For roughly the last two decades, one of the philosophies that has most influenced my thoughts and actions has been "Thelema," formulated by the late Aleister Crowley (1875-1947). As a guy who majored in religious studies, I thought I would turn the microscope on Thelema for a brief summary.
Only two qualities exist, the Observer and the Observed. These entities are not independent of one another, but polarities of the same thing. The Observer is the active pole. It actively engages with the Observed and in doing so defines it. The Observed plays the passive role, being shaped and defined by the attention of the Observer. Thelema tends to associate the Observer with the integer +n, the individual human consciousness, and the center of a circle. The Observed is associated with -n, the infinite universe, and the circumference of the circle.
“God” (and by this term we mean the sum total of all that was, is, or could ever possibly be) cannot be perceived or defined directly. To say “God” is male robs him of being “female,” and thus limits him. To say “God” is “spirit” removes the possibility of his being “matter,” etc. Thus, Thelema understands “God” by the integer 0. It neither exists not does not exist, and contains all opposites within it. Thus, the Thelemite focuses on two opposite faces of “God,” the personification of the Observer, Hadit, and the personification of the Observed, Nuit. Hadit is the Divine Within, the spark of consciousness in every human being. Nuit is the Divine Without, the universe that human consciousness reaches out to touch and merge with. Hadit is generally seen as a “god” and Nuit as a “goddess.” Again, Hadit would be represented by the center of the circle and the integer +n, while Nuit would be the circumference and -n. Their union, +n + -n, is equal to 0, the undefined and unknowable “God.”
The Universe (Nuit) is understood as being infinite. In such a situation, any single position (Hadit) within the body of the infinite would be the center. Thus both Nuit and Hadit are infinite qualities. Anything that exists in the body of Nuit can be understood as a “point-event,” a position and phenomenon in time and space. Nuit is imagined as being space, with Hadit as the stars that fill space. In all other respects, Thelema embraces all the realities embraced by the physical sciences; atomic particles, the four fundamental forces, etc. It is a strict rule in Thelemic philosophy that nothing in it must contradict logic and observation. Where Thelema differs slightly from modern materialistic science is in the notion that our ideas and thoughts are just as “real” as external, observable phenomena. These mental or “psychic” point-events are simply of a different character than the physical ones. Thus Thelema would allow for the existence of all sorts of gods, demons, angels, and thought-forms as a certain class of being, distinct from animals and plants.
Thelemic anthropology divides the human being into roughly three layers of being. There is the “divine” level, or “Hadit.” This might be understood as pure consciousness. It would be the pronoun “I” in an English sentence, stripped of defining characteristics like actions or attributes. In such a case, each individual “I” is indistinct from every other, points whose only distinction is where they exist in time and space. At the next level, which we might call “angelic,” is the human's sense of individuality and self, the “I am I and not you” level. This is also the center of our True Will (see below). Finally, there is the “animal” level, the human being as a flesh and blood creature with instincts, needs, and drives.
On one hand, Thelema would insist that reason, observation, and above all else, direct experience, are the only viable means of gaining Knowledge. On the other, there is a core belief that all Knowledge is essentially false, being the product of the Observer interacting with (and therefore shaping) the Observed. Like all things, Thelema prefers to tackle Knowledge in levels. For example, on one level a table can be proven to be a table. But on another, a table is just a physical object (“tableness” being a human concept imposed upon it). On another, the table is just an arrangement of chemical components, on yet another, atomic forces, etc. Thus the Thelemite is urged to used objective reason as the base of his understanding, but to simultaneously explore altered states of consciousness in the search for the experience of infinite nothing that the Buddhists would call “nirvana” (and the Thelemites, “crossing the Abyss”).
Thelema defines Magick as "the Science and Art of causing Change to occur in conformity with Will". In short, any time the Observer defines the Observed as a self-willed action, it is Magick. Acting is Magickal, reacting is not. Thelema vigorously strives to remove notions of the supernatural from Magick. Brushing your teeth, writing a sonnet, and balancing your checkbook are all Magickal acts. So, however, is conjuring a spirit to “visible appearance” (essentially causing yourself to see and interact with a thought-form). Everyone does Magick all the time. The only difference is that some are better than others at shaping the world in accordance with their Wills.
Central to Thelema is the notion of True Will, and the highest law is do what thou wilt. It must be clearly understood that this is NOT synonymous with “do as you please.” For the Thelemite, every individual is his or her own god, a sovereign being, a sun in his or her own solar system. However, even gods and suns possess defining characteristics and functions. Your True Will is essentially your role and place in the universe. It is “who you are” and what you are “meant to do.” True Will is the result of genetic, social, and other factors, the trajectory given to you when you manifested as a point-event in space. Discovering your True Will is essential to any Thelemite. Further, it is felt that the only sin is to interfere with the True Will of another (see Liber OZ for expansion on this).
Further reading; http://www.thelemapedia.org/index.php/Thelema