So Feynman has this path integral picture of the world, as if the world was a kind of a tapestry in which all kinds of things could go on and all you had to do in order to predict the future was start with a known state in the past, allow everything to happen in the intermediate time in all possible ways, every particle or every field could jiggle around as much as it wanted in all directions, and then at the end, in the future state, you want to calculate the probability amplitude for a particular configuration in the future, you simply add up the contributions from all the histories in between. Each history contributes a certain probability amplitude and the amplitude is just the integral of the Lagrangian over the space time volume between the past and future. So that was Feynman's picture and it made sense, it was understandable as a physical picture. But then he had a practical version of this which was a sort of a crude approximation which was the Feynman diagrams, which were very different actually, although they were supposed to be an approximation. The Feynman diagram just consisted of a set of straight line tracks which were supposed to be individual particle tracks, and joined at, vertices where two or three lines would intersect, and each vertex corresponded to an interaction and each straight line corresponded to a particle track. And then you had propagators which were telling you the probability amplitude for the particle to move from A to B, and then instead of a path integral you had just a sum over the propagators. And that was supposed to give you the answer, and the amazing thing was that it did, the amazing thing was that this very simple diagram method gave you the right answers although the connection between that and the path integral wasn't at all obvious. So what I was always trying to persuade Feynman was that it's not enough to get the right answers, you have to understand what you're doing. And so we had big arguments about that, and I told him that he ought to learn some quantum field theory if he wanted really to understand this, and he said it just was a language he never would learn and he didn't think it was worth it. As far as he was concerned he thought in pictures and he didn't think in terms of equations. I thought in terms of equations and not in pictures. So we never agreed, but we just had fun talking.
