In classifying paintings, genre categories are based on subject matter. In nineteenth-century academic theory, there was a strict hierarchy of genres, with history painting at the top and representations of inanimate forms (landscapes and still lifes) at the bottom. Cole sought to translate "landscape into history" with ambitious series such as The Course of Empire (1834-36) and The Voyage of Life (1839-40), in order to elevate the genre of painting in which he specialized.