“Occam's Razor. I'm tempted to add another principle, but it's really more of a wish than a rule. I'm referring to Occam's razor, which states in essence, "All things being equal, the simplest explanation is best." Einstein put it somewhat differently: "A physical theory should be as simple as possible, but no simpler." The last phrase is important because, as Schwinger said, "nature does not always select what we, in our ignorance, would judge to be the most symmetrical and harmonious possibility". If the theory were as simple as possible, there would be just one field (or perhaps none!), and the world would be very uninteresting-not to mention uninhabitable. I think it can be said that the equations of QFT are indeed about as simple as possible, but no simpler.”