Robert Spencer — the controversial author and founder of the blog Jihad Watch — spoke Tuesday at Stanford University at the invitation of the university's College Republicans. The event proceeded relatively peacefully, with minimal disruption.

But there were many who believed Stanford should never have allowed Spencer to speak in the first place, including a group of Stanford faculty and students who published an open letter urging the university to block Spencer's talk.

The argument of the letter's authors is that while they "fully support the principle of academic freedom that allows us to disagree about issues," Spencer's views on Islam are "not debatable" because they are "fundamentally dehumanizing."

Whenever the claim is made that an identity group is inherently less worthy of full personhood — whether that claim is made about people who are Muslim, Rohingya, Jewish, Black, trans or gender non-conforming, Bosnian, queer, immigrants, Mexican, etc. — it is always unacceptable.

This has quickly become one of the most common, insidious, and dangerously slippery-slope arguments against free speech on college campuses and beyond. Let's set aside for a moment that even most truly "dehumanizing" speech is protected by the First Amendment. (Although Stanford is not a public university, California's Leonard Law applies the protections of the First Amendment to non-sectarian private schools.) The reality on campus is that any debate over any controversial issue will, for proponents of this viewpoint, unjustly demean the value of someone's identity.

Consider students at the University of Florida who earlier this week vandalized promotional materials for an upcoming pro-life event on campus put on by the university's Young Americans for Freedom. In a Facebook message bragging about the vandalism, one student wrote: "just poured water on your lovely creations that are an insult to my entire major and life experiences!"

To others, an opposing view on immigration policy is an attack on the humanity of undocumented immigrants. As NYU professor and provost Ulrich Baer wrote last spring in The New York Times, "[s]ome topics, such as claims that some human beings are by definition inferior to others, or illegal or unworthy of legal standing, are not open to debate because such people cannot debate them on the same terms."

Meanwhile, after Laura Kipnis — the feminist Northwestern professor who was twice investigated by Northwestern for Title IX violations over her criticism of campus sexual politics — spoke at Wellesley College, the faculty on the school's Commission for Ethnicity, Race, and Equity issued a statement calling for changes to the outside speaker policy. Speakers like Kipnis, the statement said, require students to "invest time and energy in rebutting the speakers' arguments… in order to affirm their humanity."

When Heather Mac Donald, a vocal critic of the Black Lives Matter movement, spoke last spring at Claremont McKenna College, violent protesters attempted to shut down the event, forcing the Manhattan Institute fellow to give her talk via livestream. Three students from nearby Pomona College issued a statement saying "[t]he idea that the search for this truth involves entertaining Heather Mac Donald's hate speech is illogical. If engaged, Heather Mac Donald would not be debating on mere difference of opinion, but the right of Black people to exist."

And who can forget the reaction of Yale students to Erika Christakis's thoughtfully worded email, in October of 2015, questioning whether an institution of higher education should police the Halloween costumes of adult college students?

Students blasted Christakis and her husband, Nicholas, for failing to create a "safe space" for them, despite their reputations as nurturing residence mentors. In one article describing how her world was "shaken" by Christakis's "offensive" email, a Yale student wrote, "This kind of racism in disguise — where a false debate about 'free speech' is used to question people of color's humanity — needs to stop."

Two months after her email, Erika Christakis resigned from her teaching role at Yale, later explaining that she had "lost confidence" in her ability to teach in an environment where "full discussion of certain topics… has almost become taboo." In May 2016, both Nicholas and Erika announced that they resigned from their Silliman College duties to pursue academic work full time.

The "humanity denying" argument for censorship is not self-limiting, as these examples illustrate. If accepted, the argument can and will be used to shut down debate on a variety of issues profoundly important to us — including that "scholarly debate over affordable health care" the Stanford professors claim they would welcome.

Any serious health policy debate involves questions of priorities. And what might call humanity into question, if not a discussion of who is or isn't entitled to health care under some hypothetical new system? Disability activists, for example, were none too pleased with Princeton professor Peter Singer's discussion of disability in a New York Times article entitled "Why We Must Ration Health Care," accusing Singer (as they long have) of "promoting the devaluation of people with disabilities."

So the health care debate? It's probably off the table, too.