SPOILERS AHEAD - go see the film if you haven't yet. It's much better than reading any blog.
An interesting plot point in this animated pleasure is one we have seen recently - the splitting of the soul into a number of different objects. We have the same situation in the Harry Potter films, called the Horcrux by J.K. Rowling.
The Horcrux is split into seven inanimate objects, used by Lord Voldemort to insure his survival. If even one still exists, it will have the power to re-embody You Know Who. Similarly, in "9", the scientist splits his soul into nine amazingly cool little dolls, each with a different personality. In Potter, each part is capable of reproducing the whole. In "9", though each doll is joyfully different, it seems to be the same case. When some of the brave dolls die, their souls are eventually able to leave this world, with the remaining heroes remaining alive. I was expecting some twist along the lines of each doll being unable to remain when separated, but they are not simply part of the whole. With each having different talents, I expected each to need the other.
But I believe I missed a clue, because 9 has it all really - he combines in some small way the talents of 1 through 8. And maybe by the end, each one has done the same to some degree.
Can the soul be split? If we venture along the lines of each piece being able to recreate the whole, it would seem we can never really permanently lose a piece of ourselves. Somewhere inside, that energy will still be somewhere, somehow available to be pulled up for use when needed.
But it sure feels like we do lose part of ourselves at critical times - people that take a piece of us with them when they leave; hopes and dreams that fail; even decisions we make, when they eliminate possibilities. No matter that we keep going forward - something seems gone.
But maybe soul is more malleable than it might seem at first consideration. Cinema and story is filled with the idea of the team, in which each talent is needed for the completion of the whole. It is probably best to read that narrative psychologically, with each piece making up part of each of us. We seem to have all talents inside of us, just needing the courage to allow them to come to surface.