A central tenet of whiteness studies is a reading of history and its effects on the present, inspired by postmodernism and historicism, in which the very concept of race is said to have been socially constructed in order to justify discrimination against non-whites. Since the 1800s, critics of the concept of race have questioned if human races even exist and pointed out that arbitrary categories based on phenotypical characteristics are chosen, and that the idea of race is not about important differences within the human species.