The following is excerpted from Occult Paris: The Lost Magic of the Belle Époque by Tobias Churton, published by Inner Traditions.

“Tout est nombre. Le nombre es dans tout. L’ivresse est un nombre.”
(“Everything is number. Number is in all. Drunkenness is a number.”)
Baudelaire

Claude Debussy destroyed the great majority of his musical sketches, so finding personally written evidence for the impact of esotericism on his musical thinking is somewhat hampered, especially as he followed the Hermetic dictum to keep silent on such matters, nor did he relish the idea of anyone getting inside his deepest thoughts or techniques. However, he left a solitary written clue, whether an intentional leak or not, we shall probably never know.

In August 1903 Debussy wrote a letter to publisher Jacques Durand along with corrected proofs of his work Estampes. Its title inspired by artists’ concentrated use of potent images in prints and engravings, Estampes was written for solo piano in three movements: Pagodes (Pagodas), La soirée dans Grenade (Evening in Granada), and Jardins sous la pluie (Rainswept Gardens). “You’ll see,” Debussy alerted Durand, “on page 8 of Jardins sous la pluie, that there’s a bar missing—my mistake, besides, as it’s not in the manuscript. However, it’s necessary, as regards number; the divine number [elle est nécessaire, quant au nombre; le divin nombre], as Plato and Mlle. de Pougy would say, each admittedly for different reasons.”

Plato we know; Liane de Pougy (1869–1950) was one of the most famous women in France. Folies Bergères star and beautiful courtesan of Paris’s demimonde, Liane is famous now for her novelized account of her lesbian affair with American Salon hostess and talent magnet, Natalie Clifford Barney (1876–1972), Idylle Saphique (1901). Remy de Gourmont called the no less beautiful Barney, “the Amazon” for her lesbian hunting skills. Symbolist poet Renée Vivien (1877–1909) split painfully with Barney in 1901 over her lover’s infidelities while Vivien’s neighbor Colette enjoyed a brief affair with Barney after ­leaving Gauthier-Villars in 1906.

Roy Howat’s brilliant analysis of Debussy’s use of esoteric ­proportional techniques (Debussy in Proportion, 1983) recognizes at once that Debussy’s “divin nombre” probably signifies the “nombre d’or” (golden number), the French term for what is commonly called the golden section or golden mean. The joke about Mlle. de Pougy is, he thinks, a pun on the expression for “the divine few,” that is the Parisian élite: “le nombre des élus,” consistent with her ­demi-mondaine, outside-the-norm, status. Liane had the pick of the gold, so to speak, of the rich and the aristocratic men who courted her. I should suggest Debussy might also have been hinting at same-sex harmonies, since the golden section divides a single line into an internal harmony or divine ratio of greater and lesser, analogous perhaps to Liane’s idea of a “Sapphic Idyll.”

Once the missing bar is put in, Jardins sous la pluie does conform to the golden section proportion. Furthermore, since the missing bar referred to by Debussy in his letter to Durand merely repeats the previous one, we can see that the bar’s addition is intended specifically to generate Debussy’s required internal symmetrical consistency. The means to acquire that internal symmetry was, for Debussy, application of Euclid’s Elements, Book 5, Proposition No. 30, that is, how to cut a given finite line in extreme and mean ratio. This is how the Divine Proportion is established, and its discovery, before Euclid first wrote it down (ca. 300 BCE), is attributed to Pythagoras and his mystical school, a school concerned with the occult but no less practical properties of numbers.

