Civilization is a stream with banks. The stream is sometimes filled with blood from people killing, stealing, shouting and doing things historians usually record, while on the banks, unnoticed, people build homes, make love, raise children, sing songs, write poetry, and even whittle statues. The story of civilization is the story of what happened on the banks. Historians are pessimists because they ignore the banks for the river.

William James (Will) Durant (1885-1981) American historian, teacher, philosopher

Quoted in Jim Hicks, “Spry Old Team Does It Again,” Life (18 Oct 1963)
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Durant says this is what he came up with when asked to sum up civilization in a half-hour ("I did it in less than a minute, this way").

This source is Durant recounting the passage in a Life magazine interview, which is often mis-cited as the origin. The quote is also sometimes vaguely attributed to his and Ariel Durant's The Story of Civilization, their 11-volume work (1935-75) (the Life article was on the publication of Vol. 8, The Age of Louis XIV (1963)), but I cannot find it in any of the volumes to that date.