A visual of the "three principles" theory, an expansion on Aristotle's four elements theory, employed, originally by Geber (c.790) in the form of "two principles", to explain the variations of combustion of metals, wherein "sulfur" was the principle of combustibility, the more "sulfur" something contained, the more it would combust; "mercury", which explained the fluidity of metals, and "salt" which was used to explain the solidity of things, not effected by heat.

“All that burns is sulfur.”

— Paracelsus (c.1524), Publication [1]

“Know that all the seven metals are brought forth after this manner, out of a threefold manner, namely: mercury, sulfur, and salt, yet in distinct and peculiar colors. Now this is not to be understood so that of every mercury, every sulfur, or of every salt, the seven metals may be generated. Concerning the generation of minerals, and half metals, nothing else need be known than what at first said concerning metals, namely that they are in like manner produced of the three principles: mercury, sulfur, and salt, and yet with their distinct colors. The generation of gems is from the subtlety of the earth of transparent and crystalline mercury, sulfur and salt, even according to their distinct colors. But the generation of common stones is of the subtlety of water, of mucilaginous mercury, sulfur, and salt.”

— Paracelsus (1537), Of the Nature of Things [4]

“Very few have hitherto showed whence the ‘principles’ arise, and it is a hard thing to judge of any of the principles, or anything else, whose original and generation is unknown. Now the ‘principles of things’, especially of metals, according to the ancient philosophers are two: sulfur and mercury; but according to the latter philosophers, three: salt, sulfur and mercury. Now the original of these principles are the four elements. There are four elements and every one of these four hath in its center another element by which it is elementated: and these are the four statues of the world, separated from the chaos in the creation of the world by divine wisdom; and these uphold the fabric of the world by their contrary acting in equality, and proportion, and also by the inclination of celestial virtues, bring forth all things, that are within, and upon the earth. We will now descend unto the principles of things. But how they are produced of the four elements, take it thus: the fire began to act upon the air, and produced sulfur, the air also began to act upon the water, and brought forth mercury, the water also began to act upon the earth and brought forth salt. But the earth since it had nothing to work upon, brought forth nothing, but that which was brought forth continued and abided in it. Wherefore there became only ‘three principles’, and the earth was made the nurse and mother of the rest. These three things are in all things.”

— Michael Sendivogius (1607), A New Light of Alchymia [3]

“Sulfur is the only principle which makes fire.”

— Nicolas Lemery (1675), Course on Chemistry [2]

“Geber, the great eighth century Arabian chemist, who added much to the world’s knowledge of chemical facts, and who was a prolific writer, e.g. Summa Perfectionis Magisterii, De Investigatione Veritatis, etc., developed the peripatetic conception of the ultimate constitution of matter, so as to account for the observed differences between the various metals and to supply a theoretical basis for the possibility of their transmutation. The four elements of Aristotle are retained as the ultimate constituents, but the substances termed ‘mercury’ and ‘sulfur’ respectively are assumed as more proximate ones; mercury being the vehicle of the ductility, fusibility and luster, and sulfur the bearer of the quality of combustibility. The conception is, the metal exhibits the sum of the properties of its constituents, and that the differences between individual metals are due to the relative quantities of these constituents and to the degree of purity exhibited by them. This mercury and sulfur were not supposed to be identical with the substances bearing these names. According to this view the change of one metal into another should be possible and should consist in the addition or withdrawal of one of the two constituents, or in its purification. It is interesting to note how enormous an advance Geber's theorizing is on that of Aristotle. His hypothesis allows of deductive application in that it explains satisfactorily a whole number of phenomena exhibited by metals, e.g. their different combustibility, by a difference in the relative amount of sulphur contained in them. Other's doctrine was for many centuries retained in its original form and afterwards extended, first by making sulfur and mercury the proximate constituents of all substances and not of metals only, and later by the introduction of a third constituent principle or element, salt, representative of that which is permanent and unaltered by the action of heat. Basil Valentine (c.1430) was the first to definitely teach it, and the doctrine of the ‘three principles’ became for about two centuries dominant in the science. It was accepted in its entirety by Paracelsus (c.1535) and promulgated by him so vigorously as to become associated with his name.”

— Ida Freund (1904), The Study of Chemical Composition