Here's a theory: you know that voice in your head, the one that just said "what voice?", the one you talk to when you "talk to yourself"? Someone without a self doesn't have that. I say a borderline has half a self, so they do have a voice, it's just screaming "don't leave me" all the time, and obsessed with finding and attaching to another half.
Great anaylsis.
So basically they drift around without thinking, impulsively drifting towards whatever gratifies them, doing whatever it is they think will result in their emptiness and pain being taken away.
And thats it, thats their incredibly primitive existence.
Its no wonder my ex was incapable of in depth conversations, or delving deep into interests or creativity, they just don't understand it, all they seem to think about is how to survive.