Continuing with wikipedia:Thel is the allegory of the unborn spirit who has gathered experience from her own discoveries and has decided to remain forever innocent.In The Book of Thel, the Vales of Har are depicted as an edenic paradise that lived in harmony; a world where the rain feeds the flowers and the clod of clay feeds the infantile worm. The common belief in this world among the characters is that “everything that lives Lives not alone nor for itself.” Thel wishes to enter the world of experience and leave behind her innocent paradise. However, once Thel enters the world of experience, she cowers in terror at the thought of mortality and the uselessness of human beings if every action leads toward the grave. This can also be interpreted as Thel’s fear of losing innocence and virginity upon entering the world of adult sexuality. In other words, Thel’s fear of growing up is what keeps her from actually living. When she flees from the experienced world because it appears as her tombstone, she unwittingly flees life itself. William Blake has put a microscope on the conflict between innocence and experience and he has found that innocence must take on a more elevated meaning, one found through suffering, that Thel can never reach so long as she is gripped by her fear of opening herself up to risk. The idea that Thel’s future life was one of despair and death can be read as another example of Thel’s skewed perspective. Thel is surprised by her brilliance and says that the world of experience looks like a “chamber of horrors.” It has also been suggested that the Worm has a part in the conflict between innocence and experience. The Worm is speaking as a messenger for the world of experience, and his words are inaudible to Thel because the Worm is not a part of her realm. The Worm speaks of phallic sexuality and the guaranteed death of mortality. This creates a mediator when she gives the voice to the Clod of Clay. Now the Clod of Clay acts as an interface between innocence and experience.
"The Book of Thel is an allegory of the unborn spirit visiting the world of generation. Thel rejects the self-sacrificing aspects of experience and flies back to eternity. The symbols of Lily-of-the-Valley, the Cloud, the Worm and the Clod of Clay represent idealistic fancy, youth, adolescence and motherhood." —Geoffrey Keynes