“If people knew how hard I had to work to gain my mastery, it would not seem so wonderful at all.”
Found attributed to Michelangelo in non-specialist publications as early as 1929 https://books.google.com/books?id=-0YhAQAAMAAJ&dq=If+people+knew+how+hard+I+had+to+work+to+gain+my+mastery%2C+it+would+not+seem+so+wonderful+at+all.&focus=searchwithinvolume&q=michelangelo, but no source is known. Not found in any known biography of Michelangelo.
Disputed
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Michelangelo Buonarroti 27
Italian sculptor, painter, architect and poet 1475–1564Related quotes

after 2000, Gerhard Richter: An Artist Beyond Isms' (2002)

On Queen Elizabeth I of England, in 1587; reported in Emil Reich, Woman Through the Ages: Volume 2 (1908), p. 38. Alternately reported without the phrase "each other" and ending with "would have ruled the whole world".
Attributed

Benicka (1980) commented:
The frescoes of Raphael and the Pompeian murals that he saw there definitely confirmed what Renoir had begun to feel about his own art; that it was becoming too amorphous in character and was weak in design.
undated quotes
Source: ''Renoir'', by A. Vollard, Paris, 1920, p. 135; as quoted in: Corinne Benicka (1980) Great modern masters. p. 130;

As quoted in "Paris (1897-1904)", and in Bulletin, Volume 53 by the Sri Aurobindo International Centre of Education (Pondicherry, India) http://books.google.co.in/books?id=VMzWAAAAMAAJ