Modern critics tend to pass over the concerns of earlier centuries, such as formal categories or the place of moral or aesthetic value; some analyse texts as self-contained entities, in isolation from external factors, while others discuss them in terms of spheres such as biography, history, Marxism, or feminism. Since the 1950s the concepts of meaning and authorship have been explored or questioned by structuralism, post-structuralism, post-modernism, and deconstruction.