The following is excerpted from Titus Burckhardt's Introduction to Sufi Doctrine:
The term "alchemy" is very suitable as applied to the art of concentration considered in itself because, from the point of view of this art, the soul is like "a matter" which is to be transformed even as in alchemy lead is to be transmuted into gold. In other words the chaotic and opaque soul must become "formed" and crystalline. Here, form does not mean a fixation within certain limits but on the contrary a quasi-geometric coordination, and hence even a virtuality of deliverance from the limiting conditions of the arbitrary psychic tyranny, just as gold or crystal manifests on the level of solid substances the nature of light, the second both by its geometrical form—the propagation of light being rectilinear—and by its transparence.
According to the same symbolism—the nearest to alchemy properly so called—the soul, fixed in a state of sterile hardness, must be "liquefied" and then again "congealed" in order to be rid of its impurities. This "congelation" will in its turn be followed by a "fusion" and this again by the final "crystallization". In order to bring about these changes the natural forces of the soul are actualized and coordinated. They may be compared to the forces of nature—heat, cold, moistness, and dryness. There is in the soul an expansive force which normally shows itself as confident joy (basṭ) and as love and so as "heat", and there is a contractive force—a "coldness"—which shows itself as fear, its spiritual form being the extreme contraction (qabḍ) of the soul, in face of death and eternity, into the single point of the present.
As for moistness and dryness these correspond respectively to the "liquefying" passivity of the soul and the "fixing" activity of the spirit. These four forces can also be connected with two complementary principles which are analogous to the "Sulfur" and "Mercury" of the alchemist. In the Sufic method these two principles are to be identified respectively with the spiritual act—the active affirmation of a symbol—and the plasticity of the psyche. Thanks to the intervention of Grace the voluntary affirmation of the symbol becomes the permanent activity of the Spirit (ar-Rūḥ) while the plasticity or receptivity of the soul takes on a cosmic amplitude. [According to Muḥyī-d-Din ibn 'Arabī the universal meaning of Sulfur is the Divine Act (al-Amr) and that of Mercury, Nature as a whole (Tabī at al-kull).]
The fiery quality and the "fixative" quality are connected with the active pole which corresponds to Sulfur, while the contracting quality and the "moist" dissolving quality are connected with the passive pole, which is the Mercury of alchemy. Thus it is easy to see how the different "natural" qualities of the soul are combined in different states. Sterile hardening of the soul results from an alliance between the fixing quality (dryness) of the mind and the contracting quality in the psyche. Dissipation, on the other hand, comes from a link between the expansive force of desire and the dissolving power of the passive psyche. Moreover these two states of disequilibrium may be piled one upon the other, as is often the case. Equilibrium of the soul consists in a steady alternation of expansion and contraction, comparable to breathing, and in a marriage of the "fixative" activity of the spirit with the "liquid" receptivity of the soul.
In order that it may be possible for this synthesis to take place the powers of the soul must not let themselves be determined in any way by impulsions coming from outside; they must instead respond to the spiritual activity centered on the heart. [This corresponds to what in alchemy is called the "hermetic sealing" of the vessel.]
The art of concentration has been indicated here in alchemical terms because these bring out the correspondence between the powers of the soul and the natural forces—the physical forces one might say—of the human organism. The process of harnessing these powers brings this aspect of Taṣawwuf near to the methods of Raja Yoga. Clearly the technique in question can be described by means of different symbolisms. Sufi writers usually treat of this question implicitly by indicating the use of the symbols which are the object of concentration; indeed the "alchemical" work, in the sense in which it is envisaged here, cannot be separated from the nature of the symbols used as "means of Grace" and these symbols are the intermediary through which the "alchemical" aspect of spiritual work is linked with its intellectual aspect. The pre-eminent spiritual means of Taṣawwuf is the verbal symbol repeated either inwardly or aloud with or without a synchronizing of the breath; hence the various phases of the inner alchemy—the successive "liquefactions" and "crystallizations"— appear as permutations (taṣrīf) of the symbol in the soul in conformity with the different Divine Realities (ḥaqā'īq) it expresses.
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The hierarchic "placing" of the faculties of the soul is one aspect of the reintegration of the soul into the Spirit. The state of a soul which has been spiritually regenerated has already been compared to a crystal which, though solid, is akin to light both in its transparence and in its rectilinear form. The various intellectual faculties are like the facets of this crystal, each one reflecting in its own way the unique and limitless Intellect.
