Buddha’s silence on the subjects of a permanent Self and of the nature of nirvana, and his studious disapproval of metaphysical discussions, did not have the results he anticipated. Silence on questions of deep import acts as a spur to speculation.
This was what happened to Buddhist thought in subsequent centuries. A flood of metaphysical speculation on these forbidden themes ensued. And such speculation, without any guidance from the Master or his first disciples, and without any guidance from the Upanishads which were not to become widely known till Sankara a thousand years later, split up the Buddhist movement into sects upholding views many of which were clearly considered as heresies in the Master’s own time. Yamaka had a Sariputta to correct him of his nihilistic views. But Yamaka’s views did not die with his correction.
What Buddha wanted to avoid in the case of Vacchagotta -- bewildering his mind by saying there is no permanent Self -- became the source of bewilderment and confusion to his later followers.
These wrong views steadily penetrated the later Buddhist movement at the intellectual levels and in the eyes of many Buddhists and all non-Buddhists, became the main characteristic of that thought. This later intellectual movement found no point of contact with the vast mass of the Buddhistic population, which sought solace not in nirvana or its metaphysical formulations but in the worship of the Buddha through his idols, temples, processions, and pilgrimages of popular Buddhism; cut off from its intellectual sustenance, popular Buddhism slowly decayed and withered and got absorbed in the reviving Vedic religious movements of later Hinduism. And intellectual Buddhism found its challenge in Sankara in the eighth century, who succeeded in reuniting Indian philosophic thought with the thought of the Upanishads. Unlike the other post-Buddhistic movements which were merely revivalist and sectarian, Sankara’s was synthetic and constructive and inclusive.
The Advaita Vedanta of Sankara has absorbed all the essential elements of Buddhist thought; it is becoming increasingly clear that if Buddha’s teachings are to be provided with a metaphysical support, we have to search for it in Sankara’s Advaita. In the Buddhist philosophy (as presented to us in the Buddhist scriptures), we have, in the words of Dr. Oldenberg, only a ‘fragment of a circle, to complete which and to find the centre of which is forbidden, for it would involve an inquiry after things which do not contribute to deliverance and happiness’ (quoted by Edmund Holmes, The Creed of Buddha). If Buddhism had continued in the spirit in which Buddha had meant it to proceed, if it had not indulged in metaphysical speculations, but propagated only the Four Noble Truths and the Noble Eightfold Path, Sankara need not have appeared. But when against the exhortations of the Master and without his guidance, metaphysical attempts to complete the circle miscarried, resulting in intellectual confusion and spiritual anarchy, Sankara appeared and gave to humanity the priceless thought of Advaita Vedanta, which in the words of Thibaut (Introduction to The Vedanta-Sutras, p XIV), ‘is , from a purely philosophical point of view, and apart from all theological considerations, the most important and interesting one which has arisen on Indian soil; neither those forms of the Vedanta which diverge from the view represented by Sankara nor any of the non-Vedantic systems can be compared with the so-called orthodox Vedanta in boldness, depth, and subtlety of speculation.’
—This is an excerpt from Dynamic Spirituality for A Globalized World -- A commemorative volume of selections from the works of Swami Ranganathananda, late president of RK Math & Mission