[ceejay]

Don't get me wrong. I agree with the laws of thermodynamics :) , but just like we exploit a "closed loop" / "seemingly perpetual" system between the moon and the oceans, or flowing rivers, couldn't there exist other "closed loop" systems? After all, on the scale of a human life that closed loop system (between moon and oceans) is, for all practical purposes, perpetual.

[pjc50]

The water cycle is not "perpetual": it depends on input from the sun.

The tides are not "perpetual": they depend on gradually taking energy from the orbit of the moon.

[cmdrfred]

Sure you can, get me a moon and I'll show you how.

[witty_username]

Yes, but where does the energy come from for your perpetual machine?