The faculty which is specific to man is thought (al-fikr). Now the nature of thought, like the nature of man, is two-faced. By its power of synthesis it manifests the central position of man in the world and so also his direct analogy with the Spirit. But its formal structure, on the other hand, is only one existential "style" among many others; that is to say it is a specific mode of consciousness which could be called "animal" were it not distinguished, for better and for worse, by its connection with man’s unique—and intrinsically "supernatural"— function from those faculties of knowledge that are proper to animal species. In fact thought never plays an entirely "natural" part in the sense of being a passive equilibrium in harmony with the cosmic surroundings. To the degree that it turns away from the Intellect, which transcends the terrestrial plane, it can only have a destructive character, like that of a corrosive acid, which destroys the organic unity of beings and of things.
We have only to look at the modern world with its artificial character devoid of beauty and its inhumanly abstract and quantitative structure in order to know the character of thought when given over to its own resources. Man, the "thinking animal", must necessarily be either the divine crown of nature or its adversary, [In animals there does not exist, as in man, a refraction of the intellect which is at the same time subjective and active, a refraction which would stand between the intellectual essence immanent in the form of the species and the individual psychic organism. For this reason animals are more passive than man in relation to the cosmic surroundings. At the same time they more directly express their intellectual essence. The beauty of a sacred art—an art divinely inspired—heightens that of virgin nature, while the creations of a civilization that is profane and practically atheistical, such as modern civilization, are always hostile to natural harmony.] and this is so because in the mind "to be" becomes dissociated from "to know" and in the process of man’s degeneration this leads to all other ruptures and separations.
This double property of thought corresponds to the principle which Sufis symbolize by the barzakh, the "isthmus" between two oceans. The barzakh is both a barrier and a point of junction between two degrees of reality. As an intermediate agent it reverses the pencil of rays of the light it transmits in the same manner as does a lens. In the structure of thought this inversion appears as abstraction. Thought is only capable of synthesis by stripping itself of the immediate aspect of things; the more nearly it approaches the universal, the more it is reduced as it were to a point. Thought thus imitates on the level of form—and hence imperfectly—the essential "stripping bare" (tajrīd) of the Intellect.
The Intellect does not have as its immediate object the empirical existence of things but their permanent essences which are relatively "non-existing" since on the sensory plane they are not manifested. [When certain modern thinkers would see in the act of knowing a sort of annihilation—relative and subjective—of the object of knowledge considered as pure existence they merely reproduce the unreal and implicitly absurd character of thought which has turned aside from intellectual principles and ended by emptying itself of any qualitative content. The crude and undifferentiated "existence" which these philosophers oppose to the intellectual act of the subject is nothing but the shadow cast by this absence of intuition in their own thought: it is pure unintelligibility. What is real "in itself" is essence; if perception does not simultaneously grasp all aspects of a sensory object that is because both the level of manifestation and the knowledge are alike relative.]
Now this purely intellectual knowledge implies direct identification with its object and that is the decisive criterion which distinguishes intellectual "vision" from rational working of the mind. This "vision" does not, however, exclude sensory knowledge; rather it includes it since it is its essence, although a particular state of consciousness may exclude one in favor of the other.
Here it must be made quite plain that the term "intellect" (al-'aql) is in practice applied at more than one level: it may designate the universal principle of all intelligence, a principle which transcends the limiting conditions of the mind; but the direct reflection of Universal Intellect in thought may also be called "intellect" and in this case it corresponds to what the ancients called reason.
The mode of working of the mind which is complementary to reason is imagination (al-khayāl). In relation to the intellectual pole of the mind imagination may be considered as its plastic material; for this reason it corresponds by analogy to the materia prima on which the plastic continuity of the "cosmic dream" depends just as, subjectively, it depends on imagination.
If the imagination can be a cause of illusion by binding the intelligence to the level of sensory forms it none the less also has a spiritually positive aspect in so far as it fixes intellectual intuitions or inspirations in the form of symbols. For imagination to be able to assume this function it must have acquired in full measure its plastic capacity; the misdeeds of imagination come not so much from its development as from its being enslaved by passion and feeling. Imagination is one of the mirrors of Intellect; its perfection lies in its remaining virginal and of wide compass.
