For some alchemists the prima materia is something that can be found everywhere, for others it has first to be produced out of the "imperfect body" or substance. The contradiction resolves itself when one takes account of the amply documented theory of the humidum radicale: all chemical substances contain, in greater or lesser degree, the moisture, the water of the beginning that was brooded over by the spirit of God. This water was the prima ma-teria. The opening words of the first chapter can thus be understood without difficulty: The imperfect body has been changed into the prima materia, and this water, combined with our water (aqua permanens) , produces one 2 [Jung carved the Latin text on a cube of hewn stone at his "Tower" in Bollingen, in 1950. Cf. Memories, Dreams, Reflections, pp. 226L/215L] 3 [Cf. Psychology and Alchemy, par. 246, n. 125.] 4 [Cf. "The Psychology of the Transference," Fig. 1.] 798 ON THE ROSARIUM PHILOSOPHORUM pure, clear water (solvent) that purifies everything and contains within itself everything needful (i.e., for the process of self-transforma-tion). . . . Out of this water and with this water our procedure is brought to completion. But it dissolves the bodies not by means of the common solvent (solutione vulgari) , as transmitted by the ignorant who transform the body into rain-water, but by means of the true philosophical solvent in which the body is transformed into the original water whence it arose in the beginning.
— C.G. Jung
The alchemists kept circling this problem — is the prima materia something you find, or something you make? — because the question refused to resolve at the level where it was being asked. Jung's reading of the humidum radicale cuts beneath the apparent contradiction: every substance already carries the original moisture, the brooded-over water of beginning, but most of what we encounter is that moisture in its impacted, "imperfect" state. The work is not discovery and not manufacture; it is return — not to a prior innocence, but to the condition from which transformation becomes possible at all.
The distinction between solutione vulgari and the "true philosophical solvent" is the passage's sharpest edge. The common solvent dissolves things into something else — a reduction, a loss of form. The philosophical solvent dissolves a body back into what it originally was, which is to say: into the water from which it arose. This is why the alchemists insisted the process could only be done "with our water" — you cannot transform a substance using a solvent foreign to it. The medium of dissolution and the medium of origin have to be the same thing. What carries a body back toward itself cannot be imported from outside; it has to have been there from the beginning, waiting inside the imperfection for the right conjunction.
C.G. Jung·Collected Works Volume 18: The Symbolic Life·1976