For me, the problem with Rumsfeld's talk was never semantic. From, say, Chomsky, in a discussion of the philosophy of mind, then fine. But if it's the Secretary of Defense, and what he "knows they don't know" is that Iraq has WMDs (which it didn't), and that's put forward as a reason for going to war, I think there is cause for serious concern.

As Twain said, “It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so.”