A historical overview is given of the contributions of Hendrik Antoon Lorentz in quantum theory. Although especially his early work is valuable, the main importance of Lorentz’s work lies in the conceptual clarifications he provided and in his critique of the foundations of quantum theory.

The ArgumentA discussion of the circumstances of the friendship between Einstein and Lorentz using published and unpublished sources provides material for a close analysis of their relationship. In this analysis the concept of distance, both in a physical and a psychological sense, plays a central and clarifying role. It is concluded that for Einstein the distance between him and Lorentz was essential for the relationship to become as rich and intense as it was.