Steven Wienberg

Most of the world's great religions coexisted very comfortably with slavery. Part of the general moral improvement of the human race can be credited to the growth of science--a sense of rationality, a scientific view that we don't really differ that much from one another, that there is no divine right of kings and so on, there is no intrinsic racial difference that should allow us to enslave one race for the benefit of another race. People have just gotten less religious and more moral.