Shankara, to be fair to him, never misses an opportunity to mock the Buddhist anatman doctrine. That Western philosophers have found a similarity to Hume’s incoherent bundle of perceptions theory is for me no recommendation being that he was wrong about everything but all in the most stately 18th.century prose. Adi Shankaracarya is commenting on Katha Upanisad II.iii.13:
The Self is (first) to be realised as existing, and (then) as It really is. Of these two (aspects) the real nature of the Self that has been known as merely existing, becomes favourably disposed (for self-revelation).
His initial comment:
Therefore, eschewing the devilish company of those who advance the theory of non-existence, the Self should be realised as existing (i.e. immanent in all) - as productive of effects in which existence inheres, and as having the intellect etc., as Its limiting adjuncts.
The diabolism of the Buddhist position is its initial plausibility. Nothing in the furniture of the mind seems to have a permanent status; annica or impermanence pervades it. Of course that is undeniable but there is one fixed pole star; the very feeling of identity itself. Each state of consciousness is saturated by ‘I-ness’. This is not a result of an inward examination of a mental state inspected in order to discover whether it is mine or not. In the case of a memory this is clear. ‘I dug the garden’ is not arrived at by a perusal of the knowledge of a dug garden and the discovery using evidence that it was I who dug it. The personal memory is seamless. Knowledge that has the appearance of being empirical without the requirement of evidence is contrary to the Humean tradition.
What follows on for the seeker from this bare sense of personal identity in the moment and across time, is what Ramana Maharshi called atma vichara, inquiry into the self using as a tool the ‘who am I’ question. Shankara’s more theoretically complex observation and analysis of self-identity proceeds from the previous citation above:
But when the Self is devoid of all that, and is not subject to changes - and effects do not exist apart from their cause, because of the Vedic text, “All modification is mere name, being supported by speech - earth alone is real” (Ch. Up. VI.4) -the of that unconditioned, attributeless Self that is free from becoming an object of such concepts as existence and non-existence, the true (transcendental) nature is revealed.
The “earth alone is real” refers to the analogy of material identity. Various vessels made of clay are finally but clay. That is the reality behind them. Clay is their cause.
My previous observations on this point:
The various ‘shapes’ that consciousness takes are the result of limiting adjuncts or forms of limitation of pure undifferentiated consciousness - sat cit ananda. “....the limiting adjuncts that are the effects of an existing entity"......"the real aspect being that from which all limiting adjuncts have vanished, which is different from the known and the unknown, is non-dual by nature.....”