I wrote up the quarks in a very tentative manner. I also included in the letter the possibility of integrally charged... of an integrally charge triple together with a fourth particle; the… the thing I had mentioned at MIT in my lectures that spring. But I said that the quarks, I implied that the quarks were a neater solution. But I did something that has caused problems ever since. I needed a word to describe particles that actually came out and could be seen and used in the laboratory and used for industrial purposes and so on and so on. And I chose the word 'real' for that, reserving ‘mathematical’ or ‘fictitious’ for a particle that couldn't be directly, singly observed in the laboratory, utilized for industrial purposes and so forth. Well, this was a huge mistake. The reason I did it was that I imagined myself in a debate with philosophers, who would ask, 'If this particle doesn't come out to be seen singly by itself and so on, how can you say it's there? How can you say it's real?' And in fact people said that kind of thing. There's a historian of science–he must be a very silly person–called Pickering, Andy Pickering, who wrote something called Constructing Quarks. He's one of these 'social construction of science' people, I guess, or is close to it anyway. And he said in this book, written many years later, that he doesn't think quarks are part of the furniture of the universe. Anyway, it was that kind of thing that I was thinking of when I used this word 'real'. So then I said, well of course, the quarks are less likely to be real than if we have the other scheme with the three particles with integral charges and the fourth one… fourth one as a singlet. I didn't mean that the quarks were less likely to be the right solution, but they were… if they were the solution, they were more likely to be trapped inside because that would account for nobody ever having seen a fractionally charged particle, and so on and so forth. Of course, if they were very heavy, they wouldn't have been seen anyway. But then I had to have some way to describe to people what I meant by their being mathematical, so in that letter I said well, imagine the mass of the quark goes to infinity, that's the kind of thing I mean by mathematical. They're in there all right but mass being infinite they can't come out. They can be in there only bound together, they can't come out.

In 1966, to jump ahead a bit, when I was invited to give the introductory talk to the International Conference on Particle Physics at Berkeley, I used a different description of what I meant. Instead of talking about infinite mass, I said that if the quarks were not real they would… it would be like a situation in which they were trapped in a infinitely high potential barrier and couldn't get out. Well, that's exactly what we believe today to be the case, so my unreal or mathematical or fictitious quarks were precisely the kinds of quarks we believe in today. But many people have alleged that that isn't what I meant at all, that I meant I didn't believe the quarks, that they were a dumb idea and so on and so on, which was not at all the case.