Suppose the racial poles in this country were reversed, the rules for deciding someone's race. For example, suppose that everyone of mixed race was considered to be "white," and only those of exclusively African background were considered "black." "One drop" of white blood, according to these rules, would make someone "white." (Absurd, right? But no more absurd and arbitrary than the "one drop" rule of traditional American racism.) Then Obama would be another white president like all the rest.

Now imagine, furthermore (an additional contrafactual), that all our presidents up to this point have been full-blooded African-Americans, with no European ancestry at all. So in this case, when Obama is inaugurated, tomorrow, he would become the first "white" president.