Indian thought knows basically two ways to justify the validity of the Sacred Texts: the Nyāya and the Mīmāṃsā one.
In synthesis, Mīmāṃsā authors argue in favour of the validity of the Veda due to the fact that it is the only instrument of knowledge through which one can know dharma. Mīmāṃsakas divide in fact what can be known into two precincts, on the one hand common experience, which encompasses what can be known through sense-perception and the other instruments of knowledge (inference, analogy and cogent evidence, to which Bhāṭṭa Mīmāṃsā authors only add absence), which ultimately depend on perceptual data. On the other hand there is dharma, which cannot be known through sense-perception and for which, therefore, the Veda is the only instrument of knowledge. Mīmāṃsā authors also claim that all cognitions, qua cognitions, are in themselves valid, unless and until a subsequent cognition invalidates them. However, since the Veda cannot be invalidated by data of different origin, given that it is the only instrument of knowledge regarding dharma, it remains valid.
Consequently, Mīmāṃsā authors need to demonstrate that dharma is really unattainable by other instruments of knowledge.
Bhāṭṭa Mīmāṃsā authors say that the dharma cannot be known through other instruments of knowledge because it is future and sense-perception only grasps present items (see ŚV codanā 115, translated in Kataoka 2011). The Prābhākara Mīmāṃsā solution is that the dharma is trikālānavacchinna 'not delimited by the three times'. This means that the dharma does not belong to the usual temporality, which is the characteristic of common experience. It rather belongs to a different dimension, that of what has to be done, which cannot be seized by the usual instruments of knowledge, which only grasp what is present, or grasped what was present and is now past, or will grasp what will be present.
Did Prābhākaras 'anticipate' modal logic?
On the distinction of precincts of application, so that perception cannot seize dharma, see this post.