The military campaigns of the Macedonian king Alexander the Great creating an empire that stretched from Italy and Libya in the West to Northern India in the East. After his death in 323 the cultural and religious melting pot remained, and became what we know as the Hellenistic Civilisation. This civilisation survived and flourished throughout the Roman era until the coming to power of political Christianity with the conversion of the emperor Constantine in the early fourth century.
This cosmology tied in the person's pre-natal and post-mortem existence to the cosmos as a whole; to a process of descent or involution of the soul, the pure Divine essence, and its subsequent ascent. The "spheres" the soul passes through are both physical and non-physical, and indeed, pre-scientific man did not distinguish between the two. For on the one hand, the celestial spheres are each identified with the physical planets, but on the other hand they also represent psycho-spiritual zones; "planes" of existence to use the modern Theosophical-occult term. Each sphere is ruled by a god, and these gods exert their astrological influence upon the terrestrial world; this being what we call "fate". The descent through the spheres into the body is a kind of fall, which traps the soul here; the subsequent ascent a purifying process; a return to the true spiritual realm.
Central to this conception was the triad of Body, Soul (psuche), and Divine Mind (nous); with sometimes a further element, vital spirit (pnuema), added as an intermediary between the physical body and the soul. These various individual principles have their cosmic correlates. As Walter Willi explains:
"Man consists of spirit, soul, and body. The earth gave him his body, the moon his soul, the sun his spirit (or Nous). When a man dies, that is his first death...(which) separates soul and Nous from the body....Every soul, after leaving the body, must wonder for a time between earth and moon and finally, after vicissitudes (depending on its spirit nature and its life on earth) reaches the moon...Here...the souls finally die their second death; the Nous, yearning for the sun, separates from the soul; sovereign and free from all passions, it seeks the sun and unites with the primal spirit. The soul, parted from the spirit, remains on the moon and ultimately dissolves into the moon."
Here we see, alongside the Platonic separation of soul and body, and rational (here, "spirit") and irrational ("soul") psychic principles, the distinct Chaldean (Babylonian) element of the return of the soul to the sun, from which it is said to have arisen in the first place. [Franz Cumont, Oriental Religions in Roman Paganism, (Dover Publications, New York, 1956 [original edition 1911]) p.133-4]
So, according to this Chaldean-Platonic cosmology, the various levels of Self originate from the corresponding Cosmic levels ("The Earth gave man his body, the Moon his soul, the Sun his Nous"), and it is to them that they successively return. We have a sort of celestial expansion of human nature. Man is not an isolated little being, but connected in subtle ways to the entire cosmos. This was in fact a standard belief-system during the Hellenistic period of the ancient mediterranean world (4th century B.C.E. to 4th century C.E.).
Another interesting thing about this cosmology is the concept of the "second death". After the normal physical death, there is followed sometime later a second or psychic death, and the consequent release of the Nous.
Sun/Spiritual world <---- Nous -----|
| Moon/Psychic World <----Soul -|
| | [Second Death]
| | |
| | Soul + Nous
| | Earth/Physical <- Body -|
| | | [Death]
| | | |
\-------\=========\ ====> Body + Soul + Nous
The Cycle of Existence: Self and Cosmos according to Xenocrates and Plutarch
Now, the idea that the psyche is mortal and can die, and indeed does die in the normal course of things, is one which will doubtless be rather disturbing for the dualist, who accepts only the simple dichotomy of body and soul. But the esoteric-occult perspective replaces the simple two-fold metaphysic with a multi-fold one. So just as the Psyche or personality is the "soul" of the body, so there would be a further "soul" within the personality, the Nous.