There's another way to phrase that and that is that the absence of evidence is not the evidence of absence. It is basically saying the same thing in a different way. Simply because you do not have evidence that something does exist does not mean that you have evidence that it doesn't exist.
Donald RumsfeldWe do know that the Iraqi regime has chemical and biological weapons. His regime has amassed large, clandestine stockpiles of chemical weapons - including VX, sarin, cyclosarin and mustard gas. His regime has amassed large, clandestine stockpiles of biological weaponsโincluding anthrax and botulism toxin, and possibly smallpox.
Donald RumsfeldDon't automatically obey Presidential directives if you disagree or if you suspect he hasn't considered key aspects of the issue.
Donald Rumsfeld