There’s the Heisenberg Uncertainty Principle - Nothing will ever be known with ultimate precision.
That applies in general to our state of knowledge.
As much s**t gets thrown at Rumsfeld, this was an incredible statement that was more true than any of his detractors could possibly understand. Because they did not know what they did not know, and did not care.
He was also completely honest and correct about not going to war with the army you want, but the army you have. Nobody in history has ever went to war with the army they thought they wanted.
“You can’t unbake a cake.”
- D. Rumsfeld