According to Mīmāṃsā authors, texts different than the Veda are reliable insofar as they are based on the Veda. But how can one demonstrate it? The demonstration is usually based on the application of cogent evidence (arthāpatti): The Dharmaśāstras have no other root than the Veda, hence they are based on the Veda. The background of this argument is Mīmāṃsāsūtra 1.3.4 hetudarśanāc ca (''[Other texts are not valid] also in case one sees a reason [which could have lead someone to make them up, such as personal interest]''), which denies the validity of texts for which a different reason, such as greed or delusion, can be detected.
Thus, other texts are valid insofar as they are based on the Veda. This Veda-root may be, following Kumārila, either a branch of the Veda which is currently lost, or a compendium of Vedic elements which today's readers cannot recognise because they are scattered in various Vedic texts. Alternatively, the Vedic root might be the ''always inferred'' (nityānumeya) Veda, i.e., a Vedic text which is always inferrable, but has never existed as a directly perceivable texts. This claim is based on the idea that there have always been, without beginning, two sorts of Veda, one which is intrinsically available to direct perception (i.e., which can be heard) and one which is intrinsically only inferable.
In harmony with the Mīmāṃsā focus on what is currently the case, Prābhākaras do not postulate that the always inferable Veda was previously perceptible and has then at a certain point been lost. Rather, an ever inferable Veda must exist, since it inspires from time to time authors of valid texts such as the MDhŚā, but there is nothing which leads one to postulate that it has ever existed in a directly perceptible form. Tne Prābhākaras consider the two Vedas as being on the same level and say that the ever inferable one is inferred out of the proper conduct of good people or out of the fact that right Dharmaśāstras are composed.
The existence of this ever inferable Veda is one of the less-understood and more-criticised Prābhākara claims. Sheldon Pollock, for instance, makes fun of it in his The Language of Gods in the World of Men.
For the Mīmāṃā approach to the validity of the Veda, see this post.