In Book Six of his Mathesis, the fourth century CE Roman astrologer, Julius Firmicus Maternus, gives the following account of the natal chart of Plato, the fifth century BCE Athenian philosopher:
If the ascendant is in Aquarius, and Mars, Mercury, and Venus are in conjunction in that degree; Jupiter is on the descendant in Leo; the Sun is on the anafora of the ascendant in Pisces; the Moon is in the fifth house in Gemini, in trine to the ascendant; and Saturn is in the ninth house in Libra-this chart produces an interpreter of divine and celestial matters. He possesses a combination of learned speech and divine intelligence and is trained by some kind of heavenly power to give true expression to all secrets of divinity. This chart is said to have been that of Plato. (Mathesis VI.XXX.24)
Plotting this description on a chart, we get the following image:
As an historical aside, it should be noted that the placements of Jupiter and Saturn do not coincide in these signs in the ephemera of the Fifth Century BCE. They certainly do not coincide in the agreed dates for Plato's birth (circa 428/427 or 424/423). I have not explored the literature on the dating of Plato's birth in order to ascertain whether or not Maternus has been discussed in this regard. Suffice it to say that while this cannot be taken as a strictly historical chart, it nevertheless serves as an important symbolic image of Plato's character within Hellenistic tradition.
That having been said, what stands out about Plato's chart as given by Maternus is that it is ruled by Saturn (i.e. the planetary ruler of the rising sign, Aquarius). Saturn, the great malefic, is the reality principle par excellence. He brings structure and endurance whether we like it or not. Like gravity, he remains after everything else crumbles. As Philip K. Dick once said, "reality is that which, when you stop believing in it, doesn't go away".
This "perennial" quality of Saturn is deeply emblematic of Plato's role as grand architect of the theory of Forms—those eternal realities that endure beyond the genesis and corruption of the material world, which in and of itself is merely a transient mirror of the eternal structures of reality. It is precisely due to the perseverance of this idea that Plato himself has enjoyed an undeniable endurance in the Western philosophical tradition. As Alfred North Whitehead famously observed: "The safest general characterisation of the European philosophical tradition is that it consists of a series of footnotes to Plato".
Saturn is in Libra in Plato's chart, and according to the traditional system of dignities, he is said to be exalted in this position. Saturn can be an extraordinarily difficult force, but when exalted, his functions are enacted efficiently, even positively. Libra, moreover, is the home of Venus, and here Saturn's rigour works to establish beauty and harmony, two of the most important philosophical concepts in the Platonic canon.
Saturn is not only exalted in Libra, he forms the apex of what is known as a "grand air trine", formed by the signs Aquarius–Gemini–Libra. This is known as an aspect by triplicity, and its angles are significantly placed in the element of air, which is further associated with "intellect". Here it ought to be mentioned that the concept of intellect emphasised within the Platonic tradition was not the hairsplitting, analytical mentality familiar to modern academics, but the divine nous—the primordial, eternal, and ever-present intelligence underpinning the cosmos as a harmonic system. Here, "rationality" takes its integral and efficient meaning, deriving from the term ratio and referring to vivifying harmonic proportion.
It is perhaps no surprise that the apex of Plato's grand triangle hinges in the ninth house, the stead traditionally assigned to philosophy, sacred science, higher learning, and travel. Plato is said to have visited Egypt—the font of the geometricising Pythagorean tradition—where he received priestly instruction in the divine temples. Upon his return, at the age of forty (as his annual profection activated the Mercury-ruled Gemini angle of his grand trine) he placed the original academy at Athens under the rubric: "no one ignorant of geometry shall enter". If anything, this is consistent with the power of harmonic limitation represented by Saturn in Libra, as expressed in the Platonic principle of sophrosyne (wisdom guided by moderation, balance, and self-restraint). It not only accords with the Pythagorean principle, "do not go beyond the balance", but also the Delphic Oracle's most famous maxims: "know thyself" and "nothing in excess".