The fundamental thesis Jung is advancing about the relationship between Alchemy and Psychology is that for pre-scientific humans there is not a sharp distinction between subject and object and thus this leads them to unconsciously project their own inner states onto external objects (especially objects that are mostly unknown to them), so a reflective analysis of alchemical symbols becomes revelatory about the unconscious psychic life of this time period. Prior to this rational segregation of experience the world was a totally different one, phenomenologically, as people did not distinguish between the qualities of the object they were perceiving and their own values, emotions, and beliefs. It is partly for this reason that the alchemists cannot say aloud exactly what the philosophers stone really is' and why there are so many different symbols for the work.
For the alchemist trying to understand matter and develop base metals into their purest form, gold, substances are grouped as being alike based on their perceived value. Jung documents as these alchemists collectively come to understand that they themselves must embody the change they hope to effect within their materials: for instance, if they hope to achieve the philosophers stone that can redeem base or vulgar metals, then the alchemist too must become a redeemer figure. It became apparent to the alchemists that they were trying to redeem nature as Christ had redeemed man, hence the identification of the Lapis Philosophorum with Christ the Redeemer. The Opus (work) of alchemy, viewed through this interpretation, becomes a symbolic account of the fundamental process the human psyche undergoes as it re-orients its value system and creates meaning out of chaos. The opus beginning with the nigredo (blackening, akin to depression or nihilistic loss of value) in order to descend back into the manipulable prima materia' and proceeding through a process of spiritual purification that must unite seemingly irreconcilable opposites (the coniunctio) to achieve new levels of consciousness.