A profession or trade that requires practice, networking and experience more so than knowledge and/or theory. HARD for solid facts and results. ART because of a lack of quantitative theory when compared to a HARD SCIENCE. See also, Soft Science.
Journalism for instance. The knowledge and theory of Journalistic ethics and morals are imperative, but developed people, interviewing and investigative techniques, among many, will take you a lot farther than being able to recite all of Boyle's laws (Chemical/Fluid Dynamics Engineer). Experience in the field and connections (Networking) are required for a hard art.