“It is generally easier to induce an opponent to maintain a preexisting belief than to present notional evidence to change that belief.”Central Intelligence Agency, Office of Research and Development · Deception Maxims: Fact and Folklore · 1980 · CIA, «Deception Maxims: Fact and Folklore» (aprilie 1980, declasificat în 2015), Maxima 1 «Magruder's Principle — the Exploitation of Preconceptions», p. 5
The most dangerous set of clues is the one that perfectly confirms what you already believed.
In 1980 a CIA research paper distilled historical deception cases into maxims for analysts. The first, Magruder's Principle, holds that reinforcing an existing belief is easier than building a new one. The classic case is Normandy: Hitler and most of his senior military advisers expected the landing in the Pas de Calais, the Allies knew this from intercepts, and the cover plan fed exactly that belief. Jones' Lemma, named after the British physicist R. V. Jones, adds that deception grows harder as the victim's channels of information multiply — but, within limits, the more channels are controlled, the more believable the story. Counter-deception follows from both. Write down what you already believe and what the adversary would want you to believe; if they coincide, raise your guard. Check how many genuinely independent channels the confirmation comes from, since controlled channels confirm one another. Look for evidence that should exist if the story is true and is missing. In a hypothesis matrix, carry deception as an explicit hypothesis. The opposite trap is paranoia: a remark attributed to Jones, known as Crabtree's bludgeon, warns that for any set of inconsistent observations someone can devise a coherent explanation, however complicated — so a deception story that explains everything proves nothing on its own.
Why it matters An analyst is easiest to deceive exactly where they feel most certain, because there the adversary need not persuade them of anything new.