How does all this affect a piece of music? Revolutionary as Debussy was in so many ways, he still wrote his music out in conventional bars using conventional notes. In the simplest application of the golden section, you may have a piece of music of, say twenty-one bars. If at the thirteenth bar you climax the first movement, since thirteen is the golden section division of twenty-one, the piece then conforms dynamically to a “divine” proportion. What this means is that while a musical statement may not conform to expected or traditional rhythms, repeats, codas, or melodic development, it possesses a hidden, underlying, unbreakable form, an “occult” form of celestial authority and coherence that prevents mere fancy from breaking the bounds of the internal laws of nature. While it looks like a constraint, it in fact guarantees a special kind of freedom, and most importantly, it is harmonious. This freedom was dear to a man who was to many ears, initially, “breaking all the rules.” Roy Howat is quite right to point out that the combination of the letter to Durand and the analysis of Jardins sous la pluie “leaves no doubt that at least on that occasion, Debussy was consciously constructing with numbers.”

We might also slip in here the fundamental Symbolist “formula” that the artist contemplate the idea, find its symbol, then give it form. It is on account of Debussy’s general, inherent—but not too formulaic—­observation of this principle that we must once and for all drop the use of the word Impressionism when referring to Debussy. It is painfully misleading, and he rightly hated it, as we shall see. Intrigued by how painters applied geometrical proportion, including golden section proportions, to pictorial composition, Debussy applied the occult principle to music as a means of giving form to ideas perceptible to him through nature.

The Magic in Music

Let us look a little closer at what we mean by the golden section and why it had mystical and magical properties for the ancients, such that from the late 1880s, men such as Papus and Edmond Bailly could promote “occult sciences” as the “high sciences” and the keys to bringing spiritual knowledge in to revolutionize a nonmaterialist new science, integrating matter and spirit on a higher plane of consciousness.

The golden section is the point on a given line where the ratio of the shorter part of the length created by the point is in the same proportion, or ratio, to the longer part, as the longer part of the length is to the line as a whole. This is not an arbitrary piece of geometry; nature herself uses the principle in its growth patterns and in their manifestation as organic forms. The rule is inherent to the universe of things; it is an implicit not an imposed idea, and it speaks of transcendent, or if you prefer inherent, intelligence present in cosmic formation. A single line divided into its golden section also exhibits the idea of microcosm—little universe—in relation to macrocosm or greater universe. That is to say, the ratio of the shorter section to the larger section is a kind of microcosm of the ratio of the larger section to the line as a whole. This principle demonstrates the Hermetic dictum: “As above, so below.” The harmony of the greater ensures the harmony of the minor, and the two are proportionately and harmoniously interrelated. Recall how important harmony is to music; it is what color is to painting.

The golden section exhibits other, more specific, special properties.

First, the exact figure for the ratio is always irrational. That means that the figure for the ratio is always a figure whose decimal places continue indefinitely. Where we measure a given line as 1, the golden section is approximately 0.618034, or a little under two-thirds, but the decimal places ratchet on infinitely.

Another special property, which might be used to show the section’s potential to create infinite microcosms, is as follows: say we have divided a line AB at the point of golden section and called that point C. 
If we now seek the golden section of the longer section AC, and create the point D, we shall find the golden section of AC (at D) is proportionately equal to the golden section of AB, but in the other 
direction!

However, we really get to grips with what the ancients found so astonishing about the golden section when we look at its relation to the five so-called Platonic solids, and to the pentagon and magical symbol of the pentagram in particular.

Born at Samos circa 596 BCE, Pythagoras initiated a school that perpetuated and developed his doctrines. From that tradition, Plato knew of five solid geometrical bodies that embodied the principles by which the universe was formed. Constituted of atoms, these solids represented the four elements: earth (the cube), fire (the tetrahedron), air (the octahedron), and water (the icosahedron). An ultimate fifth element, often symbolized in terms of “spirit,” was represented by the dodecahedron. This twelve-sided, three-dimensional figure of regular pentagons completes the creation-palette of the cosmos.

