The break with Jung 

Carl Gustav Jung was a Swiss psychiatrist at the Burghölzli hospital in Zurich, already respected for experimental work on word association, when he began corresponding with Freud in 1906. They met in Vienna in 1907 and reportedly talked for hours on end. For Freud the attraction was practical as well as intellectual: Jung was young, energetic, hospital-based, Protestant and Swiss, which mattered to a man who feared psychoanalysis would be dismissed as a Viennese Jewish speciality. Freud made him the first president of the International Psychoanalytical Association in 1910 and spoke of him privately as his successor.

The disagreements were substantive, not merely temperamental. Freud held that the fundamental drive behind neurosis was sexual, and treated that claim as the non-negotiable core of the theory. Jung came to see libido as a more general psychic energy, took religion and myth to be expressions of something valuable rather than illusions to be dissolved, and proposed a collective unconscious containing inherited archetypes shared across cultures. In his 1912 book on the psychology of the unconscious he set out these positions in print, and in Freud's reading that placed him outside the movement.

The personal texture made it worse. Both men analysed the other's letters. Freud fainted twice in Jung's presence, in 1909 and again at a meeting in Munich in 1912, and interpreted the episodes in terms of his own conflicts. Jung came to find Freud's authority stifling and said so. Their correspondence, which survives in full and is one of the great documents in the history of psychology, drifts from warmth to pointed courtesy to accusation. In late 1912 and 1913 the letters end. Jung resigned as editor of the movement's journal, then as president in 1914, and left the association.

Both men produced significant work afterwards. Jung built analytical psychology, with introversion and extraversion, individuation and psychological types, and became a major influence on religious studies, literature and later on personality testing, some of it thin. Freud tightened the boundary of orthodoxy, and in 1914 wrote a combative history of the movement that read the dissenters out of it.

The cost was to the field. A discipline that expelled its strongest internal critics lost the mechanism by which sciences correct themselves. Adler had been dismissed rather than tested, Jung likewise, and the pattern told everyone who followed that fundamental questions were closed.

Meanwhile the therapy itself was spreading through the middle classes of Europe and America, treating patients on the basis of a theory whose central claims nobody had yet tried to check outside the consulting room.

Sooner or later somebody was going to ask for the evidence.