Altered states of consciousness (ASCs) include sleep, dreaming, hallucinations, out-of-body experiences, near-death experiences, meditation, and states induced by psychoactive drugs. Psychologist Charles Tart defines an ASC as a qualitative alteration in the overall pattern of mental functioning that feels radically different from ordinary consciousness, though this definition raises questions about what constitutes a 'normal' state. In mystical experiences and long-term meditation, people report seeing through the illusions of duality and perceiving the world as it truly is.
Consciousness might be an all-or-nothing phenomenon or a continuous variable across species. Various theories of its evolution are examined, along with the question of whether consciousness is an adaptation or a useless by-product. The work concludes that consciousness is an enticing illusion that makes us believe our minds are separate from our bodies.