On the connection between religious and racist discrimination. The concept of different human races and the resulting racism are often seen as phenomena of modernity that presuppose biological knowledge and biopolitical thinking. However, the discrimination and persecution of people based on their biological origin is much older and can be traced back at least to the Middle Ages. How can we understand the long history of these cultural demarcations, and what can we learn from it for today's manifestations of modern racism? As an internationally recognized expert on the history of Jewish, Christian, and Islamic cultures, David Nirenberg intertwines in this essay the examination of biologically driven discrimination and persecution with the religious discrimination of people. Using the example of Castilian Christians in the 14th and 15th centuries and the Muslim Almohads in North Africa in the 11th and 12th centuries, he demonstrates how different religious cultures produced concepts that bore remarkable similarities to modern racist discrimination. Ultimately, he questions the history of a connection between cultural concepts of similarity and difference with ideas of biological reproduction.