It has been ages since we checked in with Mr. Difference, our resident expert on fine distinctions. It’s been so long that he is now Dr. Difference, having earned a PhD with his soon-to-be published thesis, “Dearly ‘Departed’: How to Tell the Difference Between Matt Damon and Leonardo DiCaprio If You Are Over the Age of 30.” We caught up with him at his vacation home in Mauritius, or perhaps it was Mauritania, where he is working on a sequel: “Kiss Me Cate: Was that Blanchett or Winslett in Babel?”

Dr. Difference, is there a distinction between an idea that’s “illegitimate” and one that’s simply wrong?

That’s a good question, and I can guess why you are confused. Take the now famous essay by Alvin Rosenfeld on anti-Israel rhetoric among Jews whom he calls “progressive.” Early in the essay he makes a distinction between criticism of Israel that is “anti-Semitic” and anti-Israel rhetoric that “goes well beyond legitimate criticism.”

Criticism of Israel’s settlement policies in Gaza and the West Bank and its “sometimes harsh treatment of Palestinian Arabs” is not anti-Semitic, according to Prof. Rosenfeld. However, “to call Israel a Nazi state,” he writes, “as is commonly done today, or to accuse it of fostering South African-style apartheid rule or engaging in ethnic cleansing or wholesale genocide goes well beyond legitimate criticism.”

First let me say that, as a connoisseur of distinctions, I would have preferred that he compare apples and apples — or, in this case, anti-Semitic statements with non-anti-Semitic statements. Rosenfeld seems to be suggesting, as he does throughout the essay, that certain odious comparisons involving Israel are twice cursed: they are both anti-Semitic and illegitimate. Or perhaps they are illegitimate because they are anti-Semitic. It’s hard to tell, because Rosenfeld has since told interviewers that he wasn’t calling anybody an anti-Semite.

But back to “legitimate.” Rosenfeld can’t be using “legitimate” the way my Webster’s does, that is, as something that is “sanctioned by law or custom.” Rosenfeld also has told interviewers that it was not his intention to “silence” the debate over Israel, which suggests he wouldn’t want to see purveyors of these views punished or censored.

In that case, I would say “legitimate” is the wrong word here, and explains in part why his paper was roundly criticized, even by many who share his pro-Israel views. “Well beyond legitimate criticism” suggests that Israel’s harshest critics are not just wrong, offensive, hysterical, hate-filled, and laughable — which they too often are — but beyond the protection of the law or custom.

Oh, c’mon, Doc. Aren’t you just splitting hairs?

That’s my job. But here’s why “legitimate” is an unhelpful word when it comes to public debate. I don’t think Rosenfeld is saying that there is anything illegal about some of these opinions, but the word does seem to be suggesting that some opinions do not belong in public discourse. And that will always sound, in the ears of those who champion free speech and intellectual inquiry, like an attempt to keep certain opinions from being heard, no matter the means being advocated.

If it were me, I would have written that it is intellectually dishonest, offensive, delusional, dangerous, and revealing of some serious scholarly lapses to question Israel’s right to exist, to compare Zionism to Nazism, or to characterize Israel’s policies in the territories as “ethnic cleansing,” “apartheid,” “genocide,” “racist,” and “exterminationist.” But I would steer clear of the word “legitimate,” or the conversation will always be about free speech, and not the ideas themselves.

But can’t you argue that some ways of talking about a subject are so toxic or potentially dangerous that they don’t belong in public discourse?

Perhaps, but the problem with labeling speech “legitimate” or “illegitimate” is, who gets to decide? To paraphrase Christopher Hitchens, the British-born contrarian and free speech absolutist, whom would you trust to serve as the official arbiter of what’s legitimate and what isn’t? (It was Hitchens who pointed out the hypocrisy of Muslim leaders who demanded that the Danish cartoons be declared “illegitimate” for public display even as they sought protection for their own hate-filled rhetoric under various countries’ “hate speech” statutes.) That doesn’t give “immunity” to those who spew false ideas or lies that incite hatred. Free speech also extends to those, like Rosenfeld, who can muster the facts and attract allies to fight back in the marketplace of ideas.

But don’t different segments of society and institutions make distinctions between “legitimate” and “illegitimate” opinions all the time? When it comes to the op-ed pages of major newspapers, you can’t tell me that “anything goes.”