Sankara’s commentary on the Brahma-Sutra Bhasya I.i.1
Opponent : If Brahman be well known in the world as the Self, then It being already known, there arises the difficulty again that It is not to be deliberated on.
Vedantin : No, for there is a conflict about Its distinctive nature. Ordinary people as well as the materialists of the Lokayata school recognize the body alone to be the Self possessed of sentience. Others hold that the mind is the Self. Some say that it is merely momentary consciousness. Others say that it i~void. Still others believe that there is a soul, separate from the body, which transmigrates and is the agent (of work) and the experiencer (of results). Some say that the soul is a mere experiencer and not an agent. Some say that there is a God who is different from this soul and is all-knowing and all-powerful;others say that He is the Self of the experiencing individual. Thus there ar emany who follow opposite views by depending on logic, texts and their semblances. If one accepts any one of these views without examination, one is liable to be deflected from emancipation and come to grief. Therefore starting with the presentation of a deliberation on Brahman, here is commenced an ascertainment of the meaning of the texts of the Upanisads with the help of reasoning not opposed to the Upanisads themselves, for the purpose of leading to emancipation (through knowledge).
There you have it, an itemisation of the known theories of the Self and Self-Knowledge in the sphere of Sanskrit philosophy in the 9th.Century. Note that many of its topics did not become salient in Europe until much later. Momentary consciousness (Annica) a Buddhist doctrine had to wait for Hume and his bundle of perceptions/series of conscious states, theory. I have written about this at length previously. Follow the topics.
The vijnanavada subjective idealism is akin to that of Kant’s thing in itself and its unknowability. The whole question of the ‘external’ world and what we really are directly aware of is give a thorough examination by Sankara in the commentary on the Brahma Sutras. (B.S.B. II.ii.28)
Materialism is mentioned and so forth and so on. Suffice it to say that Sankara was not a mere theologian, an idea that is often encountered in academic circles. He wants to test those theories using rational means to uncover contradictions and fallacies. His critiques are well worth an examination by readers who have chiefly focused on the Western canon.