For years, fans of the Batman comics have puzzled over a mystery at the heart of the series: why doesn't Batman just kill his arch-nemesis, the murderous Joker?
The two have engaged in a prolonged game of cat-and-mouse. The Joker commits a crime, Batman catches him, the Joker is locked up, and then invariably escapes.
Wouldn't all this be much simpler if Batman just killed the Joker? What's stopping him?
Enter philosopher Immanuel Kant and the deontological theory of ethics.
At least, that's how the discussion progresses in a growing number of philosophy classes in the US.
Cultural and media studies have paved the way for universities to incorporate pop culture into their curriculum. These days it is not uncommon to find a television studies class alongside 17th-Century literature in the course listings of an English department.
Now, philosophy professors are finding superheroes and comic books to be exceptionally useful tools in helping students think about the complex moral and ethical debates that have occupied philosophers for centuries.
Moreover, superheroes are attracting students to a discipline often perceived as overrun by musty books, suede elbow patches and bow ties.
William Irwin, a philosophy professor at King's College in Pennsylvania, edits the Blackwell Philosophy and Pop Culture Series, which includes titles such as Batman and Philosophy, and X-Men and Philosophy.
He says there's nothing unusual about using popular references to illustrate complex theories.
"This is what philosophy has tried to do from the very beginning," he says. "Philosophy starts with Socrates in the streets of Athens taking his message to the people and speaking in their language - agricultural analogies and common mythology."
Through the centuries, though, philosophers retreated into academia, creating a convoluted vocabulary that can appear inaccessible to the average first-year university student - those "deontological" ethics for example.
Christopher Bartel, an assistant professor of philosophy at Appalachian State University, asks students to read the graphic novel Watchmen in order to explore questions about metaphysics and epistemology.
In one class, he uses the character of Dr Manhattan, who claims that everything - including people's psychology - is predetermined through all the causal laws of physics.
Mr Bartel uses this to teach theories of determinism and free will, and the moral responsibilities entailed in those world views.
Mr Bartel says his course - Philosophy, Literature, Film and Comics - is a "fantastic recruiting tool", and that more of its students go on to specialise in philosophy than students in any of his other courses.
"I usually have students read Plato, Aristotle and Hume in introduction to philosophy courses. They often find it interesting, but get scared away by just how hard it is to read the stuff," Mr Bartel told the BBC.
"Comic books can provide really good illustrations of these philosophical ideas without scaring them off."
He says there are always students who think the course will just be an easy A grade, but they soon realise that despite the fun nature of the material, the work is deeply serious.
Great power, great responsibility?
For Christopher Robichaud, who teaches ethics and political philosophy at Harvard's Kennedy School of Government and Tufts University, superhero-based thought experiments can help people grapple with ethical dilemmas in an unsentimental fashion.
Imagine for example, that you are Peter Parker (aka Spider-Man) and you have just discovered that you have superpowers. Do you have a moral obligation to use your new-found powers to help others?
In one published essay, Mr Robichaud uses that question to explore consequentialism, an approach to morality which, as the name suggests, judges the rightness or wrongness of an action based solely on its outcomes.
A consequentialist would be likely to argue that Peter Parker has a moral responsibility to be Spider-Man because that decision would bring about the greatest good.
But Peter Parker was also a talented scientist, so a non-consequentialist could argue that fulfilling his scientific vocation could be an equally valid choice for him. Perhaps being Spider-Man is above and beyond the call of duty - the answer is murky.
The conversation does not end with superheroes, of course. Mr Robichaud encourages students to take the framework they have learned and apply it to decisions in their own personal and professional lives.
But he says it is a neutral way to start talking about ethical issues that people often find provocative or confronting.
"Ethics is one of those hard things to teach because for a lot of people the answers are very personal," Mr Robichaud told the BBC. "If you make it about artificial examples at first, then it allows people to think a little bit more safely and clearly about ethical issues."
The incorporation of superheroes into a philosophy curriculum is not without critics.
When academics struggle to fill seats in their medieval poetry classes while their colleagues are turning students away from packed courses on the mythic rhetoric of the superheroes, sniping in common rooms is to be expected.
Professor Mark White of the City University of New York says he is sure his work on Batman and philosophy "arouses some chuckles in the corridors", but he is careful to point out that he is not teaching the philosophy of comic books, he is using comic books to teach philosophy.
Mr Irwin agrees, drawing a distinction between his work and that of cultural theorists.
"Cultural studies coming out of the UK took popular culture very seriously as an object of study," Mr Irwin told the BBC.
"We are not saying that the canon of Superman comic books is equivalent to Homer and Dante and you can study them for their own sake. We're not suggesting that comic books replace Plato and Descartes - not at all. The goal is always to get people interested in philosophy by speaking first in terms that people are familiar with."
Mr Robichaud has little patience for critics who say that this work cheapens the traditional study of philosophy.
"The sort of philosophy I do - analytical philosophy - uses thought experiments all the time," he says. "If the examples we are drawing from are fictional examples from popular culture, as long as that's in the service of good philosophy, who cares? Who cares if the example is from Middlemarch or Watchmen?"
Shaun Treat, who teaches at the University of North Texas, is not bothered by "highbrow" critics either. For him, the proof is in the pudding: the students lap it up.
After years of teaching traditional debates like Hobbes versus Locke, he says, "it's amazing how much more the students are interested and engaged when you them put in cape and tights and have them slug it out".