“Don't discuss yourself, for you are bound to lose; if you belittle yourself, you are believed; if you praise yourself, you are disbelieved.”
Book III, Ch. 8
Attributed
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Michel De Montaigne 264
(1533-1592) French-Occitan author, humanistic philosopher, … 1533–1592Related quotes
“If you don't believe in yourself, then who will believe in you?”
Attributed to Korda in The Power of Choice (2007) by Joyce Guccione, p. 52, the earliest occurrence of such phrasing yet located is by Martin Lawrence, in "What Up?" in Upscale : The Successful Black Magazine (February 1993), p. 79: "If you don't believe in yourself, then who will believe in you? The next man's way of getting there might not necessarily work for me, so I have to create my own ways of getting there."
Disputed

Source: Everybody’s Autobiography (1937), Ch. 2

“If you don't believe in yourself, who will?' ~Maybeck”
Source: Disney after Dark

Earliest attribution found in Who Said That?: More than 2,500 Usable Quotes and Illustrations https://books.google.nl/books?id=7mn8AwAAQBAJ&pg=PT63 (1995) by George Sweeting. Online sources always attribute the quote to Augustine, but never specify in which of his works it is to be found.
Disputed