The notion of different selves within a single person is not new. It can be found in Plato, and it was nicely articulated by the 18th-century Scottish philosopher David Hume, who wrote, “I cannot compare the soul more properly to any thing than to a republic or commonwealth, in which the several members are united by the reciprocal ties of government and subordination.” Walt Whitman gave us a pithier version: “I am large, I contain multitudes.”