Chapter 6, “A Deeper Layer of Reality,” describes my path to quarks, relating events starting as a graduate student in the spring of 1963 through the summer of 1964 when my work on quarks was essentially completed. A way of judging improbable theories is presented that, when applied to the quark model, pits the a priori likelihood that quarks exist against the difficulty of explaining the experimental data in a theory without quarks.

After a preliminary discussion of symmetry as applied to particle classification, and the constraints it places on the wave functions of particles, a detailed discussion of constituent quarks with spin is presented, based on my 1964 Erice Summer School Lectures. This takes place at two levels, first to capture the main ideas, then, with more detail, to enable the reader to decide if they would have believed that a fundamental theory based on quarks would eventually explain the strong interactions. Selection rules governing the change of strangeness in weak decays, and their relation to the change in charge of the strongly interacting particles, are derived. A graphical calculus based on quarks for calculating hadron couplings is introduced.