Origin of "7th Art"

I’ve always heard of cinema being called the seventh art, but why? What are the other six arts? And why seven? There’s definately more than seven no matter how you cut it.

http://www.news.cornell.edu/Chronicle/96/7.25.96/kammen.html

I’ve never heard movies referred to explicitly as the 7th Art, but there is a movie history by Arthur Knight in 1957 called The Liveliest Art. Maybe that led into movies given highest honors.

Seldes increased the scope of his argument later in books like The Public Arts when he included television and radio as influencing popular culture.