"Alchemical theory was essentially static throughout the medieval period. ...Paracelsus was the herald of a new era, an era of . His contribution to alchemical theory lay in the addition to sulphur and mercury of a third principle, which he called '.' Materially this was recognised as the principle of uninflammability and fixidity. ...[T]he tria prima, or three 'hypostatical principles' could be interpreted in either a material or a spiritual sense. In the words of Paracelsus himself: 'Know, then, that all the seven metals are born from a threefold matter... Mercury is the spirit, Sulphur is the soul, and Salt is the body... the soul... unites those two contraries, the body and spirit, and changes them into essence.' ...similar to the material effect of the liquid menstruum, or Hermetic Stream, in uniting sophic sulphur and sophic mercury to produce the Philospher's Stone."