What is anti-Semitism?

William I. Robinson, a professor of sociology at UC Santa Barbara, probably shouldn't have been surprised when he found himself in the news earlier this month. He had, after all, forwarded an e-mail to his students that juxtaposed images of Palestinians caught up in Israel's recent Gaza Strip offensive with Jewish victims of the Nazis. The e-mail included graphic photographs of dead Jewish children from the 1940s alongside similar photos from Gaza. In a cover note, Robinson called the images "parallel" and compared Gaza to the Warsaw Ghetto.

The outcry built slowly. First, a few students complained; then, organized groups became involved. Two national Jewish leaders accused Robinson (who is himself Jewish) of anti-Semitism, and the university's Academic Senate opened an investigation and is considering disciplinary proceedings. Articles about the controversy have been published all over the world and have given rise to fundamental questions:

Is it ever acceptable to compare Israelis to Nazis? When does criticism of Israel become anti-Semitism? And who should make these calls? Below, The Times asks and answers a few questions to help frame the debate.

Let's start with an easy question. What is anti-Semitism?

Actually, that's not easy at all; scholars, philosophers and policymakers have debated the question since the 19th century. The U.S. State Department has defined the term simply but vaguely: "Anti-Semitism is discrimination against or hatred toward Jews."

So how do we recognize it?

That was easier in the bad old days. Who could mistake the violent attacks on Jews across Europe during the First Crusade in 1096? Or the expulsion of Jews from England in 1290 and from Spain in 1492? Demonization of Jews, forced conversions, ghettoization, pogroms and the Holocaust -- all were manifestations of classic European anti-Semitism. So were Shakespeare's Shylock and Dickens' Fagin (described as "shriveled" and "repulsive," and referred to simply as "the Jew" more than 200 times in "Oliver Twist").

But today, determining what is or is not anti-Semitism is generally a more nuanced business, at least in the West. Is it anti-Semitic or merely factual to say that Hollywood is controlled largely by Jews? (Remember: Most of the big studio chiefs are Jewish.) Or to note (as some critics of the Iraq war did) that many of the neoconservatives who helped devise the war's intellectual rationale were Jewish -- and possibly harbored a dual loyalty to Israel? Or to point to the existence of a powerful "Israel lobby" that wields substantial influence on Capitol Hill?