Altered states of consciousness can only be defined if there is an understanding of an ordinary state of consciousness.

While scientists can't agree on a clear definition, altered states of consciousness are nevertheless essential to understanding the human mind.

• Excessive dancing, meditation, and mind-altering plants were used in ancient civilizations to modulate the activity of the mind.
• In 1892, the term "altered states of consciousness" was used to refer to hypnosis.
• William James introduced the scientific investigation of mystical experiences and drug-induced states into the field of psychology.

• Pharmacological. These altered states include short-term changes caused by psychoactive substances, such as LSD MDMA, cannabis, cocaine, opioids, and alcohol.
• Psychological. Hypnosis, meditation, and music can lead to altered mental states.
• Physical and physiological. An altered state of consciousness is achieved through sleep, where dreams dissociate one from reality.
• Pathological. A traumatic experience causing brain damage can lead to an altered state of consciousness. Other sources include epileptic or psychotic episodes.
• Spontaneous. Daydreaming and mind wandering can cause altered states.