As I discuss in this question, by far the most popular school of Hindu philosophy is the Vedanta school, which bases its tenets on the doctrines laid out in the Brahma Sutras, a work by the sage Vyasa which summarizes and systematizes the philosophical teachings of the Upanishads. (You can read the Brahma Sutras here.) But the Vedanta school didn't always have the dominant position in Hindu philosophy; before the time of Adi Shankaracharya the dominant school of Hindu philosophy was the Purva Mimamsa school, which I discuss here. In contrast to the Vedanta school which is devoted to analyzing the Jnana Kanda of the Vedas, i.e. the Upanishads, Purva Mimamsa focuses on analyzing the Karma Kanda of the Vedas, i.e. the Samhitas and Brahmanas.
Now as I discuss in this question, one of the differences between the Purva Mimamsa school and the Vedanta school is that the Purva Mimamsa school believed that Vedas only give knowledge about Dharma, whereas the Vedanta school believes the Vedas (specifically the Upanishads) also give knowledge about Brahman. Now in this section of the Sri Bhashya, the commentary on the Brahma Sutras written by the Sri Vaishnava Acharya Ramanujacharya, he presents one of the Purva Mimamsa school's arguments for their belief that the Vedas do not give knowledge of Brahman. The Purva Mimamsa argument goes that even if meditating on Brahman using the descriptions of Brahman given in the Upanishads (specifically the 32 Brahmavidyas) leads to Moksha, that does not mean that the Upanishads' descriptions of Brahman are correct or even that Brahman exists. After all, you can cheer up a sick person or a child by telling them something that's false, so perhaps you can give someone the eternal bliss of Moksha by telling them to meditate on some made-up entity called Brahman:
Nor must it be said that meditation is a kind of continued remembrance, and as such requires to be defined by the object remembered; and that the demand of the injunction of meditation for something to be remembered is satisfied by texts such as 'All this is that Self', 'the True, knowledge, infinite is Brahman,' &c., which set forth the nature and attributes of Brahman and--forming a syntactic whole with the injunctions--are a valid means of knowledge with regard to the existence of the matter they convey. For the fact is that the demand on the part of an injunction of meditation for an object to be remembered may be satisfied even by something unreal (not true), as in the case of injunctions such as 'Let him meditate upon mind as Brahman' (Kh. Up. III, 18, 1): the real existence of the object of meditation is therefore not demanded.... Even if [the Vedanta-texts] are viewed as independent sentences, they accomplish the end of man (i.e. please, gratify) by knowledge merely--being thus comparable to tales with which we soothe children or sick persons; it does not lie within their province to establish the reality of an accomplished thing, and hence Scripture cannot be viewed as a valid means for the cognition of Brahman.
Here is how Ramanujacharya refutes this argument:
The assertion again that a statement referring to some accomplished thing gratifies men merely by imparting a knowledge of the thing, without being a means of knowledge with regard to its real existence--so that it would be comparable to the tales we tell to children and sick people--, can in no way be upheld. When it is ascertained that a thing has no real existence, the mere knowledge or idea of the thing does not gratify. The pleasure which stories give to children and sick people is due to the fact that they erroneously believe them to be true; if they were to find out that the matter present to their thought is untrue their pleasure would come to an end that very moment. And thus in the case of the texts of the Upanishads also. If we thought that these texts do not mean to intimate the real existence of Brahman, the mere idea of Brahman to which they give rise would not satisfy us in any way.
My question is, what is the logic of Ramanujacharya's refutation of the Purva Mimamsa argument given above?
Here is the gist of Ramanujacharya's argument as I understand it:
If someone thought the Upanishads do not give valid knowledge about Brahman, then meditating on Brahman using the statements of the Upanishads would not get them Moksha.
But meditating on Brahman using the statements of the Upanishads does give Moksha.
Therefore, the Upanishads do give valid knowledge about Brahman.
But I don't understand the justification for going from step 2 to step 3. I agree that if the Upanishads didn't give valid knowledge about Brahman, and someone knew that, then they wouldn't be able to attain Moksha by meditating on Brahman using the statements of the Upanishads. But what about someone who did not know that? What if the Upanishads did not give valid knowledge about Brahman, but people thought that they did? In that case, why couldn't meditation on Brahman using the statements of the Upanishads lead to Moksha?
Is the issue that given an infinite amount of time, you would eventually learn that the Upanishads do not give valid knowledge about Brahman? Or is the issue that if the Upanishads didn't give valid knowledge about Brahman, then they would only grant Moksha in certain cases (i.e. where there is ignorance), whereas we know that they actually grant Moksha in all cases?
Do any subcommentaries on Ramanujacharya's Sri Bhashya shed light on this issue? Note that I'm a Sri Vaishnava, so I'm not trying to criticize Ramanujacharya here; I'm just trying to understand what he's saying.