[q=Sephy99]I always thought that the Sephiroth who stole the black materia was like a projection of Sephiroth or his consciousness.[/q]
Now where have I read this before?
Oh yeah.
[q=me]Yeah, I thought that Sephiroth created a 'manifestation' of his will in order to release Jenova and do whatever else. This power is a result of Sephiroth's Jenova cells and the time he spent in the Lifestream.

Basically, I've always believed that the Sephiroth who released Jenova, led Cloud around the world and (SPOILER) killed Aeris was, in fact, a powerful 'projection' of the real Sephiroth, who was entombed in that crystal in the North Cave. At one point it is said that Sephiroth can 'diffuse his will through the Lifestream'.
Sephiroth's powers, combined with Jenova's, allowed him to create a 'psychic shadow' which he could use to travel the world, transport Jenova, find the (SPOILER) black materia and interact with real people, all without having to leave the North Cave.
[/q]

So I'm not the only one who believes this outlandish notion... there may be something in it yet.