Plutarch (ca. 50–120 CE) recorded the Pythagorean belief that the heavenly sphere was formed from the dodecahedron. Jews, Christians, and pagans could see meaning in the 12-fold symbolism manifesting on Earth: 12 tribes of Israel, 12 signs of the zodiac, 12 apostles, the number of the saved: 12 squared (Revelation 7:3–8), and so on. The dodecahedron exhibits a fundamental relationship between the numbers 5 and the heavenly 12, for it is made of pentagons, and the pentagon is constructed from two lines that are in golden section proportion to each another.

This is done as follows: take a given line and adduce its golden section. The longer part of the line we call A, the shorter part B. We now take B and use it as a baseline for a triangle, where the base is B and the other two sides are the length of the longer section A. We then construct a circle around the triangle whose three points touch the circle’s circumference. We see our isosceles triangle precisely in a circle. We now take the baseline B (the shorter part of the original section) around the circle in consecutive order, marking a point where the line ends 
on the circumference. These points joined up, we see a pentagon as well as the triangle within the circle. Once we join the points diagonally across the figure, we see a perfect pentagram within the pentagon, within the circle!

This five-sided figure itself has even more properties of a special kind. It has now ten triangles within it (5 x 2), together with a proportionately reduced inverted reproduction of itself heading into the center ad infinitum, alternating in inversions. Each point on the circle occurs every 72 degrees on the 360 degree circle, as we should expect, but, surprisingly at first, all of the internal degrees are either 36, its double 72, or its triple 108, and each of these numbers reduces to 9, and all are of course made from an odd and an even number: symbolically speaking, the masculine-feminine harmony in dynamic androgynous union!

Imagine the wonder of the first person who realized all of this! It looked like the Grand Geometer’s building formula, revealed by mathematics and geometrical construction. And of course, the Pythagoreans were famous for linking these numerical relationships directly to musical intervals, scales, modes, and proportions.

I have by no means exhausted the mathematical implications of the pentagonal geometry in relation to the golden section, but I hope I have clarified why golden section carried such meaning for Debussy and his esoterically minded friends, and why he had every cause to ­experiment with it. Oh, and if one should object that the task of calculating the golden section for a proportional number of bars, or, as he did, a proportional sequence of notes, as well as sub-golden sections within golden sections would have been extremely onerous, Debussy could easily have employed Fibonacci sequences to simplify the process.

Briefly, a Fibonacci series is created by taking a number and adding the previous number to produce the next number, viz: 0, 1, 1, 2, 3, 5, 8, 13, 21, and so forth. Take three numbers in Fibonacci sequence and the figure in the middle gives you the longer part of the golden section (to round figures), the figure on the left gives the shorter part of the line, and the figure on the right gives you the length of the whole line, viz: 21, 34, 55.

Roy Howat offered many examples of Debussy’s use of, and partial use of, golden section proportions in his works. Not surprisingly, La Mer gains much of its force from Debussy’s attempt to get really inside the natural dynamics of water to locate its occulted melody, rhythm, and harmonic scope, or as the Greeks would say, its oikonomia, or proportional law. The golden section is much in evidence in La Mer’s mighty scope.

One of the clearest examples of golden section proportion can be found in the two sets of Debussy’s Images (1905, 1907) for piano. In the first piece of the first set, Reflets dans l’eau (Reflections in the Water), the principal climax occurs at bars 56–61, after 58 bars of a total 94. This is only 1 percent out and that is due to a tempo change. There are other instances of golden section proportion in this piece. Howat recognized that it had often been remarked that the opening phrases of the piece followed a wave shape. He is more specific: “the tops of these 2-bar phrases give each a Golden section shape (5 quavers out of 8), anticipating in miniature the piece’s dynamic wave form.” In the third piece of the 1905 Images, Mouvement has its principal climax at bars 109–110, conforming to golden section proportion. In the first piece of the 1907 Images, Cloches à travers les feuilles (Bells through the Leaves), there is only half a bar of fortissimo at its climax, and it is at the point of overall golden section, in the second half of bar 31.

