Boas also introduced the ideology of cultural relativism which holds that cultures cannot be objectively ranked as higher or lower, or better or more correct, but that all humans see the world through the lens of their own culture, and judge it according to their own culturally acquired norms. For Boas the object of anthropology was to understand the way in which culture conditioned people to understand and interact with the world in different ways, and to do this it was necessary to gain an understanding of the language and cultural practices of the people studied. By uniting the disciplines of archaeology, the study of material culture and history, and physical anthropology, the study of variation in human anatomy, with ethnology, the study of cultural variation of customs, and descriptive linguistics, the study of unwritten indigenous languages, Boas created the four field subdivision of anthropology which became prominent in American anthropology in the 20th century.