Aristotle noted that "equality" is the plea not of those who are satisfied but of those who seek change, and the word has long been invoked in the name of social reform. It retains its force because arguments for equality put arguments for inequality on the defensive. But why is "equality" laudatory and "inequality" pejorative? In this first book-length analysis of the rhetorical force of equality arguments, Peter Westen argues that they derive their persuasiveness largely from the kind of word that "equality" is, rather than from the values it incorporates.

By focusing on ordinary...