In the mythological writings of William Blake, Los is the fallen (earthly or human) form of Urthona, one of the four Zoas. He is referred to as the "eternal prophet" and creates the visionary city of Golgonooza. Los is regularly described as a smith, beating with his hammer on a forge, which is metaphorically connected to the beating of the human heart. The bellows of his forge are the human lungs. Los's emanation, Enitharmon, represents spiritual beauty and embodies pity, but at the same time creates the spatial aspect of the fallen world, weaving bodies for men and creating sexual strife through her insistence upon chastity. In the Book of Urizen (1794), Los and Enitharmon have the child, Orc, who is the embodiment of the spirit of revolution. It is of interest to note that the name 'Los' is, by common critical acceptance, an anagram of 'Sol', the Latin word for 'sun'. Such innovations are common in many of Blake's prophetic poems.
Blake attributed the origin of Los and many of his prophetic works to his home in Lambeth: "Los descended to me... trembling I stood... in the Vale of Lambeth; but he kissed me and wished me health".
Los is the divine aspect of the imagination. After he becomes more mechanical and regular in his actions, he falls and becomes part of the material world. In the fallen state, he becomes the creator of life and of organic systems. He also creates reproduction and the sexes, with his own partner Enitharmon soon after being created. He then creates consciousness through evolution, which leads to the creation of humans. Los is particularly focused on humans and he uses them to breed art and use their imaginations. Eventually, it is through the evolutions of the world that Orc is formed. Like Orc and Orc's cycles, Los is part of cycles as he tries to create the Golgonooza at the beginning of time and the image appears constantly in art. Los's imagination is also connected to the natural cycle, and art within the individual is developed through natural cycles. Art is mimetic of nature but order within nature. Los represents the progression through life to the conscious state.
Los appears in The Book of Urizen (1794) as an eternal prophet that binds Urizen after Urizen, the creator of the world suffers from a spiritual fall. He appears in the connected works The Song of Los, America a Prophecy and Europe a Prophecy at the same time. In these works, he begins as a prophet in Africa that describes how Urizen gave laws to the people that bound the minds of mankind. This was accomplished through Los's children with Enitharmon: "Thus the terrible race of Los & Enitharmon gave / Laws & Religions to the sons of Har binding them more / And more to Earth: closing and restraining" (lines 44–46).