 The answer seems to be that, normally, a live object has a distinctive sort of control over whether things come to be, or cease to be, part of it. The control in question is made possible by activities its constituents themselves are capable of. Contrast objects that are not alive, say automobiles. What an ordinary car is composed of is settled for the car by the mechanics who repair it (detaching some parts and affixing others), by whether it is involved in an accident and loses some parts, and so forth. Imagine a car that is not passive in this way. Imagine that its parts were somehow capable of replacing some of themselves with fresh parts, without assistance from outside, so that the activities of the parts that compose the car today were responsible for its being composed of certain parts tomorrow. That would make it quite lifelike.