The Teacher who best represented this tradition in the eyes of Śaṅkara was Gauḍapāda, author of four ‘Books’ of ‘Kārikās’ (mnemonic verses) on the short Māṇḍūkya Upanishad. Unlike the authors of the Brahma Sūtras, Gauḍapāda insists very strongly on the illusory or phenomenal character of the world, and claims that in this he is only following an earlier tradition for the interpretation of the upanishadic texts.
Three important principles used by Gauḍapāda and Śaṅkara for the interpretation of the upanishadic texts are, however, found in the earlier Mādhyamika teaching.
First, there is the principle that the transcendent is conveyed indirectly by attributing empirical characteristics to it that are subsequently denied.
Secondly there is the principle that ‘The enlightened ones (Buddhas) taught the spiritual truth through resort to two standpoints, that of the surface-truth (saṃvṛti-satya) and that of the final truth (paramārtha)’ and ‘One cannot teach the supreme truth except on the basis of the surface-truth’.
And thirdly the principle that, on the basis of the distinction between the two truths, the traditional texts may be divided into those, called nītārtha, which express the fundamental truth in terms of negations, and the rest, called neyārtha, which are not to be taken literally at their surface value but have to be interpreted as indirectly supporting the fundamental texts.
We may say, then, Gauḍapāda clearly considered that Buddhist dialectic, Buddhist methods of textual interpretation and Buddhist yoga were all powerful aids in attaining practical realization of the ancient upanishadic wisdom.
Why is it, then, that Gauḍapāda warmly acknowledges his debt to the Mahāyāna, while Śaṅkara is hostile to Buddhism in every aspect and explains most of Gauḍapāda’s references to Buddhism away? The answer to this question seems to lie in historical developments that occurred between the time of Gauḍapāda and Śaṅkara.
The mystical Inspiration that sustained the Mahāyāna Teachers of earlier centuries seems to have waned, and the leading Buddhist thinkers of the new period, speaking generally, tended to abandon the higher knowledge in their enthusiasm for the problems of logic and epistemology.
The typical Buddhist for Gauḍapāda was the author of the Laṅkāvatāra Sūtra or Nāgārjuna: the typical Buddhist for Śaṅkara was Dharmakīrti, and mutatis mutandis one might compare the transition from the Laṅkāvatāra Sūtra to Dharmakīrti to the transition from St Bonaventura to Kant.
And one is still left wondering whether Śaṅkara had any opportunity for studying the earlier Mahāyāna texts in sufficient depth to enable him to realize the extent of Gauḍapāda’s borrowing. Did he have any access at all to the earlier texts from which Gauḍapāda was quoting? Or was he dependent for his statement of Buddhist positions on contemporary Buddhist sources, eked out by an astute use of scraps of earlier Buddhist doctrine retained in Brahminical oral tradition?
After all, his prime concern was the protection of upanishadic Advaita from the attacks of Buddhist and other opponents of the Veda of his own day, and not the restitution of ancient Buddhist texts in the manner of a modern philologist.
The truth, Śaṅkara goes on to say, is ‘intuitively savoured only by those exceedingly venerable monks of the Paramahaṃsa order who have given up all desires for anything external, who depend on nothing outside their own Self, who have risen above the whole system of caste and stages of life (āśrama) and who are solely preoccupied with the knowledge proclaimed in the Upanishads. And this truth… has been formulated in four chapters of verses by one (i.e. Gauḍapāda) who followed the true tradition. And even today it is only they who teach it and no one else’.
~Alston, Absolute, pp34-44