Because we have been held in a spiritualized and Christianized alchemy, the rubedo has remained enigmatic, usually explained in terms of Christ. The reddening has been conceptualized in higher abstractions like the conjunction of opposites and the unus mundus. The brilliant blood-red vitality of the solidification has been imprisoned in a lunar albedo vision. By that I mean the unus mundus has been apprehended within the unio mentalis.[36] Therefore the yellowing, so crucial for the transition from white to red, silver to gold. The yellowing undoes the white, spoils it, decomposes it out of itself so that the psychologized mind cannot serve as vehicle for entry to the world. The rubedo can no longer be conceived to be a psychological activity only because it is as well a libidinal one, a movement in what Jung called the infrared end of the archetypal spectrum where he said "instinct" prevails.[37] "Instinct": another name for the first Adam who, legend says, had a tail. The unus mundus is therefore head-cum-tail as the Ouroboros depicts, mind as nature, image as instinct. The goal of the unus mundus occurs not necessarily at the end of the opus but whenever a mental image compels with the force of a natural phenomenon and whenever nature appears wholly as imagination.
— James Hillman
Hillman is pointing at something the albedo refuses to finish. The whitening — purification, clarity, the psychologized mind arriving at its own coherence — is not completion. It is a plateau the opus has to be spoiled out of. The yellowing is precisely that spoiling: not a failure of the white but its necessary decomposition, the citrinitas that most alchemical commentators rush past because it looks like corruption rather than transition.
What stops the rubedo from arriving is the assumption that psychological work ends in psychological terms. The unus mundus gets apprehended inside the mind that purified itself — which means the world-union becomes another form of interior achievement, another refinement of the unio mentalis. Spirit has colonized the red. The blood-tone, the libidinal heat, the first Adam's tail — these get translated upward into conjunction-language before they can do their actual work, which is not metaphorical but bodily and binding.
The move Hillman insists on is ontological, not methodological. The image that compels with the force of a natural phenomenon is not an image you are having — it is an image that is happening to you, and the nature that appears wholly as imagination is not being perceived by a mind that remains separate from it. The rubedo is when that separation closes. Not transcendence — contact.
James Hillman·Alchemical Psychology·2010