Reality is like the Internet. There are legitimate ways for souls to enter and illegitimate ways. You could be `born' under your true name, or you could get in through the subversive route and infect an outlet susceptible to Multiple Personality Disorder, or possess someone.

Haha... but seriously, I know someone who ran a BBS, and his wife had Multiple Personalities Disorder. She has 5 or 6 different accounts on the system for each of her different personalities. It was pretty funny to watch the different persoalities talk on the bulletin boards...most people did not know who those accounts actually belonged to.

It's very difficult to detect, of course! When it happens, Call 1-800-EXORCIST

Yeah and I'm a trill currently possessing this body like Dax on DS9 hahaha... But since you brought up what you call "legitimate" personalities, let me pose this philosophical question to you: Suppose a human is born healthy, but with no cerebral functions, ie braindead. Suppose I am an old man who is dying, but neurology has advanced to a point where I could transfer myself to inhabit that body, and live normally thereafter. Would that be a legitimate soul? If true names are linked to biological entities, what is my true name? How do you define a true name? It's not a trivial question because there are a lot of extropians who would do that if they had the chance...