Paracelsus was one of the first medical professors to recognize that Physicians required a solid academic knowledge in the natural sciences, especially chemistry. Paracelsus pioneered the use of chemicals and minerals in Medicine. From his study of the elements, Paracelsus adopted the idea of tripartite alternatives to explain the nature of Medicine, taking the place of a combustible element (sulphur), a fluid and changeable element (mercury), and a solid, permanent element (salt). The first mention of the mercury-sulphur-salt model was in the Opus paramirum dating to about 1530. Paracelsus believed that the principles sulphur, mercury, and salt contained the poisons contributing to all diseases. He saw each disease as having three separate cures depending on how it was afflicted, either being caused by the poisoning of sulphur, mercury, or salt. Paracelsus drew the importance of sulphur, salt, and mercury from medieval alchemy, where they all occupied a prominent place. He demonstrated his theory by burning a piece of wood. The fire was the work of sulphur, the smoke was mercury, and the residual ash was salt. Paracelsus also believed that mercury, sulphur, and salt provided a good explanation for the nature of Medicine because each of these properties existed in many physical forms. The tria prima also defined the human identity. Salt represented the body; mercury represented the spirit (imagination, moral judgment, and the higher mental faculties); sulfur represented the soul (the emotions and desires). By understanding the chemical nature of the tria prima, a physician could discover the means of curing disease. With every disease, the symptoms depended on which of the three principals caused the ailment. Paracelsus theorized that materials which are poisonous in large doses may be curative in small doses; he demonstrated this with the examples of magnetism and static electricity, wherein a small magnet can attract much larger metals.