Śaṅkara's clearest treatment of the sākṣī concept. Systematic negation of every layer until what remains is the pure witnessing awareness — not a further object to be found, but the awareness that was doing the searching all along.
After working through the five sheaths, the Vivekacūḍāmaṇi turns to what remains. The student may be clear about what they are not. But the teacher asks now to see what is doing the seeing.
You are not the body — you know the body. But who knows the body? The mind, you might say. But you are not the mind — you know the mind. You watch thoughts arise and pass. Who watches? The intellect, perhaps — the part of you that evaluates and decides. But you know the intellect too. You can observe when you are reasoning clearly and when you are confused. Who observes the intellect?
At each stage, the answering faculty — the faculty invoked to be the 'real self' — turns out to be itself observable. And anything observable is an object of awareness. The pure awareness in which all objects appear — including every faculty that claims to be the self — that is what Śaṅkara calls the sākṣī.
The sākṣī is not a new, further faculty. It is not deeper than the intellect the way the intellect is deeper than the mind. It is the awareness that was present the whole time, within which every faculty appeared and was seen. The five-sheath inquiry doesn't find it — it always was there. The inquiry removes the obstacles to seeing what was never absent.
The sākṣī section systematically applies the principle of dṛg-dṛśya viveka — the discrimination between the seer and the seen. Every object of awareness is dṛśya (seen). The dṛg (seer) is what is doing the seeing. The key move: anything that can be seen is not the ultimate seer. Apply this principle to the body (seen), the breath (seen), the mind (seen), the intellect (seen), the ego-sense (seen). What is doing all this seeing? That is the sākṣī.
Verse 319 states the principle directly: sākṣī viśuddhā jñānasvarūpaḥ — 'The witness is pure, of the very nature of knowledge.' Not a knower that occasionally knows. Not a consciousness that is sometimes present and sometimes absent. Pure knowledge itself — what Śaṅkara elsewhere calls svaprakāśa (self-luminous): unlike objects that require light to be seen, the witness requires nothing to illuminate it, because it is itself the illumination.
The philosophical core of the sākṣī verses is the argument against the doctrine that consciousness is a property of some other substance (body, brain, intellect). Śaṅkara's argument: if consciousness were a property of the body, it would arise and cease with the body's states. But consciousness witnesses the body's arising and ceasing — it is present as the witness of sleep and waking, health and disease. Whatever witnesses the arising and ceasing of states is not subject to those states. It is therefore not a property of those states' substrate but the substrate of all witnessing. This argument is the Vivekacūḍāmaṇi's sharpest statement of the case against physicalism, stated in the vocabulary available to 8th-century Indian philosophy.