When the President starts lying he begins to need evidence to back up his lies because in this democracy he is questioned on his statements. It then percolates down through the bureaucracy that you are helping the Boss if you come up with evidence that is supportive of our public position and you are distinctly unhelpful if you commit to paper statements that might leak to the wrong people.

The effect of that is to poison the flow of information to the President himself and to create a situation where a President can be almost, to use a metaphor, psychotically divorced from the realities in which he is acting...."

-Daniel Ellsburg to the US Senate on Foreign Relations, May 13, 1970