Jung went to Basel University in 1895 to study medicine, and student life, along with the early death of his father, proved to be emancipatory. His commitment to knowing the nature of the psyche through direct, personal experience and revelation resulted in the precedence he gave to dreams and visions and the idea of understanding them through investigations of philosophy, religion and literature. Jung subscribed to the idea that Heraclitus called enantiodromia (literally, 'running counter to'), namely that everything has an intrinsic tendency to turn into its opposite (this idea is also expressed in Taoist philosophy). He saw this, for example, in his observation that people show a tendency to try to compensate in their own lives for the failings of their parents. A dream about keeping a tiny light alive while being pursued by a terrifying, dark figure through dense fog and a ravaging wind brought a crucial realization - 'When I awoke, I realized at once that the figure was a "spectre of the Brocken", my own shadow on the swirling mists, brought into being by the little light I was carrying. I knew, too, that this little light was my consciousness, the only light I have. My own understanding is the sole treasure I possess, and the greatest.'

Jung was awarded his medical degree with distinction in 1900, and became assistant to Eugen Bleuler (the eminent Swiss psychiatrist who coined the term 'schizophrenia') in the Burghölzli Psychiatric Hospital in Zürich from 1900-09. His doctoral dissertation was entitled 'On the Psychology and Pathology of So-Called Occult Phenomena' and was presented at Basel University in 1902. This was based on his observations collected over the two previous years of the séances of medium Hélène Preiswerk (who was also his cousin). This study sowed the seeds of ideas that became pivotal in the practice of Jung's system of analytical psychology, namely that the unconscious contains 'complexes' or part-personalities that can manifest in dreams and visions, and that personality development and integration (individuation) occurs at the unconscious level. Jung married Emma Rauschenbach (1882-1955) in 1903, and in 1908 they moved from their flat in the Burghölzli to a house by the lake at Küsnacht. They had five children between 1904 and 1914.

Renown came first to Jung from his research on word association, in which a person's responses to stimulus words can reveal complexes: groups of related, often repressed, ideas and impulses that bring about habitual patterns of thought or behavior. After sending Sigmund Freud a copy of his book Studies in Word-Association on its publication in 1906, Jung traveled to Vienna to meet Freud in March of the following year. He became Freud's main collaborator and most probable successor as leader of the psychoanalytic movement, but his own researches led him away from Freud's emphasis on the psychosexual origins of neuroses, founding his own analytic psychology in response to Freud's psychoanalysis. This differed from the Freudian model in downgrading the importance of sexuality and childhood conflicts in the treatment of neuroses, and concentrating more on a patient's current conflicts.

After his break with Freud, Jung suffered a crisis that came close to madness, lasting from a recurring vision of Northern Europe sinking in a sea of blood in 1913 through to a dream of a luminous magnolia tree after the Armistice in 1918. During this dark time he heard voices in his head and conversed with imaginary figures, but Jung used his breakdown as an experiment in which, as he wrote, he could experience first-hand 'the fund of unconscious images which fatally confuse the mental patient, but which is also the matrix of a mythopoeic imagination which has vanished from our rational age.' Looking back, Jung considered these years to be the most important of all. At the end of his life, he wrote of them, 'It all began then; the later details are only supplements and clarifications of the material that burst forth from the unconscious and at first swamped me. It was the prima materia for a lifetime's work.'

Emerging from his creative illness, Jung went back to studying myth, philosophy and religion to seek parallels of his experiences. He published Psychological Types in which the concepts of introverted and extroverted personality types are introduced - habitual outlooks which determine a person's experience of life. He refined these ideas according to four functions of the mind: thinking, feeling, sensation and intuition, and considered that, in each person, one or more of these functions predominate, and that the others require development through application if that person is to become whole. Jung put it like this: 'For complete orientation all four functions should contribute equally.'

'My life is the story of the self-realization of the unconscious,' wrote Jung on the very first line of his autobiography, and this process that he called 'individuation' - the idea of continual, lifelong personal development - was an important part of his approach to psychology and to life. Few contemporary psychologists shared his view that psychological development, the growth towards the realization of an individual's true potential, continued throughout the whole of life rather than being limited to childhood. Such self-realization could occur, he argued, by treating the unconscious as a living, daemonic presence: by confronting and examining what the unconscious has to say, a person can come to know themselves more truly and personal transformation can occur.

Richard Wilhelm sent Jung The Secret of the Golden Flower in 1927. This was Wilhelm's translation of a Taoist alchemical discourse to which Jung provided a foreword and afterword. More importantly, it was this work that led Jung to the discovery that alchemy, symbolically interpreted, was 'the historical counterpart of my psychology of the unconscious'. He also read the works of Jacob Boehme, observing in Psychology and Alchemy that 'Boehme's mysticism is influenced by alchemy in the highest degree' and 'Paracelsus and Boehme between them split alchemy into natural science and Protestant mysticism.' Jung found direct parallels between the stages of analysis and the stages of alchemical transformation.

Jung made the significant step of defining the unconscious of a person as comprised of both a personal unconscious (proceeding from the experiences of the individual) and a collective unconscious (issuing from the inherited structure of the brain, and common to humanity). This is important to esoteric study in that it goes some way towards explaining the power of archetypal, symbolic systems like the Tarot. Indeed, the concept of archetypes - potent universal symbols appearing in myths, fairytales and dreams - is an important part of Jung's concept of the unconscious. He considered the complexes existing in the personal unconscious to be personifications or manifestations of archetypes from the collective unconscious. Jung made it clear that an archetype is not 'an inherited idea', but rather 'an inherited mode of functioning, corresponding to the inborn way in which the chick emerges from the egg, the bird builds its nest… In other words, it is a "pattern of behavior".' Thus Jung's collective unconscious does not, as it may appear at first, require belief in the discredited theory of 'inheritance of acquired characteristics', first propounded by Jean-Baptiste Lamarck (1744-1829).

Jung proposed an underlying, unitary reality that gives rise to the archetypes, and to this he applied the ancient term unus mundus ('unitary world'). This is, of course, an idea found in much mystical and religious thought. He also turned his thoughts to parapsychology, and developed a theory of 'meaningful coincidence' which he called synchronicity. He described this as 'a coincidence in time of two or more causally unrelated events which have the same or similar meaning.'