“Fear arises from the second; where there is no second, there is no fear.”
— Adi ShankaraCited by Shankara from Brihadaranyaka Upanishad 1.4.2 in his Brahma Sutra commentary; paraphrase of the Upanishadic verse

Commentary

This statement reproduces the logic of the Brihadaranyaka Upanishad, which Shankara cites to argue that fear is a consequence of the perception of duality. Where the self perceives something other than itself, it experiences the insecurity of separation; where no second exists, fear has no ground. Shankara uses the passage to support the non-dual teaching that the Self, being without a second, is free from fear. Eliot Deutsch notes that the verse is a favourite of Advaita writers because it links ontology with the psychology of liberation. Sengaku Mayeda shows how Shankara's use of the Upanishadic citation is shaped by his concern to make the scriptures consistent with the non-dual reading. Richard King observes that the passage was debated by Vedantic rivals, who interpreted the second differently. Paul Hacker's study of Shankara's exegetical method emphasises the care with which he selects scriptural evidence. The statement is a faithful rendering of a passage that Shankara treats as authoritative, and its use in his commentary is well documented.