Adhyasa, “superimposition,” is the concept Adi Shankara places at the very opening of his commentary on the Brahma Sutras, treating it as the single root error underneath every other confusion a person has about who they are. The classic illustration is a rope mistaken for a snake at dusk: the snake was never really there, but the fear it produces is completely real for as long as the mistake lasts.
Applied to the Self, Adhyasa names the same structural mistake happening constantly and invisibly: qualities of the body (“I am tall,” “I am tired”) and qualities of the mind (“I am anxious,” “I am clever”) get superimposed onto the changeless, attributeless witness-consciousness underneath them, and qualities of that witness get superimposed back onto body and mind, producing the tangled, ordinary sense of being “a person” — an apparent fusion of the real (consciousness) and the unreal (its ever-changing coverings).
Shankara’s point is not that the body and mind are illusions in the sense of not existing at all, but that mistaking them for the Self is exactly as structurally mistaken as mistaking a rope for a snake — a confusion that produces real consequences (fear, grief, ego, craving) despite resting on a category error. Vedantic self-inquiry is, in this framework, mostly the slow work of noticing and undoing this one superimposition, layer by layer, until what is left is what was never actually confused in the first place.