Obviously, Newton and Leibniz and many other Mathematicians (and other people) understood the chain rule before back propagation. But unfortunately I am very far from a Newton or Leibniz, so it took me a lot longer to grasp why the chain rule is the way it is. And back propagation just made it click for me. I was really just talking about me personally.
What clicked for me was drawing the chain rule as a graph. When I was in school I just applied the chain rule without thinking about it. I really didn't mean this to be some deep insight or anything. Just an anecdotal comment.
