During long periods of "self-experimentation" he met heroes and goddesses, wise men and serpents, battled his demons and re-discovered his faith in God.
"My entire life consisted in elaborating what had burst forth from the unconscious," Jung said of this experience later in life.
He documented his imaginary journey, illustrating the most powerful revelations by hand, in a leather bound tome he named the Liber Novus, or new book, but which came to be known as the Red Book.
But, wary that the mystical tale of self-exploration would damage his reputation as a serious psychologist, Jung kept the book under lock and key until his death in 1961.
Now, nearly 90 years after he started on his journey, the Jung family has agreed to bring the book out of its bank vault exile, giving a long-awaited insight into the great psychologist's own mind.
"This is the only authentic, first-hand account we have of what went on in the most crucial period of his life," says Jung historian Sonu Shamdasani, who edited the Red Book.
"He navigated through a mid-life crisis and, in so doing, created a form in which others have managed to do the same."
On the edge
The publication of the Red Book has led to speculation about Jung's sanity at the time of his existential crisis.
Are these the hallucinations of a man on the verge of psychosis? Was he overpowered by his own imagination?
The visions should not been seen as hallucinations, but as "waking fantasies" says Mr Shamdasani. Jung plumbed the depths of his unconscious in an intentional, controlled state - much as you might allow yourself to daydream.
Jung just went one step further and had a conversation with the people he met there.
Archetype - An inherited pattern of thought, sometimes personified as mythical individuals
Persona - The psychological mechanism that conceals a person's true feelings
Collective unconsciousness - A part of the unconscious mind that is shared by a group of people
Introvert/extrovert - Personality types that indicate a person's relationship to the world
Synchronicity - Coincidental events that seem to be meaningfully related
These characters became the archetypes of his later psychological theories, which Jung argued were a manifestation of a common human inheritance he termed the "collective unconscious".
They form the backbone of the school he founded known as analytic psychology, which is practiced around the world to this day.
Despite having always made it back to reality, Jung continually stressed the danger of his encounters, says Professor Robert Segal, editor of Jung on Mythology.
"Madness is exactly what one risked - getting so far into the unconscious so as not to be able to get back," he says.
Man of science
It is easy to see how Jung might have worried that revealing his mythical voyage of discovery would dent his reputation as a serious psychologist.
But, even without the Red Book, there are few people who would consider Jung a scientist in the normal sense.
Jung wrote the Red Book between 1914 and 1930
"Nobody really thinks that Jungian psychology is based on what people today would think of as science," argues Professor Segal.
"You can't study it in a lab, you can't subject it to anything systematic."
Moreover, it was precisely the subjective, anti-scientific, mystical aspect of Jung that attracted many of those who embraced his theories during his lifetime, says Segal. So if he was trying to keep it under wraps, "it was a lost venture".
Nevertheless, for Jungian analyst Andrew Samuels, a professor of Jungian psychology at Essex University, the theories that were inspired by these experiences "have stood the test of time".
"All great psychologists make their theories out of their own experiences. Their ideas come from their lives," he says.
"The justification is that, in psychology, the subject and the object are the same thing.
"You're looking systematically, deeply - you could say scientifically, if you wanted - at yourself."
The popularity of the Red Book, which entered Amazon's top sellers list at number four when it was released in the US, may come as a surprise to those who are not familiar with the Jung's theories.
But, though some of the concepts of Jungian psychology, such as extrovert and introvert and the idea of archetypical personality types, have survived in mainstream culture, the legacy of his ideas is still in the balance.
"There are plenty of people who think Freud was a great thinker who are not Freudians," says Professor Segal. "There are very few people who think Jung was a great thinker who are not Jungians."
But for those who follow his ideas, to overlook his insights is to ignore an important voice countering the modern, rational age.
"The thing he discovered is that there is an internal world, just as large as the external world," says Professor Samuels.
"To cut off from this is to cut off from one half of ourselves."
Do you think there is still a place for psychological theories based on introspection? Do you think Jung should be considered a great thinker? Let us know using the form below.
Our obsession with the human animal has greatly distracted from our experience of the human person. My son who has a mental illness is certainly helped by medication, but the light in his life has come from his courage to explore his own person and his place in the human community and the universe (and, beyond). His interior life is a choice he has made. St Theresa of Avila called it "the interior castle". Jamie Ballenger, Charlottesville Virginia USA
There will be a need for psychological theories as long as man exist. All brains are not equal. As soon as man through science can adjust human brains to an average logical behaviour then the need for psychology will decrease but never eliminated. I don't believe that Mr. Jung was a great thinker that will stand the test of time. As long as man keeps banking on an after life spiritual existence then his logic is no better than a worm. Basil Aftousmis, US
I discovered Carl Jung at 19 but really got into his work 20 years later. For me his 'Individuation' theory can be applied so easily to human beings and his focus on mythical connections is just fascinating. At university no-one seemed to know about archetypes and I had to study it all myself. I then applied it to virtually everything in my studies and in my life. I think was a brilliant thinker. Jan Christopher, London
Thought he was minimal then, think he still is, been a behaviorist since the begining and while I follow "some" of the works of Freud, Ericson still leads me where I need to go. Dave Briscoe, Clarksville, IN, US
Peering into our levels of consciousness/innermost selves will open revelations of knowledge both scientifically and spiritually which could help us, the human race, survive and transcend our present wonderful but self destructive animal selves. Think Jung! Kinsey, Edith Ann, Jacksonville, FL USA
It is hard to believe how anybody could not understand that "objectivity" is a sole product of subjectivity. They will understand at the point of death though - once all the outer senses are shut, they will also see that a "millisecond" can as well feel like infinity, and that all the "objective" constructs that talk about whatever will be entirely meaningless if not non-existent. Jung was a clever guy. Werner, London
Jung's remarks seem somehow related to some of the eastern streams of knowledge regarding self-knowledge. I wonder if it is coincidence that different people in different ages have jumped to similar conclusions. His point of view will definitively sounds plausible to some people. In fact, the lack of that inner knowledge of ourselves is a sensible explanation of many mistakes the world continually makes, including the environmental disasters we are already experiencing. Our perspective of the world tends to be quite rational, leaving behind that part of the heart that has definitively been with us since thousands of years ago. Jose Luis Araya, Costa Rica
Freud and Jung tell us a great deal about how Freud and Jung thought. And that's all. Two self-absorbed idiots who excelled at PR. Thomas Worthington, Bangor, NI
I guess Jung is a "great thinker" in so far as the uniqueness of his ideas and the sheer breadth of them are concerned. However the practical utility of his work in actually helping distressed people is another matter. C. Manson, London
C.G. Jung is the greatest modern day thinker that has emerged from a scientific route. Of course there is Sigmund Freud and others, but Jung's discoveries had the depth and courage that were lacking in such scientific circles. Not only was he way out in front of the others, he was also forging the pathway for others to follow, he was a pioneer. Philip Price, Lancaster
Seems to me he was an artist first and foremost. Bravo, London
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