The First Book of Urizen was evidently intended to form a part of that "Bible of Hell" promised to the world "whether they will or no," in The Marriage of Heaven and Hell. Blake's account of Urizen's creation of the Newtonian solar system takes up the theme of the Bible of Hell, and describes the building of a heaven by the Messiah of Reason with "what he stole from the Abyss." Saurat suggests that Blake was of Urizen's party without knowing it; but this is to underrate the thoroughness of his thought. Urizen creates the "Mundane Shell" by the Swedenborgian process of "Petrifying all the Human Imagination into rock & solid." The "Vortexes" over which Urizen travels in his arduous exploration of his world come from another system of scientific speculation, all but forgotten. Descartes describes the nature of the vortexes: each is "a kind of Firmament, at whose Centre will be an Astre or fix'd Star.".