There is no future or past. Everybody is living in this point of time forever. When you die, you just live some other life from somebody else. You might even see you as a previous life while living another life.

Being insane is the next step. You might start screaming that you've lived another life, it wasn't a past life though... it was just another life in the same time period, at the same place. Eventually everybody will live the life of this one crazy person.

The next step would be knowing you've lived all these other lives, but not telling anybody because you would be locked away. Ever have anybody look at you strange for no reason at all? They're probably remembering when they were you.

Of course, the steps aren't progressive. When you've been looking at somebody you once were, your next life could be a stage one of life. Maybe even the insane stage, and remember a life you never actually lived. The life you remember though would have been a real lived life.

The last step, who knows... maybe we're on a different planet in the universe... at this exact same time.