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… That is what the alchemists called the fifth essence, the quintessentia or the philosopher’s stone. It means a consolidated nucleus of the personality
Von Franz identifies the quintessence psychologically as the philosopher’s stone — a consolidated core of personality that transcends identification with any of the four functions, representing the goal of individuation.
, Lectures on Jung’s Typology, 2013thesis