Imagine a snowball that's rolling down an infinite slope. As it descends, it picks up more snow, rocks, sticks, maybe some bugs, I don't know. Maybe there are dry patches, too, and the snowball loses some snow. Maybe the snowball hits a boulder and loses half of its snow, and what remains is less than 10% original snow material. But it still can be said to be this snowball and not that snowball because its composition and history are unique to it - it can be identified by its past travels, its momentum, and the resulting trajectory. If this can be taken to be one's life (an analogy that I hope was obvious), then the "I" that we refer to in our own lives isn't even the whole snowball but merely the place where the snowball touches the ground.