Pauli grew up under the influence of Ernst Mach, but – like Einstein – he turned away from the radical positivism of most of his contemporaries quite early. Even though he was a rigorous and systematic thinker, he always devoted much attention to paradoxes and to the mystical background of science. Pauli tried to reconcile this attitude with both modern physics and Jung's archetypal psychology. While his publications present the results of more or less longsome searches for insight, his methodical flow of work and the gradual emergence of understanding become visible only in his rich correspondence.