Possibly Debussy’s most famous piece, Clair de Lune (published in 1905, but dated 1890 by the composer) included in the Suite Bergamasque, uses the golden section throughout its duration, in the sequence 36:22:14. The much-loved Clair de Lune was composed then, at the high tide of the explosion of Occult Paris, when Debussy regularly played at Bailly’s l’Art Indépendant bookshop, that period immediately preceded by what Howat describes as “arguably the largest evolutionary leap in Debussy’s career” (1885–88) when we see “expressive chromaticism suddenly taking a dominating role in their forms, replete with tritonal and other chromatic relationships.” Howat marks the leap with Debussy’s Ariettes oubliées (Forgotten Arias), and the work entitled “Spleen” in particular. From that point, Debussy was particularly open to new ideas that expanded liberty of composition, and the ideas were to hand, presented in a charmed intellectual setting.

The composer experimented carefully, until, from about 1894, “virtually all Debussy’s works show proportional organization, although to varying degrees of structural importance.” With some works, the pattern is not obvious to locate. For example, La Cathédrale Engloutie only revealed its golden section proportion when Debussy’s own playing of it was heard on an authentic piano roll, where it was discovered he doubled in tempo some of the bars, demonstrating how he intended the proportions to work, even though this doubling was not present on the published score.

Debussy was struck by natural phenomena such as the pebble in the pool that seem to speak of curious, dormant energies, but it was essentially the spiritual beauty, not the quantity that attracted him. Likewise, golden section proportion is not merely mathematical—though some mathematicians might think so—it has poetry about it; it is suggestive of a deeper, hidden order of subtle intelligence, inherently aesthetic. In other words, we find Debussy full of the Symbolist atmosphere and intellectual fervor of those who gathered, as he did, at Bailly’s bookshop. Michelet isn’t exaggerating when he tells us that Debussy absorbed the Hermetic universe. Since this universe has been unfamiliar to most commentators on the arts until recently perhaps, it is not surprising it has passed unnoticed, and people have been misled into the Impressionist fallacy. Debussy himself said that he professed “une religion de la mystérieuse nature,” a religion of mysterious nature. What is this religion but the Hermetic religio mentis, the religion of the mind whose axiom held that mundus imago dei: “the world is the image of God” (Corpus Hermeticum)?

Influenced by Impressionism, Symbolist poet Jules Laforgue ­(1860–1887) wrote an essay of that name in which Howat is wont to locate the perceptual eye of Debussy in works such as Reflets dans l’eau. According to Laforgue “the basic characteristic of the impressionistic eye” is one of “seeing reality in the living atmosphere of forms, decomposed, refracted, reflected by people and objects in endless variations.” I have to say I find this a bit woolly, its being merely descriptive, not revelatory, wholly confined to the visual plane. Nevertheless, Howat considers this reference to “seeing reality” might have influenced Debussy when he described his orchestral Images as “realities—what imbeciles call impressionism.” I should have thought that Debussy’s tone and specific denigration of Impressionist categories here makes it abundantly clear that the “realities” he was referring to were not primarily visual impressions, however spun out through enchanting plays of light and color, but the living spiritual principles that animated the perceived universe: the source of the enchantment. The beauty is essentially not in or of the object, but in the mind of the perceiver linking to the beauty of the mind of the creator, that is, the reality behind the visible reflection of that reality,not the mere reflection, as in Impressionism, which, when all is said and done, is fundamentally representational and illustrative.

