Edward Snowden. Bernard Madoff. Vidkun Quisling. Palestinians who sell their land to Israeli Jews and inform on other Palestinians to security forces.Husbands who sleep with their wives’ best friends.These people allegedly committed acts of betrayal. They betrayed security-conscious American nationalists; otherwise sophisticated investors who belonged to Madoff’s “tribe”; Norwegians opposed to the Nazis; fellow Palestinians; spouses committed to their marital vows.In On Betrayal, Avishai Margalit, an emeritus professor of philosophy at the Hebrew University of Jerusalem, draws on incidents from the Bible, centuries of political history, novels, biographies and yesterday’s headlines in a meditation on a complex and contested concept. Citing E.M. Forster’s classic comment (“If I had to choose between betraying my country and betraying my friend, I hope I should have the guts to betray my country”) and the experience of whistle-blowers, Margalit points out that claims of more than one loyalty are at the core of betrayal.Distinguishing between “thin trust” (as expressed in the request to a stranger in an airport to “keep an eye on my stuff while I go to the bathroom”) and “thick trust” (as embodied in the promise to “love, honor, and obey, in sickness and in health, and forsaking all others, to be faithful as long as you both shall live”), Margalit maintains that betrayal is an act that undermines the latter.Sophisticated and stimulating, On Betrayal is at its best raising questions – and courting controversy. Along the way, it helps unpack a term that is often used and frequently misunderstood.On Betrayal is by turns abstract and practical. The “Jewish project,” Margalit writes, separates ethics (what you owe your own people) from morality (what you owe humanity) while Jesus and his disciples sought to forge a model of a loving community of the faithful “that supersedes conventional thick relations, and even the thickest of all relations, family relations.” Thick relations, Margalit demonstrates, can also be a matter of choice. Borrowing the term “elective affinities” from Johann Wolfgang von Goethe (who used it to explain the tendency of different substances to form chemical compounds), he indicates that family, friends, ethnicity, religion and social class can form strong human bonds. In such cases, he writes, betrayal hurts because it reveals that the person who has been wronged is, apparently, not the significant other – and not even all that special – in the eyes of the betrayer. Margalit also points out that while one act of treason (like one crime of murder) turns the actor into a “traitor,” betrayal (like lying) “seems to vacillate between the dispositional use and the normative use.”Most important, perhaps, Margalit emphasizes the importance of context in assessing acts of betrayal. Depending on the setting – military occupation, colonial rule, annexation – collaboration, for example, can be “a necessity of life, not a good option, but an acceptable one.”Margalit sees “no comparison” between “the horrendous coercion” the members of the Judenräte went through during World War II and what officials of Vichy France faced. For members of Jewish councils, of course, the threat was existential: do what the Nazis tell you to do or die. Many of them, moreover, thought they were postponing the danger to members of their family, “with the reasonable hope that they would be rescued by the advancing Allies.” Marshal Pétain, by contrast, “made the wrong ethical judgment on top of the wrong moral judgment by siding with a regime that was set on destroying the very idea of shared humanity” – and he betrayed people “with whom he felt he shared thick relations by debasing the values of their shared past.”Margalit concludes with a defense – of sorts – of betrayal. Transparency and privacy, he indicates, are perennially in conflict, with the pendulum often swinging in one direction and then another. It may well be impossible to govern (or for that matter, to live a fulfilled life) entirely innocently or transparently.“Like all good liberal values,” Margalit writes, transparency goes over well in times of peace, but badly during emergencies (perceived and real) and war.In this way of thinking, unmasking myths and revealing to the fickle masses how the sausage is made is not a “heroic service to humanity of handing over the divine secret of fire but a pyromaniac disservice to humanity of dangerously playing with fire.”At the same time, these security-centered political thoughts are often opposed by those who view “secrets as manipulative mechanisms of control.” By their account the real betrayers are “the people in power, the ‘gatekeepers’ of state secrets, whereas the saints are the whistle- blowers.”Viewed in this way, both “betrayal and hypocrisy are necessary by-products of civilized life the way urinating is a necessary by-product of drinking.” Glenn C. Altschuler is the Thomas and Dorothy Litwin Professor of American Studies at Cornell University.