition described above, a condition in which, according to Scientology, he is happier and more capable than he would otherwise be (Scientology: The Fundamentals of Thought, p. 65). More will be said about this in a later section. For the present it is sufficient to note that a fundamental element of the doctrines of Scientology is that man consists of three parts: the body; the organized physical substance; the mind -- which consists essentially of pictures; and the thetan -- the soul or spirit which animates the body and uses the mind as a communication and control system between himself and the physical universe. The thetan is the most senior of these three parts, for "without the thetan there would be no mind or animation of the body, while without a body or a mind there is still animation or life in the thetan" (p. 147; see also Scientology: The Fundamentals of Thought, pp. 63-79, and The Scientology Handbook, p. xxiii).