--- Carl von Clausewitz, quoted by Colin Powell, in a profile on Powell by Peter Grier
From the CSMonitor story:
Asked in an interview how he dealt – for years and years – with the firehose of news, opinions, advice, and criticism that washes over any top U.S. official, General Powell quoted one of the U.S. Army’s favorite military theoreticians, 19th-century German Gen. Carl von Clausewitz.
“There’s a great Clausewitzian expression which says ‘beware the vividness of transient events,’” General Powell said. “There are lots of transient events out there, and I am trying to beware of their vividness.”
Lenni Brenner in an Oct 2001 piece ascribes the quote to Colin Powell’s (with Joseph Persico), autobiography, My American Journey, 1995, p. 419:
I consoled myself with the words of Clausewitz: ‘The vividness of transient impressions must not make us forget that such truth they maintain is of a lesser stamp.'
I haven't been able to track down the original.