The Intermediate zone, a term found in Sri Aurobindo's philosophy, refers to what is described as a spiritually dangerous and misleading transitional spiritual and pseudospiritual region between the ordinary consciousness and true spiritual realisation. Paul Brunton also uses the term, correlates the term intermediate zone with a perilous and deceptive psychological region also given various other names in mystical literature, such as the astral plane, the hall of illusion, and so on. Prior to Aurobindo's use of the term, a similar conception, termed "astral intoxication", was described by the Theosophist W. Q. Judge.
Original use of the term
Sri Aurobindo first describes The Intermediate Zone in a lengthy letter in reply to a disciple, written (as were most of his letters) in the early 1930s. It was then published in 1933 in The Riddle of this World, a small booklet that includes several essays. The letter later appeared in Letters on Yoga. More recently, a number of copies have been posted on the Internet.,.
A shorter but similar reference to a misleading intermediate consciousness, but without the distinguishing qualifier "zone", is also found in some of the later strata of The Synthesis of Yoga which dates to the early 1940s
In his posthumously published book, Vernal Blooms, Theosophist W. Q. Judge describes the dangers of "astral intoxication". He asserts that the astral plane, which is the same as that of our psychic senses, by nature has endless powers of delusion, and has to be well understood before the student can stay there long without danger. He states that while interesting phenomena such as astral lights, moments of peace and revelation, come, it does not indicate spiritual advancement. He states that to regard every picture seen in the astral light as a spiritual experience is like becoming drunk, and that such indulgence while neglecting true progress, which is always dependent upon purity of motive and conquest of known or ascertainable defects, only results in becoming satiated with a store of illusory appearances. 
An early reference is made in 1934 in the Theosophist Magazine (edited by Annie Besant), in a review by a certain A.E.A. to "'the Intermediate Zone' and its perils impossible to overpass without the guidance of the guru", as well as to other aspects of Sri Aurobindo's teachings.
Characteristics of the Intermediate Zone
Aurobindo asserted that spiritual aspirants may pass through an intermediate zone where experiences of force, inspiration, illumination, light, joy, expansion, power, and freedom from normal limits are possible, which can become associated with personal aspirations, ambitions, notions of spiritual fulfilment and yogic siddhi, and even be falsely interpreted as full spiritual realisation. He asserted that one can pass through this zone and the associated spiritual dangers without harm by perceiving its real nature and seeing through the half-lights and tempting but imperfect and often mixed and misleading experiences. But he taught, those who go astray in it, following false voices and mendacious guidance, may end in a spiritual disaster, or may remain stuck there and adopt some half-truth as the whole truth or become an instrument of lesser powers of these transitional planes, which he stated happens to many sadhaks and yogis., Likewise, Paul Brunton taught that between the state of ordinary man and the state of the matured mystic lies a perilous and deceptive region known as the astral plane, the intermediate zone, the hall of illusion, and so on, which spiritual aspirants can reach through concentration, meditation, self-conquest, and study. Brunton asserted that the danger is that once there, then their egoism becomes stimulated by the subtle forces they have evoked, their emotional nature becomes more sensitive and more fluid, their imaginative power becomes more active and is less restrained, and if they then fall victim to spiritual error regarding their state, the result is swollen vanity, superstitious credulity, emotions run riot, and wild imagination. He considered this a major factor in explaining the human wreckage found on the spiritual path. W. Q. Judge likewise points out the dangers of astral intoxication or delusion are greatest for the person who revolves selfishly around himself because they lack the assistance that comes from being united in thought with all other sincere seekers, that one "must first dispel the inner darkness before trying to see into the darkness without; we must know ourselves before knowing things extraneous to ourselves." 
These things, when they pour down or come in, present themselves with a great force, a vivid sense of inspiration or illumination, much sensation of light and joy, an impression of widening and power. The sadhak feels himself freed from the normal limits, projected into a wonderful new world of experience, filled and enlarged and exalted; what comes associates itself, besides, with his aspirations, ambitions, notions of spiritual fulfilment and yogic siddhi; it is represented even as itself that realisation and fulfilment. Very easily he is carried away by the splendour and the rush, and thinks that he has realised more than he has truly done, something final or at least something sovereignly true.
— Sri Aurobindo, The Riddle of the World, 35
This is in fact an intermediary state, a zone of transition between the ordinary consciousness in mind and the true yoga knowledge. One may cross without hurt through it, perceiving at once or at an early stage its real nature and refusing to be detained by its half-lights and tempting but imperfect and often mixed and misleading experiences; one may go astray in it, follow false voices and mendacious guidance, and that ends in a spiritual disaster; or one may take up one’s abode in this intermediate zone, care to go no farther and build there some half-truth which one takes for the whole truth or become the instrument of the powers of these transitional planes, - that is what happens to many sadhaks and yogis.
— Sri Aurobindo, The Riddle of the World, 36-7
The pathway of the mystical goal is strewn with human wreckage. Why? Several reasons would be needed to give a complete answer but one of the most important is this: Between the state of ordinary man and the state of the matured mystic there lies a perilous and deceptive psychological region which has been given various names in mystical literature. It has been called the astral plane, the intermediate zone, the hall of illusion, and so on. The early efforts of all aspirants in concentration, meditation, self-conquest, and study, bring them into this region. But once here their egoism becomes stimulated by the subtle forces they have evoked, their emotional nature becomes more sensitive and more fluid, their imaginative power becomes more active and is less restrained. The consequence of failure to negotiate these changes properly is swollen vanity, superstitious credulity, emotions run riot, and imagination gone wild...
