The theistic traditions posit that it is possible to understand the Sacred Texts by the will of God, who reveals them. In the atheistic Mīmāṃsā, on the other hand, the possibility of understanding is based on our linguistic expertise: We have to rely on worldly meanings of words when reading the Sacred Texts since we have no other key to their interpretation. Hence, the mastery of worldly meanings is a pre-condition for the understanding of a Sacred Text. But what if that text prescribes a kind of duty which is fully new (apūrva), unprecedented, that is, non-attainable through any other (worldly) kind of knowledge? Should it not remain beyond any possible grasp?
This issue raises thought-provoking questions for all theological discourse. How can the non-human be expressed in terms accessible to human beings?
Let me now sketch the problem and its solution within Tantrarahasya IV.
Rāmānujācārya lets a Prābhākara propose the option that even in worldly sentences prescribing something to be done, what one grasps is the pure ``ought", so that one can grasp the ought in itself even in mundane commands (TR IV 9.6). Rāmānujācārya dissents. According to him, it is only through its link to action, which can be experienced even in this world, that we can understand what an ought means, and, hence, understand it even in its apūrva-garb in the Sacred Texts. Thus, Rāmānujācārya confirms the Mīmāṃsā commitment to our common experience even in regard to the Veda.
This final position of Rāmānujācārya has linguistic roots. According to both the Bhāṭṭa and the Prābhākara school of Mīmāṃsā, the relation betweeen a word and the entity it means is fixed (nitya). Nonetheless, this does not mean that everyone, upon hearing for the first time a word, automatically understands its meaning.
Rather, one needs first to acquire proficiency in language through exposure to the usage of elders and through witnessing the connection of this verbal usage to physical actions (both these aspects may be referred to as vyavahāra). E.g., after having heard one's grandfather ordering:“Bring [the] cow!," one sees one's father bringing a cow. Through many similar instances, one eventually learns the meaning of the words “Bring!” and “cow”.
However, according to the Prābhākara Mīmāṃsā, the meaning conveyed by the Veda is a duty (kārya) that is unprecedented (apūrva). Hence, how could it be possible to learn the relation between a word and a meaning in the case of an unprecedented duty (apūrvakārya) through the usage of one's seniors? And if this is not possible, how could one understand the meaning of the Vedic words referring to it?
In fact, the relation between Vedic words and the unprecedented duty is fixed, but a meaning can be grasped only by people who have previously understood, by means of the linguistic usage of senior speakers, the relation between the signifiying word and its meaning. Hence, a relation lying beyond the possibility of being grasped in the usage of senior speakers would be fixed, but unattainable and useless for human beings. Nor can it be said that one can learn the meaning of Vedic words referring to an apūrvakārya through the Veda itself, as in this case there would be a vicious circle (the elders' usage would depend on the Veda, whose understanding depends on the elders' usage).
For some tables on this topic, see here.