Hello! I hope this is the proper forum for my question(s). English is not my native language, so I apologize in advance for language mistakes. In Feynman Lectures on Physics, there is a chapter about the conservation of energy. The first part, which deals with the definition of energy, I understand quite well. Energy is an abstract mathematical concept; It is a quantity that must be calculated, it has many forms, and in total - it never changes. In the next part of the article, Feynman discusses the various forms of energy, in particular - gravitational potential energy. He opens the discussion on gravitational potential energy with a number of statements about weight-lifting machines, especially - perpetual motion weight-lifting machines. To rule out the existence of perpetual motion machines, Feynman begins with the definition of perpetual motion: So here are my questions (I will probably have more when you will answer those two ): What does "...when we have lifted and lowered a lot of weights..." mean? If it means that we have added weights to the machine, doesn't it mean that we put energy into the system? Doesn't it contradicts with "it has not received the energy to lift that weight from some external source"? "...raised and lowered a lot of weights..." suggests that the process is repeated several times. How is that possible? Suppose we have on one side of the machine 5kg, and on the other side 1kg. The scale with 5kg will be lowered, and the scale with 1kg will be lifted (provided that the axis is located in the center of the scales). The final state of this process is 1kg up and 5kg down. How can the machine continue the process and miraculously lift the 5kg? Thank you very much!