Unfortunately, critics have been easily misled by the poetic titles Debussy placed before and, more significantly, after—as with the Préludes—his music. This has had the misleading effect of making critics think the music is purely descriptive of the content of the title, partly on account of the symbiosis between the dapples of color in Impressionist painting and Debussy’s gentle and dramatic piano effects, the merging of tonalities in echoes being taken as an aural analog of the Impressionists’ blurring of lines. However, it would be well to bear in mind that Debussy shared the specific attitude of Symbolist poets and painters with regard to titles, as exemplified by Debussy’s friend, who was also a musician and an architect (geometry again!), Odilon Redon. One is to look beyond the obvious; ambiguity is always to be preferred to specificity. If meaning is obvious, one has missed the nature of a symbol. As critic Edward Lockspeiser has put it: “A title is justified only by its vague indeterminate nature, suggesting a double meaning.”20—to which I should only add that we should not forget triple or quadruple meanings, for a true symbol implies infinite correspondences. However, the image is there, and there is nothing wrong with listening to “The Engulfed Cathedral” and quite naturally imagining an engulfed cathedral. But what does it mean? What does it portend? What does it evoke? What does it symbolize? The answer to those questions is expressed in the music, in the spirit, not in titles. Meaning is not exhausted by rational statements or descriptions, but limited by them. Such is the nature of mystery.

Since Pico della Mirandola announced in 1486 in his Oratio de dignitatis homini (Oration on the Dignity of Man) that Man the Magus had arrived, the Magus was one who sought in the “hidden recesses of nature” for the transcendent principles that sustained living things that, while accessible to the enlightened man, were closed to the dense. Debussy’s beauty is not superficial but ideal and divine. As Debussy would say to Michelet with regard to the success of his opera Pélleas et Mélisande, his music was “supported, but not digested.” This is still the case, judging by the DJ’s sentimental sighs that invariably accompany Debussy’s better-known melodies when played on 
classical radio.

Other than the books and assembled company of Bailly’s bookshop, where else might Debussy have encountered esoteric influences? Debussy’s education was very inconsistent. Julia d’Almendra interviewed Debussy’s sister Adèle in 1948, and from her she learned that many 
of Debussy’s earliest years were spent with his aunt in Cannes where he received training in the cathedral liturgy. D’Almendra wondered if this Provençal period might well have stirred in him a love for old modes as well as an attraction to Palestrina and his contemporaries.

Aged nine, during the events of the 1871 Paris Commune, Debussy and his father met the eccentric Charles de Sivry (1848–1900), whose mother taught Debussy piano. Through de Sivry, Debussy experienced the artistic avant-garde, for de Sivry was Verlaine’s brother-in-law, having married Verlaine’s half sister Mme. Mauté de Fleurville, who would also teach Debussy music. De Sivry was himself a musician, teaching piano, conducting, and composing songs for cabaret, while maintaining an interest in Kabbalah and the occult sciences. The January 1892 issue of Papus’s L’Initiation includes Charles de Sivry in its list of “principal editors and contributors” for its “Literary Section,” along with 
E. Goudeau, Catulle Mendès, Émile Michelet, and others. In the 1890s de Sivry also ran the shadow theater at le Chat Noir, where the French chanson tradition was born. With this experience, it is not surprising that Debussy entertained disrespect for the academic musical establishment, evinced when he went to the Conservatoire in 1882.

At the end of his student days, Debussy was naturally caught up in the Symbolist movement, enjoying many opportunities to learn about the golden section and kindred occult subjects of harmony and symmetry through his association with painters who were themselves using occult proportional systems to advantage, seeking what Paracelsus called the “divine signatures”: the encoded memory, if you like, in nature of the divine source of creation. These interests would manifest in works such as Paul Sérusier’s ABC de la peinture—see also his oil painting Tetrahedrons (ca. 1910). A large part of the work’s thirty-five pages is dedicated to proportional techniques, with four pages devoted to the golden section. Though not printed until 1921, the work contained ideas entertained by Sérusier, Maurice Denis, and their Nabi friends since their youth. Denis, you may recall, designed the beautiful cover for Debussy’s La damoiselle élue, published by Edmond Bailly’s l’Art Indépendant in July 1893.

