“I know that many will call this useless work.”
The Notebooks of Leonardo da Vinci (1883), I Prolegomena and General Introduction to the Book on Painting
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Leonardo Da Vinci 363
Italian Renaissance polymath 1452–1519Related quotes

Ai Weiwei, interview in “ Change http://www.pbs.org/art21/watch-now/episode-change,” Episode 1, Season Six, Art: 21—Art in the Twenty-First Century, PBS, April 2012.
2010-, 2012

Source: Down and out in Paris and London (1933), Ch. 31
Context: Beggars do not work, it is said; but then, what is work? A navvy works by swinging a pick. An accountant works by adding up figures. A beggar works by standing out of doors in all weathers and getting varicose veins, bronchitis etc. It is a trade like any other; quite useless, of course — but, then, many reputable trades are quite useless. And as a social type a beggar compares well with scores of others. He is honest compared with the sellers of most patent medicines, high-minded compared with a Sunday newspaper proprietor, amiable compared with a hire-purchase tout-in short, a parasite, but a fairly harmless parasite. He seldom extracts more than a bare living from the community, and, what should justify him according to our ethical ideas, he pays for it over and over in suffering.

Adams as portrayed in the musical 1776 (1969); this has sometimes been cited as an actual quote of Adams.
Misattributed

To Leon Goldensohn (27 May 1946)
The Nuremberg Interviews (2004)

In Question Time, c. March 2012
"Labor cleans up after aftermath" http://www.abc.net.au/insiders/content/2012/s3445116.htm, in Insiders (ABC), 4 March 2012

A Path to Freedom (2010), p. 64