Feynman's lecture on physics: chapter 4 derives equation for gravitational potential energy by a "lifting 3 balls" example. The book notes: "But the strange thing is that, in a certain way of speaking, we have not lifted two of them at all because, after all, there were balls on shelves 2 and 3 before. The resulting effect has been to lift one ball to a distance of 3X." How is this argument true? I am a physics student. And I have no rights to question Feynman's point of reasoning. But, this line of reasoning seems very strange to me. Just because we can not see that the other two balls have also moved, does not mean that they have not been moved. If seeing is everything we could have used 3 different colored balls. What am I missing here?