* * *

Perhaps the most potentially significant writer on proportion whose works surely did not pass Debussy by was the mathematician Charles Henry, author of Introduction à une ésthetique scientifique (Introduction to a Scientific Aesthetic, 1885). Henry’s interest lay in demonstrating numerical relationships that related to sensations of harmony, whether produced by shapes, colors, or angles in pictures. He attempted to establish a scientific basis for aesthetic pleasures, happy to think in terms of correspondences more familiar to the world of occult philosophy. He could see how a musical line could correspond to a geometrical angle. He believed proper understanding could help artists avoid disharmonious elements in proportion as well as disharmony in music. His ideas justified the Symbolist search for deeper knowledge of the universe and seemed to put occult insights onto the level of an advancing science of the mind and spirit. Henry’s theories on number and proportion were appearing in Parisian Symbolist journals at precisely the time Debussy was composing his Ariettes oubliées (1885–88).

During the 1880s and ’90s, Charles Henry was mentor to the “Hydropathes,” a young men’s drinking club devoted to intoxication with drink, poetry, and ideas; de Sivry was an associate. Avant-garde artists constituted the membership, in whose company Henry bonded with Jules Laforgue, thanks to an introduction from poet Gustave Kahn. Laforgue’s critical works were much influenced by Charles Henry’s scientific approach; Debussy was familiar with them.

Debussy could also encounter Henry’s ideas through reading the review La Vogue, which Henry, also a poet, cofounded, while contributing articles to La Revue Blanche. Neo-Impressionists Georges Seurat and Paul Signac were also interested in applying Henry’s theories. According to William Homer, Neo-Impressionism may be laid in part at Henry’s door. Camille Pissaro dipped in but was afraid of theory becoming formula: a concern shared by Debussy.

Debussy read deeply and remembered what he read, and let it be said, if it is not obvious, that simply writing a piece of music according to golden section proportions isn’t necessarily going to light anyone’s candle! Debussy had the essential spark of genius in himself, and like others so gifted, he sought ways of enhancing his talent and taking it via depths and profundities to the heights through inspiration, originality, knowledge, and experience. Given what we now know of the genuine Occult Paris, it would be extraordinary, if not incredible, had a man like Debussy not taken advantage of the spiritual movement of Hermetism that was, for an epoch, alive and radiant in France’s capital. That he kept it to himself indicates I think not lack of interest, but depth of understanding, loyalty to its precepts, and innate seriousness about his vocation.

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Another source :

https://www.theosophical.org/publications/quest-magazine/1289-concerto-for-magic-and-mysticism-esotericism-and-western-music

Across the Channel, France produced its own esoteric musicians. Claude Debussy, that master of musical impressionism, was familiar with the occult circles of fin-de-siecle Paris and incorporated many esoteric ideas in his works (Orledge 46, 47, 49, 124 -7). Ethereal compositions like Prélude à  l’Aprés-midi d’un Faune (1894) evoke the astral realm of nature spirits and elementals. Debussy wrote his opera Pelleas et Melisande (1902) based on the play by the Belgian symbolist and esotericist Maurice Maeterlinck, and used phi, the golden section, part of the canon of ancient sacred geometry in his compositions. (Another composer who used the golden section was the Hungarian Bela Bartok.)

Claims for Debussy’s occult pedigree run high. In his book Music: Its Secret Influence throughout theAges (first published in 1933), Cyril Scott, a composer and Theosophist, remarked that Debussy was used by the Higher Ones to introduce ancient Atlantean music into the modern age. More recently, the authors of the best selling Holy Blood, Holy Grail (Baigent, Leigh, and Lincoln) claimed that Debussy was one of the Grand Masters of the mysterious esoteric society the Priory of Sion coming in between Victor Hugo and Jean Cocteau. Erik Satie, Debussy’s contemporary, was a member of the occultist Josephin Peladan’s Salon de la Rose+Croix and Ordre de la Rose+Croix Catholique, and hob-nobbed at Edmond Bailly’s occult bookshop, a famous rendez-vous for Parisian esotericists at the turn of the last century.