No one feels called upon to explain, or explain away, his good deeds or qualities, which are taken to arise from his essential being as naturally and irresistibly as a river from its source, but most people feel the necessity when it comes to their bad deeds or moral failings. That is why the problem of evil is so much more compelling than the problem of good, for evil—especially one’s own—is assumed to be against the natural order of things. Ever since Rousseau, man has been born not with original sin, but with original virtue. The question therefore arises as to how beings so inherently good often turn out to be so horribly bad.

One way of solving the problem is by appeal to the phenomenon of doubling. At least once a week, usually more often, a patient in my clinic describes himself to me as a Jekyll and Hyde. The assumption always is, of course, that the Jekyll is the real him or her, while the Hyde is an intruder,...