This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Adi Shankara's account of how error works and his argument for why scripture is indispensable to liberation are two sides of the same problem, namely how a philosophy that declares the everyday world of distinct selves and objects to be ultimately unreal can also explain, without contradiction, how that unreal world seems so convincingly real in the meantime. Shankara's answer to the first question turns on the concept of superimposition, adhyasa, illustrated by cases where some feature genuinely present becomes overlaid with a false character borrowed from memory: a scratch on a mirror mistaken for a mark on one's own face, a clear crystal mistaken for red because a red flower happens to sit nearby, and, in the tradition's single best known image, a rope glimpsed in dim light mistaken for a snake. In each case the perceiver does not invent the error out of nothing, nor does the underlying object simply vanish from perception, some real feature is misattributed to the wrong thing until better light or a closer look dispels the error. Shankara treats ordinary experience of the world, and even the sense of being a limited individual self bounded by a particular body and mind, as an instance of exactly this structure. Consciousness, which is really unbounded and identical with brahman, gets superimposed onto the mind body complex, and the mind body complex is in turn superimposed back onto consciousness, so that a person mistakes the borrowed light of pure awareness for a property of the body, much as the perceiver mistakes a mark on the mirror for a mark on the face.
What makes the error metaphysically peculiar, and what much of the later Advaita tradition spent centuries arguing over, is the status of the false snake, or the false world, while the error lasts. Shankara will not say the illusory object is simply nonexistent, since it is perceived and can produce real effects, fear in the case of the snake, an entire life of striving and suffering in the case of the world, but he equally will not grant it the same order of reality as brahman, the one thing that never turns out, on further inspection, to have been mistaken for something else. His term for this in between status is anirvacaniya, indeterminable, neither straightforwardly real nor straightforwardly unreal, a category invented specifically to hold a place for appearances that perception undeniably delivers but that inquiry ultimately dissolves.
That dissolving is where scripture enters. Shankara does not attempt to prove nonduality through independent reasoning of the kind that settles an ordinary empirical dispute; perception and inference are instruments suited to the world of distinct objects standing in causal relations to one another, and veridical cognition by these means is itself ultimately dependent on the nature of the object known, which is exactly what an inquiry into a reality lying beyond all objects cannot supply. Action fares no better as a path, since ritual and meditation are things an agent chooses to do, and whatever an act can produce is itself a further conditioned state, not the unconditioned identity that was already true prior to any effort. Shankara instead accepts nonduality on the authority of the Upanishads alone, and the great Upanishadic utterances, sentences such as tat tvam asi, that thou art, function differently from an ordinary argument precisely because they are not asked to establish some further fact. They remove a false belief already held, the way correcting someone's mistaken belief about a fact that was already true does not create that fact but simply cancels the error obscuring it. Scripture, on this argument, is not one more source of information alongside perception and inference, competing with them the way one witness might contradict another. It is the instrument suited to a target that consists entirely in canceling a superimposition rather than establishing some further fact, which is why Shankara can rest liberating knowledge on the authority of the Upanishads while still granting perception and inference full authority within the ordinary business of living in the world the rope and the snake both belong to.