  Du Bois also holds that the spiritual distinctiveness of a socio-historical race cannot be explained in terms of physical, biological facts. Like Wilhelm Dilthey, whose Introduction to the Human Sciences (1883) had appeared a decade before Du Bois heard him lecture in Berlin, he expressly questions the possibility of causal explanations that reduce spiritual facts and differences to biological facts and  differences.[9]  Thus, Du Bois rejects the physio-biological reductionism characteristic of nineteenth century racial science: the thesis that physical racial differences causally explain the spiritual and cultural differences between racial groups. Spiritual differences have historical and social causes (law, religion, and so on), which Du Bois takes to be causally independent of biological racial facts (1897a,  54–55).[10]