The original Shankara, Adi Shankara, made it an absolute necessity that whoever sought to realize the spiritual Truth must seek out a guru. This injunction has hypnotized the Indians who came after him as it hypnotized those before his time because it was laid down in the Mundaka Upanishad long, long before. Shankara even warned his readers and hearers that even an expert student of the Vedas should not engage in such a search by himself. Yet there are several cases in Indian history where men have experienced this realization without any guru whatever.
-- Notebooks Category 1: Overview of the Quest > Chapter 6: Student-Teacher > # 76