Perpetual motion does not equal perpetual energy. Newton's 1st Law of Motion states that an object in motion stays in motion unless an external force is applied to it. In the world today, virtually all energy capturing devices rely on the principle of asymmetry. However all asymmetric systems will entropically devolve towards a balanced energy state. This is the principle of the conservation of energy.<br /><br />So if you're designing an energy capturing device, you need to think about where the energy is coming from, where it's going, and how entropy will affect the overall system. While a perpetual energy machine may not be possible, it could be possible to design a very efficient machine that has an extremely long asymmetric energy state and that could be "perpetual" on human time-scales.