The ability to foretell what is going to happen tomorrow, next week, next month, and next year – and to have the ability afterwards to explain why it didn't happen.
Source, in full
Newspaper interview (1902), when asked what qualities a politician required, Halle, Kay, Irrepressible Churchill. Cleveland: World, 1966. cited in Churchill by Himself (2008), ed. Langworth, PublicAffairs, p. 489 ISBN 1586486381