“The question whether moral requirements are universal comes up not only when we compare the motives of different individuals, but also when we compare the moral standards that are accepted in different societies and at different times. Many things that you probably think are wrong have been accepted as morally correct by large groups of people in the past: slavery, serfdom, human sacrifice, racial segregation, denial of religious and political freedom, hereditary caste systems. And probably some things you now think are right will be thought wrong by future societies. Is it reasonable to believe that there is some single truth about all this, even though we can't be sure what it is? Or is it more reasonable to believe that right and wrong are relative to a particular time and place and social background?”