Intrinsic validity means that each cognition is in itself valid, unless and until the opposite is proven. I do not need to prove that I am typing in order to know that I am. I know that I am typing unless and until something shows me that I am wrong (e.g., I wake up and realise I was only dreaming of typing).
One of the main arguments in favour of intrinsic validity is that if we needed a further cognition Y to know that X is right (that I am in fact typing), we would then need a cognition Z to know that cognition Y is right and so on. Veṅkaṭanātha discusses the issue in its original context, namely the commentary on the Pūrva Mīmāṃsā Sūtra 1.1.3. Interestingly enough, he frames his objector as if he were quoting from a specific text:
As for the said regressus ad infinitum (anavasthāna) in the investigation about the instruments of knowledge, namely, “the instrument of knowledge which has to be investigated should be investigated through another [instrument of knowledge] and that other by an [instrument of knowledge] other than itself [and so on ad infinitum]”, that too is stupid, because the investigation operates [only] in regard to topics which are doubted (whereas no one would doubt the validity of the instrument for a thorough investigation of the instrument to know dharma), and because we do not agree about the rule (niyama) according to which doubt [must] regard each element respectively following (i.e., it is not agreed upon that doubt regards again and again each following instrument, as assumed by the objector), and because distrust (aśaṅkā) ends up (avadhi) in contradictions (with one’s own actions, since nothing would be possible if one had to verify everything), and because the reasoning about possible alternatives (tarka) operates only as long as there is doubt, or it (reasoning) ceases (viśrama) in the case of whatever instrument of knowledge, etc., which brings about (nirvah-) [knowledge about] itself and another (its object) (svaparanirvāhaka).
yat tu pramāṇaparīkṣāyām anavasthānam uktam— pramāṇaṃ parīkṣyamāṇam anyena parīkṣyeta, taccānyat tato ‘nyeneti— tad api mandam; sandigdhaviṣaye parīkṣāpravṛtteḥ | uttarottareṣu ca sandehaniyamānabhyupagamāt | āśaṃkāyāś ca vyāghātāvadhikatvāt | yāvadāśaṃkam eva ca tarkapravṛtteḥ | svaparanirvāhake vā kasmiṃścit pramāṇādau viśramāt (SM ad PMS 1.1.3, 1971 pp. 44–45).
The passage is quite dense, since Veṅkaṭanātha updates the discussion through a reference to Udayana’s Nyāyakusumañjali 3.7 and the contradictory nature of doubt (śaṅkā ced anumāsty eva na cec chaṅkā tatastarām | vyāghātāvadhir āśaṅkā tarkaḥ śaṅkāvadhir mataḥ || 7 ||). Udayana appears to say that doubt is contradicted by the fact that one in fact undertake actions.
The frame yat tu…iti “As for the said…” makes me think that Veṅkaṭanātha is repeating either from his own text or from another specific one (as it is the case whenever he uses this frame), but I could not locate any source for the passage. Similar ideas are found both in the section preceding Udayana’s verse quoted above (where they regard inference), and (in nuce) in the Vṛtti on the PMS quoted by Śaṅkara, to which Veṅkaṭanātha refers some lines before and some lines after the ones reproduced here.
Did Veṅkaṭanātha know further portions of it?