Some Sufi writers, including 'Abd al-Karīm al-Jīlī, have said that the dark pole of the mind is al-wahm, a term which means conjecture and also opinion, suggestion, and suspicion and so mental illusion. This is the reverse of the speculative freedom of the mind. The power of illusion of the mind is, as it were, fascinated by an abyss; it is attracted by every unexhausted negative possibility. When this power dominates the imagination, imagination becomes the greatest obstacle to spirituality. In this context may be quoted the saying of the Prophet that "the worst thing your soul suggests to you is suspicion".
As for memory, this has a double aspect; as the faculty of retaining impressions it is passive and "earthly" and it is called al-ḥafẓ in this relationship; in so far as it is the act of recollection (adh-dhikr) it is directly connected with the intellect, for this act refers implicitly to the timeless presence of the essences, although they cannot appear as such in the mind. The recapitulation of perceptions in recollection may be inadequate and in a certain sense even must be so since the mind is subject to the attrition of time, but, if recollection were not implicitly adequate, it would be only pure illusion—something which does not exist. If recollection can evoke the past in the present it is because the present contains in virtuality the whole extension of time; all existential "flavors" are contained in the "flavorlessness" of the present moment. This is what is realized by spiritual recollection (dhikr): instead of going back "horizontally" into the past it addresses itself "vertically" to the essences which regulate both past and future.
The Spirit (ar-Rūḥ) is both Knowledge and Being. In man these two aspects are in a way polarized as the reason and the heart. The heart marks what we are in the light of eternity, while the reason marks what we "think". Seen from one angle the heart (al-qalb) also represents the presence of the Spirit in both aspects, for it is both the organ of intuition (al-kashf) and also the point of identification (wajd) with Being (al-Wujūd). According to a divine saying (ḥadīth qudsī) revealed through the mouth of the Prophet, God said: "The heavens and the earth cannot contain Me, but the heart of my believing servant does contain Me." The most intimate center of the heart is called the mystery (as-sirr), and this is the inapprehensible point in which the creature meets God. Ordinarily the spiritual reality of the heart is veiled by the egocentric consciousness; this assimilates the heart to its own center of gravity which will be either mind or feeling according to the tendencies of the particular being.
The heart is to the other faculties what the sun is to the planets: it is from the sun that these receive both their light and their impulsion. This analogy, which is even more clear in the heliocentric perspective than in the geocentric system of the ancients where the sun occupies the middle heaven between two triads of planets, was developed by 'Abd al-Karīm al-Jīlī in his book Al-Insān al-Kāmil ("Universal Man"). According to this symbolical order, Saturn, the most distant of the planets that are visible to the naked eye, corresponds to intellect-reason (al-'aql). Just as the heaven of Saturn includes all the other planetary heavens, intellect-reason embraces all things; moreover the "abstract", cold, and "saturnian" character of reason is opposite to the solar and central nature of the heart, which marks intellect in its "total" and "existential" aspect. Mercury symbolizes thought (al-fikr), Venus imagination (al-khayāl), Mars the conjectural faculty (al-wahm), Jupiter spiritual aspiration (al-himmah), and the moon the vital spirit (ar-rūḥ). Anyone with some knowledge of astrological "aspects" can readily deduce from this outline both the beneficent and the harmful "conjunctions" of the different faculties represented by the planets.
From another point of view the heart is compared to the moon which reflects the light of the divine sun. In this case the phases of the moon correspond to the different states of receptivity of the heart direct "enunciation" of Being. Both these aspects are to be found in the Greek word Logos which means principle and also idea and speech; in the same way man is defined either as a "thinking animal" or as an "animal endowed with speech" (ḥayawān nāṭiq).
From the principial point of view the idea is dependent on the Word, inasmuch as it is an intellectual reflection of Reality, but in man the idea precedes speech. In the rite of invocation (dhikr) the principial relationship is symbolically re-established since the revealed speech— the sacred formula or the Divine Name which is invoked—affirms the ontological continuity of the Spirit whereas thought is—practically speaking—cut off from its transcendent source through being the seat of individual consciousness. In this way the faculty of speech, which is a faculty of action, becomes the vehicle for knowledge of Being.