Hi Beanu,
Thanks for raising this interesting topic for conversation and exploration.
Of course, Robert O'Neil on his website has made an exhaustive list of NeoPlatonic influences on the Latin Middle Ages and early Renaissance. Part of Plato's Timaeus was available in a Latin translation by Calcidius from the late 4th Century.
In the Timaeus the opposites are harmonized through the workings of the World Soul. The "artificer" (sometimes called the demiurge) accomplishes this correlating of psychic substance with the harmonic and arithmetical means (Stephen Gersh, Concord in Discourse: Harmonic and Seminiotics in Late Classical and early Medieval Platonism). There is a formula for this "harmonizing" that incorporates the musical ratios of the 8 tone scale (folks, correct me on this if I am wrong).
The Artificer (Magician?) "established as the limits six and twelve which is two times six together with the two means eight and nine. Thus, he connected the interval between the limits of six and twelve with the force of ratios 4:3 and 3:2, after the same fashion in which the world's body has been connected through insertion of the elements of air and water between the limits of fire and earth. This was so that intelligible divisions of the soul could be joined by insertion of numerical potencies as if by elements and substances, and that there could be a likeness between soul and body."
It is a controversial topic on the Forum how the number of Trumps evolved-that is, whether by design or by accident. However, as a footnote it is worth mentioning here that 6 added up equals 21, and 12 added up equals 78.( like this 1+2+3+4+5+6=21) Referring here to the portion of the quote describing the limits-- six and twelve.
The card historians on the forum could tell us when the name World Soul is given to the 21st Trump. I can think of two versions of this Trump. One version is the two putti catching a city descending in a kind of ball, almost like catching a baby out of the womb. I have often correlated this with Revelations 21, "And I saw the holy city, new Jerusalem, coming down out of heaven..."
The other version is of the figure dancing within the "vesica Piscis." Here is a web connection of the vesica piscus that I think is relevant to this discussion.
http://en.wikipedia.org/wiki/Vesica_Piscis The vesica piscus is identified as a ratio of 265/153, which is a close approximation of the square root of three. That is, the vesica is a metaphysical concept expressed as a numerical formula. Do these numbers reflect back to something in the Tarot?
Finally, in Calcidius's Commentary on Timaeus he describes exactly how it is that the elements are put together to form the world body, or animus mundi. I am here copying the material from Gersh's book. First there is fire: sharp, subtle, and mobile. Next is air which shares the subtle and mobile part of fire but is blunt. Then comes water which like air is blunt and mobile, but has corpulence. Finally, there is earth, which is blunt, corpulent, and immobile.
Here is what Calcidius wrote in relation to this model: "So this diagram which sketches out the generation or combination of those parts in which soul is held to subsist, reveals the basis of soul's connection to the body. For the body of a living thing which is animated by the force of soul certainly has surface and also solidity. Since soul was to permeate both surface and solidity with vital force, it has to possess powers similar to the solid and also to the surface, given that like collects with like." (Taken from Gersh)
I think the 21st card of the Trumps, World Soul, as it came to be known, might be a clue. Or perhaps Trump 1 which came to be called the Magician with the four elements on the table. Is Force, force? I feel more optimistic about the 21st Trump as World Soul, something that has harmonized like descent of perfection.. The others are wild guesses. My thoughts..