"It is rather a pity, considered from the standpoint of the professional politician or opinion-taker, that nobody knows exactly what "credibility" is, or how one acquires it. "Credibility" doesn't stand for anything morally straightforward, like meaning what you say or saying what you mean. Nor does it signify anything remotely quantifiable — any correlation between evidence presented and case made. Suggestively, perhaps, it entered the language as a consensus euphemism during the Vietnam War, when "concerned" members of the Eastern Establishment spoke of a "credibility gap" rather than give awful utterance to the thought that the Johnson administration was systematically lying. To restore its "credibility," that administration was urged — not to stop lying, but to improve its public presentation. At some stage in the lesson learned from that injunction, the era of postmodern politics began. It doesn't seem ridiculous now to have "approval ratings" that fluctuate from week to week, because these are based upon the all-important "perception" factor, which has in turn quite lost its own relationship to the word "perceptive.""