The term "psychedelic experience" is vague – characterized by polyvalence or ambiguity due to its nature – however 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. In particular hallucinogens, many entheogens and specifically 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.
 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 and with 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, and common doses of MDA.
 Level 3
Very obvious visuals, everything looking 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 splitting 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, 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 (10+ dried grams), strong to heavy doses of ayahuasca and common to heavy doses of DMT and salvinorin A.
 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