The materia prima should not be understood merely as a primordial condition of the substance but also as an inner experience of the alchemist. The reduction of matter to its original condition of absolute indifferentiation, corresponds, on the plane of inner experience, to the regression to the pre-natal, embryonic state. The theme of rejuvenation and longevity by means of the regresses ad uterum is a leitmotif of Taoism. The most usual method is “embryonic respiration” (t'ai-si). But the alchemist also achieves this state by the smelting of various ingredients in his furnace.
— Mircea Eliade, The Forge and the Crucible: The Origins and Structures of Alchemy