Critic John Rockwell says that art rock is one of rock's most wide-ranging and eclectic genres with its overt sense of creative detachment, classical music pretensions, and experimental, avant-garde proclivities. In the rock music of the 1970s, the "art" descriptor was generally understood to mean "aggressively avant-garde" or "pretentiously progressive". "Art rock" is often used synonymously with progressive rock. Historically, the term has been used to describe at least two related, but distinct, types of rock music. The first is progressive rock, while the second usage refers to groups who rejected psychedelia and the hippie counterculture in favor of a modernist, avant-garde approach defined by the Velvet Underground. Essayist Ellen Willis compared these two types: