Something I've recently figured out. The final piece of a soul, and it's always the final piece, after the creation of a soul. It might sound a bit odd, or might not make any sense. But the best way to phrase it is:
The ability to stand up on its own.
With no form of assistance. If you really want to call yourself a "living being", you should be able to do things on your own, with no help from anyone whatsoever.