the fifth essence, which is not another additional element, but is, so to speak, the essence of all four and yet none of the four; it is the four in one. That is what the alchemists called the fifth essence, the quintessentia or the philosopher’s stone.
Von Franz defines the quinta essentia as the unified transcendence of the four functions/elements, equating it with the philosopher’s stone and with the consolidated, non-identified nucleus of the integrated personality.
, Lectures on Jung’s Typology, 2013thesis