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Many ancient scripts have been deciphered (see "The great decipherments" and The ancient scripts), but some significant ones have yet to be cracked. These fall into three broad categories: a known script writing an unknown language; an unknown script writing a known language; and an unknown script writing an unknown language. The first two categories are more likely to yield to decipherment; the third – which recalls Donald Rumsfeld's infamous "unknown unknowns" – is a much tougher proposition, though this doesn't keep people from trying.