This is my transcription of a video where Christopher Warnock of Renaissance Astrology talks about the philosophy and spiritual path of Hermeticism.
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Hermetic wisdom consists of the writings attributed to the ancient figure of Hermes Trismegistus. Modern scholarship divides Hermetic wisdom into two main categories: the philosophical Hermetica and the technical Hermetica. Hermeticists themselves, both then and now, would not make such a strong distinction.
Hermes Trismegistus
The focus of the philosophical Hermetica is on the underlying principles of the teachings. The technical Hermetica, in contrast, provides instruction on how to practice the Hermetic arts of astrology, alchemy, and magic. Common to both, however, is the fundamental Hermetic principle of unity: all things proceed from Unity, from the One, and thus maintain a connection to each other and to the One.
Typically, reality was seen by Hermeticists as a series of emanations. The term the One can be misleading; it refers not to a spirit, a being, or a thing, but rather to that which precedes being, perceives existence, contains all things, and underlies reality. It is similar to the Chinese concept of the Dao. The first differentiated thing or being to emanate from the One was a thought of the One. This is often seen as the Logos or the Demiurge—the Creator God, which is the highest level of the Divine that can be conceptualized. Each succeeding emanation becomes more individualized and less perfect, since it contains less of the One.
All things exist undifferentiated in the Logos or Demiurge as ideal forms—the Platonic Ideas. The next level of manifestation is then thoughts in the mind of the Divine, or the Anima Mundi—the Soul of the Cosmos. These thoughts then manifest in the stars and planets in the Imaginal or Celestial World. Finally, all things manifest in material form in the material world. Because all things have come from the Divine realm, from the One, all things remain connected to it and are completely supported in each moment by the One. Without this support, creation would instantly cease. The emanation of all things from the One establishes chains of spiritual sympathy and connection. This is succinctly expressed in the maxim: As Above, So Below, from the Emerald Tablet of Hermes Trismegistus.
Everything is connected not only to the One but also to the particular intermediate ideas and archetypes that gave rise to it. Justice, for example, exists first as an archetype or Platonic Idea in the Divine realm, then in the planet Jupiter, and finally in the material world as acts of justice and just people. A spiritual connection thus exists between all acts of justice, just people, the planet Jupiter, and the Divine. Of course, Jupiter here refers not to the physical planet but to its archetype, its soul, and its intelligence.
Another aspect of spiritual sympathy is the correspondence between macrocosm and microcosm, which holds that the patterns of a greater system are wholly contained within any smaller system. A simple example of this correspondence is the similarity between a nation and a family: each has leaders and members, rules and codes of conduct, and mores.
The most well-known correspondence is that between the macrocosm of the Great World, the cosmos, and the microcosm, or the Little World of each individual. Just as a human being is composed of a Divine Spirit, a material body, and an intermediate Soul—which various traditions conceptualize as the astral body, the Life Energy, chi, or prana—so the cosmos has the Divine realm as its Spirit, all matter in the universe as its body, and the Anima Mundi, the Soul of the World, as the intermediate Soul connecting the Divine to the material world. Hermeticists did not see these sympathies as merely metaphor or poetic conceits but as an accurate model of reality, one that could be put to use in a variety of ways. Astrology, one of the three Hermetic Arts, was employed to this end.
Horary astrology, for example, which was much more commonly practiced during the Renaissance than today, was used to answer specific questions. Typical questions might concern matters such as whether a marriage will take place, whether land will be sold, or whether a debt will be paid. By reading the chart for the "birth" of the question—that is, the moment when the question was posed—the astrologer could determine the answer. Horary astrology could thus yield more concrete and specific information than natal astrology, or birth chart astrology, which is more suitably used to reveal a person's general character and course of life.
There are some minor attempts to explain astrology as the effect of sunspots, magnetism, or other physical phenomena upon a person's body at the time of birth. Such conjectures, however, cannot explain the efficacy of horary astrology, where the "birth chart" of a question, rather than a person, is examined. The Hermetic view—that the cycles in heaven reflect all events on Earth—offers a more cogent and elegant explanation. Whether the event on Earth is the birth of a person or a question, knowledge concerning it will be mirrored in the cycles of the heavens.
We can take the use of spiritual sympathy and connection one step further with astrological magic. Astrological magic is the art of using astrological timing and the creation of talismans, typically in the form of rings, pendants, or other small objects. Although astrological magic can be used to create statues, gardens, and even whole cities as talismans to capture the influence of the celestial realm at a given moment—and so confer it on the wearer—the astrological magician makes active use of the chains of correspondence rather than simply observing their effects on Earth. Traditional astrological sources often focus on practical, worldly uses of astrology and magic. Astrologers and their clients typically sought guidance and help regarding issues such as wealth, length of life, love, marriage, career, enemies, and friends.
How does this relate to spirituality? Inherent in their use of astrology was a worldview with Spirit underlying and unifying all of reality. Magic and astrology could not work without the spiritual connection of all things. Thus, working with traditional astrology and astrological magic—and seeing the results—becomes a profound experience. Of course, the practical results are useful, but even more important is the realization that the Hermetic view of reality is valid: that the cosmos is truly one great unified being. In addition to seeking help and guidance for their daily lives through the Hermetic Arts, Hermeticists used astrology and magic to directly deepen their connection with the spiritual realm.
The basis of the Hermetic spiritual path is Unity. Since all things descend from the One, we can ascend through higher and higher levels of reality—from the material to the celestial, to the Divine Ideas, and ultimately back to the One. Mystic union of the individual soul with the One is the goal of Hermetic spirituality. There never was an organized Hermetic church. Instead, the teachings appear to have been propagated in loose circles consisting of a master and several disciples. From our surviving traditional sources, it seems that while there was a definite intellectual component to the teachings, at the higher levels the master initiated the disciple through various visionary experiences, which included magic and ritual.
We have a description of Hermetic visionary initiation in The Discourse on the Eighth and the Ninth, in which astrology plays a key role. The initiate, having already attained the wisdom of the first seven spheres—that of the planets—is instructed and then initiated into the mysteries of the eighth sphere, that of the fixed stars, and finally into the ninth, the sphere of the One. The initiate learns the subject matter intellectually, purifies himself, and then receives a mystic initiation that confirms his intellectual knowledge, encompasses it, and transcends it.

Geocentric model of the Universe
When he had finished praising, he shouted: "Father Trismegistus, what shall I say? We have received this light, and I see myself— the same vision in you. I see the eighth and the souls that are in it, and the angel singing a hymn to the ninth and all its powers. I see Him who has the power of them all, creating those that are in the Spirit." Hermes Trismegistus then tells the initiate to record this teaching in a book, writing it on turquoise in hieroglyphs, and to choose its time using astrology. Even while describing a spiritual and visionary experience, the astrological setting remains important.
Hermeticism, like many Eastern spiritual paths, is founded on the principle of unity. However, having evolved in the context of Western culture, it is more amenable to Western spiritual seekers. Hermeticism embraces a worldview that sees Spirit diffused throughout all of reality. Each moment and movement on the earthly, material plane emanates from above. Power and wisdom regarding mundane affairs can be discerned from the cycles above, and the spiritual seeker can attain mystic union by transcending the interconnected realms of matter, soul, and spirit.