Machines which extract energy from "perpetual sources" such as ocean currents, are indeed capable of moving "perpetually" but are not considered in this category, as they are clearly absorbing energy from an external source and are not "closed systems".
Machines, are indeed capable of moving "perpetually". These Machines extract energy from "perpetual sources" such as ocean currents. As they are clearly absorbing energy from an external source. This was what Machines which extract energy from "perpetual sources" such as ocean currents are not considered in this category. Machines, are not "closed systems". These Machines extract energy from "perpetual sources" such as ocean currents.