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The definition of "psychedelic experience" is characterized by polyvalence or ambiguity due to its nature. In modern psychopharmacological science as well as philosophical, psychological, neurological, spiritual-religious and most other ideological discourses it is understood as an altered state of awareness often distinct to, and induced by the consumption of certain psychotropics. Psychedelic compounds are known to cause this change in mental state.
In essence a psychedelic episode is, like other ontological notions of unique states of being (compare "enlightenment", religious experience, mystical experience, ego death, ecstasy, etc.) considered ineffable and rather a solely experiential phenomenon. However on some level the experience is communicable through more concrete or familiar effects on the senses: it has variously been characterized by the perception of aspects of one's mind usually believed to be unavailable to ordinary, waking consciousness, normally by the creative exuberance of the mind liberated from its ordinary restraints, or products of artificially-induced chemical imbalances in the human nervous system. Psychedelic states are one of many of the experiences elicited by sensory deprivation, as well as illusions, delusions, changes of perception, and hallucinations in general – whether associated with a mental disorder, psychoactive drugs, etc.
Reportedly there is a common theme of "connectedness" or "unboundedness" which seems unique to many transcendent states of mind, and no less by the state of psychedelia – ranging from a sense of connectedness to everything in the immediate vicinity, to a sense of oneness with everything in the universe. This phenomenon can be juxtaposed with various metaphysical, spiritual and religious concepts such as ataraxia, monad, gnosis, henosis, kenosis, transcendence, the "Absolute" or the penultimate of self-actualization or authentication, or even theosis in Western thought – as well as rigpa or mahamudra, nirvana, cosmic consciousness, moksha, sunyata, dharmakaya, dharmata, etc. in the Orient.
Some who undertake psychedelic experiences come to see them as an ordeal, and mentally overbearing – in which case the result is often known as a "bad trip" or psychedelic crisis, closely linked to the psychological turmoil of panic attacks, depersonalization/derealization, hysteria and dysphoria. For others, such experiences come to be seen as personal re-enactments of a hero's journey. Spiritual practices and psychedelic drugs are the usual context when discussing means to achieve states of mind in which novel perceptions can arise, unhindered by everyday mental filters and processes.
Research that was done during the 1960s suggested that psychedelic drugs might have medical uses. More recently, the Multidisciplinary Association for Psychedelic Studies (MAPS), the Heffter Research Institute, and the Beckley Foundation have continued studying the effects of the psychedelic experience.
Dynamics of psychedelic experience 
An attempt (in no way capable of exhaustively covering the "tremendous range" of psychedelic experience) to give "a more general view of their [psychedelics] effects on perception, thought, and feeling" follows:
First, sensory perceptions become especially brilliant and intense. Normally unnoticed aspects of the environment capture the attention; ordinary objects are seen as if for the first time and acquire new depths of significance. Esthetic responses are greatly heightened: colors seem more intense, textures richer, contours sharpened, music more emotionally profound, the spatial arrangements of objects more meaningful. People may feel keener awareness of their bodies or sense changes in the appearance and feeling of body parts. Depth perception is often heightened and perspective distorted; inanimate objects take on expressions, and synesthesia (hearing colors, seeing sounds, etc.) is common. Time may seem to slow down enormously as more and more passing events claim the attention, or it may stop entirely, giving place to an eternal present. When the eyes are closed, fantastically vivid images appear: first geometrical forms and then landscapes, buildings, animate beings, and symbolic objects.
The emotional effects are even more profound than the perceptual ones. The drug taker becomes unusually sensitive to faces, gestures, and small changes in the environment. As everything in the field of consciousness assumes unusual importance, feelings become magnified; love, gratitude, joy, sympathy, lust, anger, pain, terror, despair, or loneliness may become overwhelming, or two seemingly incompatible feelings may be experienced at once. It is possible to feel either unusual openness and closeness to others or exaggerated distance that makes them seem like grotesque puppets or robots. The extraordinary sensations and feelings may bring on fear of losing control, paranoia, and panic, or they may cause euphoria and bliss.
Short-term memory is usually impaired, but forgotten incidents from the remote past may be released from the unconscious and relived. Introspective reflection with a sense of deep, sometimes painful insight into oneself or the nature of humanity and the universe is common; often the experience seems somehow more real or more essential than everyday life. There are also profound changes in the sense of self: the ego may separate from the body (dissociation), or the boundary between self and environment may dissolve.
At deeper levels, drug users may regress to childhood as they relive their memories, or they may project themselves into the series of dreamlike images before their closed eyelids and become the protagonists of symbolic dramas enacted for the mind's eye. Actions, persons, and images in this dream-world or even in the external world may become so intensely significant and metaphorically representative that they take on the character of symbols, myths, and allegories. Loss of self may be experienced as an actual death and rebirth, undergone with anguish and joy of overwhelming intensity. In some cases the culmination is a mystical ecstasy in which for an eternal moment all contradictions seem reconciled, all questions answered, all wants irrelevant or satisfied, all existence encompassed by an experience that is felt to define the ultimate reality, boundless, timeless, and ineffable.
Some of these effects are more common than others, but none is guaranteed to occur. Many recreational users have probably never experienced the more profound and extraordinary effects, which are usually produced by larger doses, closed eyes, and deep introspection. At any dose, a great deal depends on the time, the place, and the persons involved. Each drug experience is a unique journey of exploration into the mind.
