We now come to changes of personality which imply neither enlargement nor diminution but a structural alteration. One of the most important forms is the phenomenon of possession: some content, an idea or part of the personality, obtains mastery of the individual for one reason or another. The contents which thus take possession appear as peculiar convictions, idiosyncrasies, stubborn plans, and so forth. As a rule, they are not open to correction. One has to be an especially good friend of the possessed person and willing to put up with almost anything if one is to attempt to deal with such a condition. I am not prepared to lay down any hard and fast line of demarcation between possession and paranoia. Possession can be formulated as identity of the ego-personality with a complex.
A common instance of this is identity with the persona, which is the individual’s system of adaptation to, or the manner he assumes in dealing with, the world. Every calling or profession, for example, has its own characteristic persona. It is easy to study these things nowadays, when the photographs of public personalities so frequently appear in the press. A certain kind of behaviour is forced on them by the world, and professional people endeavour to come up to these expectations. Only, the danger is that they become identical to their personas – the professor with his text-book, the tenor with his voice. Then the damage is done; henceforth he lives exclusively against the background of his own biography. For by that time it is written: “…then he went to such and such a place and said this or that,” etc. The garment of Deianeira has grown fast to his skin, and a desperate decision like that of Heracles is needed if he is to tear this Nessus shirt from his body and step into the consuming fire of the flame of immortality, in order to transform himself into what he really is. One could say, with a little exaggeration, that the persona is that which in reality one is not, but which oneself as well as others think one is. In any case, the temptation to be what one seems to be is great, because the persona is usually rewarded in cash.
– Carl Jung. Collected Works Vol. 9 (1) (p 122-3)
Cosmic Flotsam 