How to put it, it is not impossible to say what this sentence insists on. After all, the conservation law of energy conservation is generally only available in systems with constant time translation. In general relativity, space-time can be bent, and a system with non-conserved energy (material part) can always be found. For example, during the inflation period of the very early universe, the total energy of matter in the universe E~a³ (a is the scale factor of the universe), and the energy increases exponentially with the expansion of the universe. In the radiation-dominated universe, the total energy is E~1/a, and the energy decays as the universe expands. Today’s universe is dominated by dark energy, and the total energy of the material part is also increasing with the expansion of the universe. Therefore, if a person can create a space-time dominated by “vacuum energy” and make effective use of the increased energy, it can really be regarded as a perpetual motion machine. Or if this person uses some means to convert dark energy into energy that can be used by humans, then it is considered that he has created a perpetual motion machine. But, can these really be done by human science and technology? I am afraid that this will not be possible until mankind disappears. Let me add one more point. Some friends who have studied general relativity may have the following questions: The energy of the matter part can not be conserved, but the energy of the matter + gravitational field is conserved. How can it be regarded as a perpetual motion machine? Indeed, the increased energy of the matter can be regarded as borrowed from the gravitational field, but,,, it seems that there is no law that prevents matter from borrowing energy from the gravitational field indefinitely. For example, dark energy. The total energy has been increasing for more than 10 billion years, and there is no sign of stopping at all. Since energy can be borrowed infinitely without repayment, what is the difference between this and perpetual motion machine? Let me talk about this first, and I will add if there are many people watching.