Earlier quoted context omitted.
You're the one talking about purpose. The rest of us are talking about mechanism. You're handwaving away the problem, but that doesn't mean there still isn't a problem there we don't understand yet.
what is the problem?
Suppose we eventually know everything about the body and mind, that some day science can simulate and predict the motion of every atom in your body. Thus, all your behavior is deterministic. That still doesn't explain why there's an "ego" there to experience it all.