“If you want creative workers, give them enough time to play.”

Three Years in a Curatorship, By One Whom It Has Tried, 1886

Speech at UCLA, May 9, 2002.
Attributed

Speech in the Hippodrome, Darlington (25 May 1929), quoted in The Times (27 May 1929), p. 9
1920s

Interview with Inc. Magazine for its "The Entrepreneur of the Decade Award" (1 April 1989) http://www.inc.com/magazine/19890401/5602.html
1980s
Variant: You can't just ask customers what they want and then try to give that to them. By the time you get it built, they'll want something new.

Part One, chapter 5, page 27
Why Government Doesn't Work (1995)

Gerald Ford in a Presidential address to a joint session of Congress (12 August 1974)
Ford has also been quoted as having made a similar statement many years earlier, as a representative to the US Congress: "If the government is big enough to give you everything you want, it is big enough to take away everything you have."
"If Elected, I Promise…" : Stories and Gems of Wisdom by and About Politicians (1960) p. 193
Unsourced variants attributed to Goldwater include:
A government that is big enough to give you all you want is big enough to take it all away.
Remember that a government big enough to give you everything you want is also big enough to take away everything you have.
However, Karl Hess, a speechwriter for Goldwater, quoted Goldwater as having "repeatedly" said during the 1964 campaign that "the government strong enough to give you what you want is strong enough to take it all away." See The Death of Politics http://fare.tunes.org/books/Hess/dop.html, a Playboy article from 1969.
Misattributed

Presidential address to a joint session of Congress (12 August 1974)
Ford has also been quoted as having made a similar statement many years earlier, as a representative to the US Congress: "If the government is big enough to give you everything you want, it is big enough to take away everything you have."
"If Elected, I Promise…" : Stories and Gems of Wisdom by and About Politicians (1960) p. 193
Similar assertions have often been attributed to Barry Goldwater and Ronald Reagan. Some of the inspiration for such expressions may lie in "The Criminality of the State" by Albert Jay Nock in American Mercury (March 1939) where he stated: "You get the same order of criminality from any State to which you give power to exercise it; and whatever power you give the State to do things for you carries with it the equivalent power to do things to you."
1970s, Address to Congress (12 August 1974)

Attributed to Reagan from informal remarks to reporters 10 October 1984,and to students and faculty at St. John's University 28 March 1985.[citation needed] The statement was made a decade earlier by Gerald Ford in an address to a Joint Session of the Congress, 12 August 1974. It is sometimes attributed to Thomas Jefferson and Barry Goldwater.
Similar assertions have often been attributed to Barry Goldwater. Some of the inspiration for such expressions may lie in "The Criminality of the State" by Albert Jay Nock in American Mercury (March 1939) where he stated: "You get the same order of criminality from any State to which you give power to exercise it; and whatever power you give the State to do things for you carries with it the equivalent power to do things to you."
Misattributed
Variant: A government that is big enough to give you everything you want is also big enough to take everything you have.

Secrets of Closing the Sale (1984)
Variant: You can get everything in life you want if you will just help enough other people get what they want.