Emptiness
The only thing required to engage in the alchemical work is a "free and empty soul." (Psychology and Alchemy, par. 381.) According to Emma Jung, "emptiness is the condition for the Self to emerge." (The Grail Legend, p. 133.)
In a philosophical sense, emptiness is a prerequisite for being. Jung argues (Dream Symbols of the Individuation Process, p. 324) that nature fears emptiness; as soon as emptiness arises, it is filled with something, or something tries to fill it.