If an elderly but distinguished scientist says that something is possible he is almost certainly right, but if he says that it is impossible he is very probably wrong.
Source: In New Yorker, 9 Aug 1969.
-- Arthur C Clarke, (Dec 16 1917-2008), English science writer, science-fiction writer; He was an early advocate of space flight and cosmic unity; wrote 2001: A Space Odyssey, 1968.