Read what is printed on the front of every Federal Reserve Note since FDR outlawed private ownership of gold in 1933. (Funny, before that creditors could and DID sometimes insist in payment in GOLD rather than paper.)
Also see my post at #63 on this thread.
ML/NJ
From 1933 to 1974 it was illegal for U.S. citizens to own gold in the form of gold bullion, without a special license. On January 1, 1975, these restrictions were lifted and gold can now be freely held in the U. S. without any licensing or restrictions of any kind.
So I could insist you pay me in gold if I wanted to!
Allowing other forms of barter or currency would allow the economy to function, at least on some level.