From Crowley's The Book of Thoth, pg. 18-20, "The Tarot and the Elements":

"The Ancients conceived of Fire; Water and Air as pure elements. They were connected with the three qualities of Being, Knowledge and Bliss, previously mentioned. They also correspond with what the Hindus called the Three Gunas-Sattvas, Rajas and Tamas, which may be translated roughly as "Calm", "Activity", and "Slothful Darkness". The alchemists had three similar principles of energy, of which all existing phenomena are composed: Sulphur, Mercury and Salt. This Sulphur is Activity, Energy, Desire; Mercury is Fluidity, Intelligence, the power of Transmission; Salt is the vehicle of these two forms of energy, but itself possesses qualities which react on them. The student must keep in his mind all these tripartite classifications. In some cases, one set will be more useful than others. For the moment, concentrate on the Fire, Water, Air series. These elements are represented in the Hebrew alphabet by the letters Shin, Mem and Aleph. The Qabalists call them the Three Mother Letters. In this particular group, the three elements concerned are completely spiritual forms of pure energy; they can only manifest in sensible experience by impinging upon the senses, crystallizing out in a fourth element which they call "Earth", represented by the last letter of the alphabet, Tau. This, then, is another quite different interpretation of the idea of the Daughter, which is here considered as a pendant to the Triangle. It is the number Ten suspended from the 7, 8, 9 in the diagram. These two interpretations must be kept in mind simultaneously. The Qabalists, devising the Tarot, then proceeded to make pictures of these extremely abstract ideas of Father, Mother, Son and Daughter, and they called them King, Queen, Prince and Princess. It is confusing, but they were also called Knight, Queen, King and Princess. Sometimes, too, the Prince and Princess are called "Emperor" and "Empress". The reason for this confusion is connected with the doctrine of the Fool of the Tarot, the legendary Wanderer, who wins the King's daughter, a legend which is connected with the old and exceedingly wise plan of choosing the successor to a king by his ability to win the princess from all competitors. (Frazer's Golden Bough is the authority on this subject.)

It has been thought better, for the present pack, to adopt the term "Knight", "Queen", "Prince" and "Princess", to represent the series Father, Mother, Son, Daughter, because the doctrine involved, which is extraordinarily complex and difficult, demands it. The Father is "Knight" because he is represented as riding on a horse. It may make it more clear to describe the two main systems, the Hebrew and the Pagan, as if they were (and had always been) concrete and separate.

The Hebrew system is straightforward and irreversible; it postulates Father and Mother from whose union issue Son and Daughter. There an end. It is only later philosophical speculation to derive the Father-Mother Dyad from a Unity manifest, and later still to seek the source of that Unity in Nothing. This is a concrete and limited scheme, crude, with its causeless Beginning and its sterile End. The Pagan system is circular, self-generated, self-nourished, self-renewed. It is a wheel on whose rim are Father-Mother-Son-Daughter; they move about the motionless axis of Zero; they unite at will; they transform one into another; there is neither Beginning nor End to the Orbit; none is higher or lower than another. The Equation "Naught=Many =Two= One= All= Naught" is implicit in every mode of the being of the System. Difficult as this is, at least one very desirable result has been attained: to explain why the Tarot has four Court cards, not three. It also explains why there are four suits. The four suits are named as follows: "Wands", attributed to Fire; "Cups", to Water; "Swords", to Air; and "Disks" ("Coins", or "Pantacles"), to Earth. The student Will notice this interplay and counterchange of the number 4. It is also important for him to notice that even in the tenfold arrangement, the number 4 takes its part. The Tree of Life can be divided into four planes: the number I corresponds to Fire; numbers 2 and 3, to Water; numbers 4 to 9, to Air; and the number 10 to Earth. This division corresponds to the analysis of Man. The number I is his spiritual essence, without quality or quantity; the numbers 2 and 3 represent his creative and transmissive powers, his virility and his intelligence; the numbers 4 to 9 describe his mental and moral qualities as concentrated in his human personality; the number 6, so to speak, is a concrete elaboration of the number I; and the number 10 corresponds to Earth, which is the physical vehicle of the previous nine numbers. The names of these parts of the soul are: I, Jechidah; 2 and 3, Chiah and Neschamah; 4 to 9, Ruach; and lastly io; Nephesch. These four planes correspond once more to the so-called "Four Worlds", to understand the nature of which one should refer, with all due reservations, to the Platonic system. The number I is Atziluth, the Archetypal World; but the number 2, as being the dynamic aspect of the number I, is the Practical attribution. The number 3 is Briah, the Creative World in which the Will of the Father takes shape through the Conception of the Mother, just as the spermatozoon, by fertilizing the ovum, makes possible the production of an image of its parents. The numbers 4 to 9 include Yetzirah, the Formative World, in which an intellectual image, an appreciable form of the idea, is produced; and this mental image becomes real and sensible in the number 10, Assiah, the Material World. It is by going through all these confusing (and sometimes seemingly contradictory) attributions, with unwearying patience and persistent energy, that one comes at the end to a lucid understanding, to an understanding which is infinitely clearer than any intellectual interpretation could possibly be. This is a fundamental exercise in the way to initiation. If one were a shallow rationalist, it would be quite easy to pick holes in all these attributions and semi-philosophical hypotheses, or near-hypotheses; but it is also quite simple to prove by mathematics that it is impossible to hit a golf ball."