Levels of psychedelic experience 
The Vaults of Erowid discuss the psychedelic experience in a FAQ that provides a partial overview of ideas expressed in Timothy Leary's book The Psychedelic Experience: A Manual Based on the Tibetan Book of the Dead. They classified five levels of psychedelic experience.
Level 1 
This level produces a mild 'stoning' effect, with some visual enhancement (i.e. brighter colours). Some short term memory anomalies. Left/right brain communication changes causing music to sound 'wider'. Can be achieved with common doses of cannabis and MDMA, light doses of MDA, and light doses of psilocybin mushrooms.
Level 2 
Bright colours and visuals (i.e. things start to move and breathe). Some 2-dimensional patterns become apparent upon shutting eyes. Confused or reminiscent thoughts. Change in short term memory leads to continual distractive thought patterns. Vast increase in abstract thought becomes apparent as the natural brain filter is bypassed. Can be achieved with strong doses of cannabis, light doses of LSD, light to common doses of psilocybin mushrooms, light to common doses of peyote, strong to heavy doses of MDMA, and common doses of MDA.
Level 3 
Very obvious visuals, everything looks curved and warped, patterns and kaleidoscopes seen on walls and faces. Some mild hallucinations such as rivers flowing in wood grain or 'mother of pearl' surfaces. Closed-eye hallucinations become 3-dimensional. There is some confusing of the senses (synesthesia). Time distortions and 'moments of eternity'. Movement at times becomes extremely difficult (too much effort required). Can be achieved with common doses of LSD, psilocybin mushrooms, peyote, and ayahuasca.
Level 4 
Strong hallucinations, i.e. objects morphing into other objects. Destruction or multiple splittings of the ego. Things start talking to you or you find that you are feeling contradictory things simultaneously. Some loss of reality. Time becomes meaningless. Out-of-body experiences and ESP type phenomena. Blending of the senses. Can be achieved with strong doses of LSD, strong doses of psilocybin mushrooms, strong to heavy doses of peyote, and common to strong doses of ayahuasca.
Level 5 
Total loss of visual connection with reality. The senses cease to function in the normal way. Total loss of ego. Merging with space, other objects or the universe. The loss of reality becomes so severe that it defies explanation. The earlier levels are relatively easy to explain in terms of measurable changes in perception and thought patterns. This level is different in that the actual universe within which things are normally perceived, ceases to exist. Can be achieved with heavy doses of LSD (1,000+ µg), heavy doses of psilocybin mushrooms, strong to heavy doses of ayahuasca, common to heavy doses of DMT, 5-MeO-DMT, and Salvia divinorum, and high but sub-anesthetic doses of dissociatives such as ketamine.
Huxley's "Mind at Large" 
In his book The Doors of Perception, author and psychonaut Aldous Huxley presents the idea of the Mind at Large. This is Huxley's theoretical state of mind which humans are normally oblivious to, due to learned social norms and partially due to their biology. Huxley believed that the central nervous system's main function was to filter through irrelevancies and useless knowledge, by shutting out the majority of what we could actually perceive at any given point in time.
Through the pages of his book, Huxley talks about the business of survival, and the information that is the most useful for survival. He believed that this was one element which was forcing the brain to filter out these perceptions. Huxley also believed that man was partially responsible for it, by asserting that society has made a symbolic system which structures our reality, in order to achieve a "reduced awareness".
Aldous Huxley discusses thousands of other worlds that were in some sense interconnected with our own. He said that humans dynamically make contact with these other worlds, all of which are with the Mind at Large. He believed that there were multiple ways of contacting these other worlds such as genetics, hypnosis, and the use of psychedelic drugs.
He then summarizes the psychedelic experience for himself, using the four statements below:
- The ability to remember and to "think straight" is little if at all reduced. (Listening to the recordings of my conversation under the influence of the drug, I cannot discover that I was then any stupider than I am at ordinary times.)
- Visual impressions are greatly intensified and the eye recovers some of the perceptual innocence of childhood, when the sensum was not immediately and automatically subordinated to the concept. Interest in space is diminished and interest in time falls almost to zero.
- Though the intellect remains unimpaired and though perception is enormously improved, the will suffers a profound change for the worse. The mescaline taker sees no reason for doing anything in particular and finds most of the causes for which, at ordinary times, he was prepared to act and suffer, profoundly uninteresting. He can't be bothered with them, for the good reason that he has better things to think about.
- These better things may be experienced (as I experienced them) "out there," or "in here," or in both worlds, the inner and the outer, simultaneously or successively. That they are better seems to be self-evident to all mescaline takers who come to the drug with a sound liver and an untroubled mind.
See also 
- Grinspoon, Lester, & Bakalar, James. B. (Eds.). Psychedelic Reflections. (1983). New York: Human Sciences Press. p. 13-14 ISBN 0-89885-129-7
- "Gnosis". "The Psychedelic FAQ". Erowid Psychoactive Vaults. 1996. Erowid.org
- Huxley, Aldous (1954) The Doors of Perception. Reissue published by HarperCollins: 2004. p. 22-25 ISBN 0-06-059518-3