John Toll was a graduate student at Princeton, and he and I got interested in this game of what you can predict about a system from the fact that you can't get a signal to go through it faster than the speed of light. And this says - and two Dutch physicists, Krämers and Krönig, had shown this - that the absorption of the light in the substance, and the speed of the light through the substance, must be connected in a certain way, if Causality is to be upheld. What does causality mean? It means really that no signal should come out of the device before it goes in. If you apply that simple argument, you can draw a lot of conclusions about the connection between the absorbing power of the material and the speed of conduction of the material. And John Toll got into that, and was writing it up for his thesis. Why did I go to France to do my work? What was my work anyway? This fellowship I had applied for, I wanted to trace out this idea of action at a distance, further. Feynman and I had worked on Electrodynamic Action at a distance, where we swept out the electromagnetic field between the elementary particles and talked through the action, directly from one particle to another. I wanted to sweep out the gravitational field between one particle and another and talk directly of the action, one on another. What is the gravitational field? Well, it's space-time structure, so I want to sweep out space and time from between elementary particles and have just direct action. And I thought I could get some helpful insights from Niels Bohr, so I made various visits to Copenhagen. Why hadn't I gone to Copenhagen in the first place? Well, I would have if it weren't for my children, but I didn't think that learning Danish would be, for them, so useful as learning French, so I put them into a French school about a block from where we lived, in Paris, and they made out very well. But in Copenhagen, on my visits, the Bohrs were kind enough to put me up in their house. And Bohr did not particularly take to this idea of sweeping out the space and time between the elementary particles, that was not his kettle of fish, that was fried by myself. But we talked to Bohr about nuclear physics, and we started writing an article about a unified view of nuclear physics. There's that lovely room in Bohr's house in Copenhagen where there's a Pompeian Court, a set of columns, arranged in an arch and water, maybe a foot thick, between them. And then a walkway around, and we'd [go] round and round that, talking, and then we'd go into the little work room beside the place, where usually Bohr would dictate and I would write down. He would dictate the result of our discussion.