In June 1968, the International Centre for Theoretical Physics in Trieste was privileged to hold an extended symposium on contemporary physics — an event made somewhat unique by its aim to range over and review not just one specialised aspect of modern physics but its entire spectrum.

The most memorable lectures of the symposium were the ones given in an evening series entitled “From a Life of Physics” by some of those to whom we owe the creation of modern physics as we know it. There were six lectures in all: the first five delivered by P. A. M. Dirac, W. Heisenberg, H. A. Bethe, E. Wigner and O. Klein on their own lives of physics; the sixth delivered by E. Lifshitz, commemorating L. Landau's life (who died that year).

The written record of the lectures was published by the International Atomic Energy Agency (IAEA) as a special supplement to the IAEA Bulletin and has not been accessible generally. Since two of the most exciting lectures were those delivered by P. A. M. Dirac and Werner Heisenberg (presided over by Dirac himself) and since both these lectures concern their ideas about the methodology in theoretical physics, it is appropriate that these should also be published in this volume.