This is the earliest lecture by Paul Dirac available in the Mediatheque. He also participated in the first physics meeting 1953, with a lecture entitled “Quantenmechanik und der Äther”, however, there is no tape recording of that lecture. Instead, it can be found in the journal Naturwissenschaftlische Rundschau, volume 6, page 441, 1953. Together with the eight more tape-recorded Dirac lectures, which will eventually be published in the Mediatheque, this gives a unique picture of Dirac’s varying interests over a period of 30 years. Among the participants of the 1956 meeting were Born, Heisenberg and Yukawa. The latter two gave their lectures the day before Dirac and he makes reference to all three. Heisenberg appears over and over again in Dirac’s lecture, because Heisenberg had lectured on a unified theory of elementary particles, a theory that would solve many problems for many particles. Dirac’s line of attack was different, he advocated taking one problem for one particle at the time. So his lecture is quite different from Heisenberg’s, but both are extremely interesting. Speaking slowly in a very clear English, Dirac critizizes the then current technique of first making a theory of the naked electron and then adding its dress, the electromagnetic field, as a perturbation. Instead he wants to construct the theory directly for a physical electron and goes through a number of steps in this direction. He then raises the old problem of the vacuum, the negative energy states and the fluctuations and reports some partial findings. He also refers to his 1953 lecture and speculates that by introducing an ether, the theory might be able to avoid producing naked electrons and positrons by pair creation. It seems that the tape recording was made with a stationary microphone, since when Dirac is using the blackboard, there are difficulties hearing his voice. But one can definitely hear his footsteps as he walks to the blackboard and back!