Unity & Interconnectedness

Unity — also called Oneness, interconnectedness, nondual awareness, or the oceanic feeling — is the phenomenological experience in which one's ordinary sense of self as a bounded, separate entity is temporarily replaced by a felt identification with a much larger system: another person, one's whole physical body, one's immediate environment, the natural world, or (in the deepest presentations) the entire universe experiencing itself through the specific point in space and time that this body happens to occupy.

PsychonautWiki's Subjective Effect Index formalises the state into four progressive levels: (1) unity between specific external systems; (2) unity between the self and specific external systems (e.g. an object one is interacting with, or another person during sexual or musical intimacy); (3) unity between the self and all perceivable external systems (the classic 'one with the surroundings' state); and (4) unity between the self and all known systems (the deepest state, commonly interpreted as 'becoming one with the universe'). Each level involves progressively more of the ordinary self-vs-world boundary dissolving.

The state has been described across nearly every mystical tradition. William James's Varieties of Religious Experience (1902) catalogued it as one of the four defining features of mystical states. Romain Rolland's letter to Sigmund Freud coined the term oceanic feeling. Vedantic advaita ('non-two') is the philosophical elaboration of it. The Chan/Zen satori, the Sufi fana, the Buddhist śūnyatā, the Christian unio mystica, the Taoist recognition of ziran, and modern nondual traditions (Advaita Vedanta, Kashmir Shaivism, Dzogchen, contemporary teachers like Rupert Spira and Adyashanti) all point at the same experiential territory.

In contemporary psychedelic research, unity is one of the specific dimensions measured by the Griffiths Mystical Experience Questionnaire (MEQ-30) and correlated in imaging studies with disruption of the default-mode network — the brain circuitry associated with self-referential thought. Experiences reaching the higher levels are among the most durably positive across the whole psychedelic literature: participants routinely rate them among the most meaningful events of their lives, and follow-up studies document lasting changes in openness, wellbeing, and orientation to death.

Sources:
PsychonautWiki — Unity and interconnectedness
Wikipedia — Nondualism