— Paul Brunton, Notebooks
The placid surface of the sea of spirit is the only mirror in which can be caught undisturbed the reflections of spiritual things. When a student starts upon the path and begins to see spots of light flash out now and then, or balls of golden fire roll past him, it does not mean that he is beginning to see the real Self—pure spirit. A moment of deepest peace or wonderful revealings given to the student, is not the awful moment when one is about to see his spiritual guide, much less his own soul. Nor are psychical splashes of blue flame, nor visions of things that afterwards come to pass, nor sights of small sections of the astral light with its wonderful photographs of past or future, nor the sudden ringing of distant fairy-like bells, any proof that you are cultivating spirituality. These things, and still more curious things, will occur when you have passed a little distance on the way, but they are only the mere outposts of a new land which is itself wholly material, and only one remove from the plane of gross physical consciousness.
The liability to be carried off and intoxicated by these phenomena is to be guarded against. We should watch, note and discriminate in all these cases, place them down for future reference, to be related to some law, or for comparison with other circumstances of a like sort. The power that Nature has of deluding us is endless, and if we stop at these matters she will let us no further. It is not that any person or power in nature has declared that if we do so and so we must stop, but when one is carried off by what Boehme calls “God’s wonders,” the result is an intoxication that produces confusion of the intellect. Were one, for instance, to regard every picture seen in the astral light as a spiritual experience, he might truly after a while brook no contradiction upon the subject, but that would be merely because he was drunk with this kind of wine. While he provided with his indulgence and neglected his true progress, which is always dependent upon his purity of motive and conquest of his known or ascertainable defects, nature went on accumulating the store of illusory appearances with which he satiated himself...The astral plane, which is the same as that of our psychic senses...has to be well understood before the student can stay there long without danger. While we can overcome the dangers of a forest by the use of human inventions, whose entire object is the physical destruction of the noxious things encountered there, we have no such aids when treading the astral labyrinth. We may be physically brave and say that no fear can enter into us, but no untrained or merely curious seeker is able to say just what effect will result to his outer senses from the attack or influence encountered by the psychical senses.
And the person who revolves selfishly around himself as a centre is in greater danger of delusion than any one else, for he has not the assistance that comes from being united in thought with all other sincere seekers. One may stand in a dark house where none of the objects can be distinguished and quite plainly see all that is illuminated outside; in the same way we can see from out of the blackness of our own house—our hearts—the objects now and then illuminated outside by the astral light; but we gain nothing. We must first dispel the inner darkness before trying to see into the darkness without; we must know ourselves before knowing things extraneous to ourselves......
— W. Q. Judge, Vernal Blooms
- Sri Aurobindo's Letters on Yoga - The Intermediate Zone
- http://singingmountain.org/y2004jul15.html Paul Brunton on the Intermediate Zone - The Source of Psychic Delusion and the Subtle Ego
- Vernal Blooms by W.Q.Judge
- Sri Aurobindo, Letters on Yoga, Part 3, Section 3 "Experiences of the Inner and the Cosmic Consciousness", Subsection 5, Pondicherry: Sri Aurobindo Ashram
- Grey Lodge Occult Review :: Issue #9 :: The Intermediate Zone ::
- Sri Aurobindo, The Synthesis of Yoga, fifth edition, Pondicherry: Sri Aurobindo Ashram, ISBN 81-7058-615-1 p.283 - "Another error that has to be guarded against is...to take some higher intermediate consciousness or even any kind of supernormal consciousness for the supermind. To reach supermind it is not enough to go above the ordinary movements of the human mind; it is not enough to receive a greater light, a greater power, a greater joy or to develop capacities of knowledge, sight, effective will that surpass the normal range of the human being. All light is not the light of the spirit, still less is all light the light of the supermind; the mind, the vital, the physical itself have lights of their own, as yet hidden, which can be very inspiring, exalting, informative, powerfully executive..." - for the date see "note on the text", pp.915-6
- Theosophist Magazine, May 1934-August 1934 issue, p.359
- The Notebooks of Paul Brunton (Published in 1989; 16 volumes) - Volume 11: THE SENSITIVES - ch.12. THE INTERMEDIATE ZONE - online text at http://wisdomsgoldenrod.org/notebooks/
- The Intermediate Zone - Notebooks of Paul Brunton
- Barzakh, the intermediate zone in Islam.
- Sri Aurobindo The Riddle of This World online
- ----- Letters on Yoga, Sri Aurobindo Ashram Press, Pondicherry (pp 1039–1046 of the third edition 1971).
- Sri Aurobindo The Synthesis of Yoga, fifth edition, Pondicherry: Sri Aurobindo Ashram, 1999, ISBN 81-7058-614-3 (paperback) ISBN 81-7058-615-1 (hardcover) online
- Paul Brunton, Notebooks of Paul Brunton, 1989 online
- W. Q. Judge, Vernal Blooms, Canadian Theosophical Association, Bombay India, 1946 online