This paper has traced Bion’s discovery of alpha function and its subsequent elaboration. His traumatic experiences as a young tank commander in World War I (overlaid on, and intertwined with, childhood conflicts) gave him firsthand exposure to very painful emotions that tested his capacity to manage. Later, in the 1950s, after his analysis with Melanie Klein and marriage to Francesca Bion, he undertook the analysis of psychotic patients and learned how they disassembled their ability to know reality as a defense against unbearable emotional truths in their lives. This led Bion to identify an aspect of dreaming that was necessary in order for reality experience to be given personal meaning so that one may learn from experience. Simultaneous with working out this new theory of dreaming, Bion also revisited his World War I experiences that had remained undigested and all these elements coalesced into a selected fact – his discovery of alpha function. In subsequent writings, Bion explored the constituent factors of alpha function, including the container/contained relationship, the PS↔D balance, reverie, tolerated doubt and other factors which I have termed the ‘Constellation for Thinking’.