Translation is an art .（不可数）翻译是一门艺术.既然art是不可数名词为什么还要用an来修饰啊?

作"艺术品;美术品,艺术;美术"时不可数,如
There has never been an exhibition of African Art ever held in our small town.
在我们小镇上从没举办过非洲美术品展览.
She teaches art history.
她教授艺术史.
作"技术,技艺;技巧"时可数,如
Language teaching is both a science and art.
语言教学既是一门科学也是一门艺术.