They were not concerned merely with an intellectual quest for the self or
reality, the quest which the sages were set was experiential. There are
passages of sophisticated philosophical analysis and argument in the
Upanishads, but in the end it is not intellectual conviction but experience and
the resultant knowledge or realisation that is the aim of Upanishadic
teaching. Given their focus on the experience of meditation and on the
disclosure of being that it offers, it is no surprise that the Upanishads were
used as a sourcebook and reference point not only by orthodox thinkers
throughout religious history, but also by nastika dissidents. Buddhist texts,
for example, are rich with material drawn from the early Upanishads.
The Upanishadic tradition did not go unchallenged. While the earliest
Upanishads predate the Buddha, many of the later Upanishads are from his
period and later. Even before the major attack on orthodoxy was mounted by
Buddhists and Jains, other teachers were promulgating dissident doctrines
which denied the truth of Upanishadic teachings. We need to see the
Upanishads not as the products of a serene, self-confident, unchallenged
spiritual tradition, but as those belonging to an early age, presenting the
spiritual manifesto of orthodoxy under siege. In which case the position of
the sages seems all the more interesting in that they then represent not a
defensive rearguard action, but rather seems to have a critical response to
their own tradition while still being loyal to it. Like the nastikas they could
oppose the priestly evaluation of rituals and rites, but unlike them remained a
loyal part of the Vedic community.