Opponents coming from the Advaita field figure often in Yāmuna’s Ātmasiddhi, which shows that even before Rāmānuja Vaiṣṇava authors were taking seriously the challenge of Advaita. Even more interesting is the way Yāmuna answers to them. Let us see some examples concerning the concept of self (ātman):
[Obj.:] But the fact of being a cogniser is the fact of performing the action of cognising and this implies modifications and is (typical of) insentient things and belongs to the sense of Ego. The self is the enjoyer of the result of this (cognition), it does not act, it does not undergo modifications, it is a witness, pure light (with no content). […] And in the same way it is correct to say that the self is a witness and that it is different from the knower and the entity meant by the word ”I”.
(nanu jñātṛtvaṃ jñānakriyākartṛtvaṃ vikriyātmakaṃ jaḍam ahaṅkāragranthistham. tatphalabhug akartāvikriyaḥ sākṣī prakāśamātra ātmā. […] tathehāpi pramātur ahamarthād vilakṣaṇaḥ sākṣī pratyagātmeti yuktam)
The connection of this objection with the ontology of substances is made immediately evident in another statement by the objector:
For, the fact of being a doer, etc., since they are commonly experienced like colour and other (qualities), are not qualities of the self. (ĀS 1942 p. 38)
(kartṛtvādir hi dṛśyatvād rūpādivan nātmadharmaḥ)
In other words, this Advaitin imagines a self which is pure, i.e. contentless, consciousness. In order to preserve this purity, everything is precluded to it, even knowledge.
Yāmuna, by contrast, answers that the self is the entity denoted by the word “I” and that this is also the knower, as proved by expressions such as “I know” (p. 39). Why is it so important for him that the self can know? An answer can be found a few pages below:
And cognition makes nescience disappear only as far as its specific content is concerned. (ĀS 1942 p. 42)
(jñānaṃ ca svaviṣaya evājñānaṃ nivartayati)
This is a precise attack against the Advaita soteriology. In fact, even in order to defeat nescience, which is the soteriological goal of Advaitins, one needs not a void consciousness, but an intentional knowledge. In fact, nescience is always nescience of something, and it can be defeated only by the cognition of the corresponding thing.
On the Viśiṣṭādvaita Vedānta concept of qualities and substances see this post.