As a rhetorical ideal, greater opportunity is hard to beat. Just about all candidates for high elected office declare their commitments to promoting opportunity – who, after all, could be against it? But opportunity is, to borrow a term from the philosopher and political theorist Isaiah Berlin, a "protean" word, with different meanings for different people at different times.

Typically, opportunity is closely entwined with an idea of upward mobility, especially between generations. The Amer...