Usually alchemy is only known for its belief to transmute lead into gold. But the metamorphosis of different metals into gold can be seen as a metaphor for the soul being freed from its leaden status and, therefore, being enabled to realize its own light, the light of pure spirit, symbolized by the 'philosopher's stone'.
Of course there were many alchemists who really believed in the transmutation of physical lead into gold. But it was their enthusiasm in this task and the projection of all kinds of fantasies, wishes and hopes onto matter that enabled them to discover the unconscious. From the 12th century on the alchemical work, whose roots go back to the ancient Egyptian mysteries, evolved more and more into explorations of the inner universe.
In the 20th century the depth psychologist C. G. Jung became aware of the transformative aspect of alchemy. He realized that many dreams and states of consciousness of his clients resembled the motives described in alchemical texts. And he discovered that there were similar traditions in remote cultures too, for example in Chinese daoism.
There seems to exist an ancient knowledge about human transformation processes, which was used by mystics and alchemists of former times and cultures. Pir Vilayat Inayat Khan, the son of the SOI-founder, was inspired by this treasure of experience and built on it the structure for the alchemical Sufi retreat.