Sulphur is one of the tria prima, the three philosophical principles that the physician and alchemist Paracelsus (1493-1541) set at the root of all substances, alongside Sulphur, Mercury and Salt. It stands for the soul, the active and combustible principle; the fiery, expansive quality that gives a thing its character and its power to burn and to change.
The three principles
Paracelsus taught that every material thing is composed of these three, whose balance accounts for its nature: Sulphur the soul, Mercury the spirit and Salt the body. In the scheme of correspondences the principle is linked with the Sun.
These principles are not the ordinary substances that share their names; they are qualities or philosophical elements. They are described here as part of the documented tradition of European alchemy and Paracelsian medicine, in which they became a standard framework for thinking about matter and transformation.
Sources The Secrets of AlchemyLawrence M. Principe with Paracelsus: An Introduction to Philosophical Medicine in the Era of the RenaissanceWalter Pagel