God.

Maybe, just maybe, its something that's akin to the concept of the number zero. In otherwords, a symbol that rejects the lack of meaning that is necessary in order to designate the system as such. The symbol's analog version is God, whereas the digital version is zero. What do you think? And then if you think about it, our (Tachikoma) primary structure is digital, right? So, no matter how much information we accumulate, a ghost will never dwell within us. But since Batou-san and the others have a primary analog structure, no matter how much you increase your digital factors in your structural makeup by adding cybernetic and cyberbrain implants, your ghost will remain intact. In addition, you are capable of death because you have a ghost. Lucky you.