[T]here are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns – there are things we do not know we don't know.

I'm no fan of Donald Rumsfeld but when he said the above he embedded a pearl of wiki wisdom that doesn't get discussed as much as it should.

there are known unknowns

When writing in a wiki, it is easy, possible and appropriate to LinkAsYouThink, creating a link to a page that doesn't yet exist.

This creates a known unknown: something for which you have an awareness of ignorance that ought to be remedied.

Update 20120504:

Our physics doesn't even hint at how they did it. We call it a quantum snapshot, but that doesn't mean we have clue one about what was involved in producing it. That's just a name we give it to hide our ignorance.

That's Verity Auger speaking in the book Century Rain by Alastair Reynolds. She's wrong.

We don't give names to things to hide our ignorance. We give names to things so we can fix our ignorance.

The name gives us a lever to use to work against the ignorance.

Overcoming ignorance is what wikis are for. It's what they do. They make us less dumb. Large wiki systems like Wikipedia are great because they can make someone less dumb by reading something somebody else knows. Small wiki systems (for individuals and collaborating teams) are greater because they can help make someone less dumb about something nobody knows by revealing gaps and making a place where the gap can be filled.