Human-Engineered Self-Replicating Machines

How close are we really to a human-engineered self-replicating machine? A couple of thought experiments are proposed which help explain why such machines are still far beyond current human technology. And I conclude that we are unlikely to be successful in explaining how the self-replicating machines we see everywhere in the living world could have arisen through entirely natural causes, when we still do not know how such machines could be designed.