We played with the sigma model and used it as an example of many different things. I don't think we were the first ever to use it, but we certainly made a great deal more use of it than had been made before. We also developed the non-linear sigma model which somebody else may have had before, I doubt whether it was absolutely original, but that's - but again we did much more with it than anyone else and possibly we discovered it, I don't know.
Feynman didn't want to have anything to do with the non-linear sigma model. He said he didn't believe in square roots of fields, he didn't understand square roots of fields, he didn't want to discuss square roots of fields - and so he wouldn't sign any paper that included them. So we had to leave him off of the ones that mentioned it, and the ones that didn't mention it he - as far as the ones that didn't mention it were concerned - he hadn't contributed to those. So he didn't co-sign any of the papers I wrote in Paris.