As to the soul's immortality then we have said enough, but as to its nature there is this that must be said. What manner of thing it is would be along tale to tell, and most assuredly a god alone could tell it, but what it resembles, that a man might tell in briefer compass. Let this therefore be our manner of discourse. Let it be likened to the union of powers in a team of winged steeds and their winged charioteer. Now all the gods' steeds and all their charioteers are good, and of good stock, but with other beings it is not wholly so. With us men, in the first place, it is a pair of steeds that the charioteer controls; moreover one of them is noble and good, and of good stock, while the other has the opposite character, and his stock is opposite. (246a-b)
(I am using Hackforth's translation, as fairly clear, compared to the old ones I find on the Web. It would be of interest to know how the 15th century Latin version read--I forget by whom, probably a Florentine, but it has been mentioned on THF somewhere; it was a partial translation, but it included this part. I expect that Filelfo would have assigned it as homework to his young Sforza charges. I give the standard pagination so that people can look it up in other languages.)
The PMB chariot, with its white, winged horses, is a chariot of the gods. The Issy and Maseille-style chariots, with their horses either of different colors or pointing in different directions, are the chariots of humans. In a later section Plato describes the humans' horses more fully:
He that is on the more honorable side is upright and clean-limbed;...in color he is white, with black eyes...and needs no whip, being driven by the word of command alone. The other is...with thick short neck, snub nose, black skin, and gray eyes; hot-blooded, consorting with wantonness and vainglory; shaggy of ear, deaf, and hard to control with whip and goad.(253d-e)
From scholarly discussions of the Phaedrus not presently at hand, I understand that it is not exactly clear what the Greek word translated as "black" actually means; it might be "dark" or "dun." (Well, I will never forget the time in Figueres when I was trying to clarify what kind of wine I was ordering, and the waiter pointed to his black shoes while saying the word in Catalan. We got a red wine.) So we have the two horses, one darker than the other. Since the darker one is hot-blooded, it is natural to paint him red. Some designers, perhaps not noticing the section on the colors, or being unclear about it, simply had them the same color.
The noble horse needs only the voice of the charioteer to direct him, no need for a whip; the Charioteer, Reason, commands the noble horse with his voice (perhaps assimilated to the Word of the Gospel of John). And although Plato does have a whip for the other horse, I imagine that the Marseille tarot designer has the noble horse controlling the ignoble one, or trying to, given that they look in the same direction even as their bodies point opposite. Since the horses have lost their wings, they represent the souls of humans who have fallen from heaven to incarnate in earthly bodies.
All that, however, pertains to the Marseille and Issy designs. The PMB is portraying a chariot of the gods or archetypes, as seen from afar by the charioteer of a human chariot still in the heaven. While in the heaven, some humans' chariots rise high enough that the highest part of the soul, reason, can catch glimpses of the gods in their chariots.
Some lucky souls even catch a glimpse of the eternal archetypes themselves.And behold, there in the heaven Zeus, mighty leader, drives his winged team. First of the host of gods and daemons he proceeds, ordering all things and caring therefor, and the host follows after him, marshaled in eleven companies. For Hesita abides alone in the gods' dwelling place, but for the rest, all such are ranked in the number of the twelve as ruler gods lead their several companies, each according to his rank. (246e-247a).
And while she is borne round she discerns justice, its very self, and likewise temperance, and knowledge, not the knowledge that is neighbor to becoming and varies with the various objects to which we commonly ascribe being, but the veritable knowledge of being that veritably is. (247d-e.)
Then the humans' chariots fall, thanks to the ignoble horse and to wounds from inexpert handling, and the threesome incarnate in human bodies. Even then there is a dim memory of the other world, and especially of the sight of Beauty:
Now in the earthly likenesses of justice and temperance and all other prized possessions of the soul there dwells no luster; nay, so dull are the organs wherewith men approach their images that hardly can a few behold that which is imaged, but with beauty it is otherwise. Beauty it was ours to see in all its brightness in those days when, amidst that happy company, we beheld with our eyes that blessed vision, ourselves in the train of Zeus, others following some other god, then were we all initiated into that mystery which is rightly accounted blessed beyond all others....(250b-e)
While Beauty is what attracts the soul, the memory of other gods is also sought, depending upon which of the gods he takes after in his character.
Thus the followers of Zeus seek a beloved who is Zeuslike in soul; wherefore they look for one who by nature disposed to the love of wisdom and the leading of men, and when they have found him and come to love him they do all in their power to foster that disposition. (252e)
(The suggestion of homosexuality is one of the things that provoked controversy--and interest-about this passage. The way around it is to go directly to the appropriate goddess, rather than her earthly carrier.)
Who is the PMB charioteer? She is Beauty, i.e. celestial Venus, but, since Venus was usually painted in a state of undress, also other celestial archetypes, in particular, for a ruler, Sapienta, Wisdom, i.e. Minerva. For it is she who must guide leaders. Specifically, she is "Minerva Pacifica," who has put down her shield and lance and taken up the scepter and globe, symbols of her rulership in the well-governed state, perhaps after a hard-won victory. I don't know of any Milanese examples of her with these attributes, but in Florence there was a medal of her on one side in that way, as the goddess governing the city. The caption doesn't say she is Minerva, but the book (whose name I conveniently forget) does.
There was a Roman statue of Minerva as Dea Roma (scroll almost halfway down at http://www.digital-images.net/Gallery/S ... enery.html). Perhaps Florentia is modeled on that one. In the late 16th century, there was a "Britannia Minerva."
There is also this detail, from Mantua early 16th century, with a globe-like helmet and a broken lance substituting for the scepter; a broken lance also suggests peace. It is Corregio’s “Allegory of Philosophy" (philosophy = love of wisdom) or "Allegory of Virtue”; this time Minerva, being crowned, presumably for chasing out, or at leas sitting on, some Vices. She is above another lady who represents the four cardinal virtues: the lionskin is for courage, the serpent for prudence, the bridle in her left hand for temperance, and I would guess the sword for justice.
As for the amalgamation of Minerva and Venus, there is an excellent article by Rudolf Wittkower, "Transformations of MInerva in Renaissance Imagery," originally in the Journal of the Warburg Institute II, 1938-1939, reprinted in his Allegory and the Migration of Symbols. For Minerva on the card, we have the dress, the globe, and the scepter. For Venus, there remains the face.
Minerva, of course, was famous for her chastity, even her virginity. The celestial Venus, Aphrodite Urania in Plato's Symposium, likewise is without carnal stain, as opposed to Aphrodite Pandemos, the common Venus. Thus we have, for the common people who know only the Petrarchan parades, a symbol of chastity even in as we celebrate the victory of beauty and wisdom. But I am only talking about the PMB in this assignment of a Petrarchan triumph.