There are six types of known.

The JC’S favourite Big Ideas™
An unknown known, yesterday
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The Rumsfeld three:

And the Jolly Contrarian three:

Say what you like about the neocons, but this, from Donald Rumsfeld was a modern classic:

Reports that say that something hasn't happened are always interesting to me, because as we know, there 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—the ones we don't know we don't know. And if one looks throughout the history of our country and other free countries, it is the latter category that tend to be the difficult ones.

Rumsfeld’s knowns and unknowns

The JC’s further categories of known

Those who cannot remember the past are condemned to repeat it.

George Santayana

Not included in Rumsfeld’s taxonomy (perhaps because it doesn’t make much sense at first) are these JC coinages:

The unknown known. Now, before you write this off, well — see theFinancial Services Modernization Act of 1999 for the sorts of things a well-ordered market can forget over sixty-odd years. The JC — who likes a pet theory, as readers will know — feels that unknown knowns might be the riskiest bucket of the lot.

The “suspected known”: The embarrassed press officer’s best friend, the “unsubstantiated allegation”. This is one that the press officer knows to be true but believes nobody else can prove, and which everyone else knows perfectly well to be true, but knows they can’t prove.

See also