[jeremysmyth]

It's nothing at all like that. Rumsfeld (much as it pains me to support anything he said) was categorizing knowledge in a way that's actually quite useful.

Example: In education it's important for students to learn a wide range of things about a subject, even things they didn't know that they needed. This is why self-taught students often have large but important holes in their knowledge; they simply didn't know what they had to study, so they didn't look for it.

If you know that you don't know something, that's a "known unknown", and you can plan to research it. If you don't know that you don't know something, then that's an unknown unknown, and that's a much harder problem to fix without external help or accidental discovery.