Jung's split with Freud in 1913 was costly. He was on his own again, an experience that reminded him of his lonely childhood. He suffered a breakdown that lasted through the years of the first world war. It was a traumatic experience. But it was not simply a collapse. It turned out to be a highly inventive period, one of discovery. He would later say that all his future work originated with this "creative illness".

He experienced a succession of episodes during which he vividly encountered the rich and disturbing fantasies of his unconscious. He made a record of what he saw when he descended into this underworld, a record published in 2009 as The Red Book. It is like an illuminated manuscript, a cross between Carroll's Alice in Wonderland and Blake's The Marriage of Heaven and Hell.

Its publication sparked massive interest in Jungian circles, rather like what happens in Christian circles when a new first-century codex is discovered. It is of undoubted interest to scholars, in the same way that the notebooks of Leonardo are to art historians. And it is an astonishing work to browse, for its intricacy and imagination. But it is also highly personal, which is presumably why Jung decided against its publication in his own lifetime. So, to turn it into a sacred text, as some appear inclined to do, would be a folly of the kind Jung argued against in the work that followed his recovery from the breakdown.

In particular he wrote two pieces, known as the Two Essays, that provide a succinct introduction to his mature work. (He can otherwise be a rambling, elusive writer.) On the Psychology of the Unconscious completes his separation from Freud. He shows how tracing the origins of a personal crisis back to a childhood trauma, as Freud was inclined to do, might well miss the significance of the crisis for the adult patient now.

In The Relations between the Ego and the Unconscious, he describes a process whereby a person can pay attention to how their unconscious life manifests itself in their conscious life. It will be a highly personal and tortuous experience. "There is no birth of consciousness without pain," he wrote. But with it, the individual can become more whole.

By way of illustration, Jung considers the example of a man whose public image is one of honour and service but who, in the privacy of his home, is prone to moods – so much so that he scares his wife and children. He is leading a double life as public benefactor and domestic tyrant. Jung argues that such a man has identified with his public image and neglected his unconscious life – though it won't be ignored and so comes out, with possibly explosive force, in his relations with his family. The way forward is to pay attention to this inner personality, literally by holding a conversation with himself. He should overcome any embarrassment in doing so and allow each part of himself to talk to the other so that both "partners" can be fully heard.

A non-judgmental attitude is critical. If one side judges the other, then the other side actually gains power because it feels wronged, and so justified in its complaints. This is where therapy can help. "The course of therapy is thus rather like a running conversation with the unconscious", Jung writes. And when properly heard, the tensions between the inner and outer personality should subside. The result will be a more honest saint who is a lot easier to live with. Moreover, he will find that he has more energy for life because he will be less at war with himself and those around him.

The Red Book, then, can be interpreted as Jung's conversation with his unconscious. The devotee of Jung who reads it as if it were a conversation with their own unconscious diverges from the particular path towards individuation that they themselves must forge.

Needless to say, a discussion with the unconscious is not straightforward. If conscious life is not wholly rational, driven as much by emotions and intuitions, then the patterns and instincts of the unconscious are even more buried and obscure. Worse, Jung argues that the modern world has developed a positive fear of the unconscious because it escapes the precise determination, analysis and control promised by modern science. The natural language of the unconscious is not exact like mathematics; it is flexible like mythology.

It is at this point that the links between Jungian psychology and religion emerge particularly clearly, because if symbolism and mythology are the natural languages of the unconscious, they are the natural languages of spiritual traditions too.

Jung found continual inspiration for his psychology in spiritual writings. The Talmud says that "The dream is its own interpretation." Jung agreed. Heraclitus, influenced by Eastern philosophies, wrote, "Out of discord comes the fairest harmony". Jung adopted this principle of enantiodromia as his own.

Further, Jung understood spiritual traditions to be a kind of psychotherapy avant la lettre. (Or to put it the other way round, he thought that psychotherapy emerged in the 19th century precisely because religious systems had begun to fail.) Human beings cannot stand meaninglessness in life, he argued. The decisive question we pursue is whether we are "related to the infinite or not?" Religious traditions provide frameworks within which this question can be approached, not primarily in an empirical or rational sense, but rather in an experiential and practical one.

For the Christian, the symbol of Christ represents complete humanity. The Buddha holds the same hope for the Buddhist. "The Christ-symbol is of the greatest importance for psychology in so far as it is perhaps the most highly developed and differentiated symbol of the self, apart from the figure of the Buddha," Jung averred.

Further still, he argued that Christ and the Buddha had both experienced their own confrontations with the unconscious, respectively in the stories of the temptations of Jesus and the Mara episode in the Buddha's legend. They are experiences that individuals shouldn't seek to imitate, but might expect and follow.