U.S. Supreme Court Associate Justice Potter Stewart once said that he could not define pornography. “But I know it when I see it,” he famously added. Others have also remarked on the difficulty of coming up with a satisfying definition. Why should this be such a difficult task?

Mostly, it is the same philosophical issues that arise when trying to define anything. But pornography touches some tender nerves – whatever you think of it, it’s clear that some pretty basic moral and legal questions are involved. It matters where we set boundaries, so the inherent difficulties in coming up with a definition are more poignantly felt.

I certainly cannot do any better than Justice Stewart in this short space, but a survey of some attempts at a definition might be instructive.

So, what is pornography? An answer that springs to mind immediately is: “Material that is intended to cause arousal for the purpose of masturbation.” There is wisdom in this answer; pornography is a way of viewing something, and many things can become pornography when viewed from certain angles. This definition will not stand up, however, because it invokes the problematic concept of intent.

Intentions are inaccessible. Nothing is to stop me from filming a hardcore porno and then claiming it was actually intended as a study in suburban architecture and the camera’s focus on a housewife and pizza delivery boy having disgusting amounts of sex on the couch is a merely accidental feature of the film. You may not believe me, but you have no way of proving me wrong.

Turning to that sexiest of all reference works, the Stanford Encyclopedia of Philosophy, we find that “pornography is sexually explicit material designed to produce sexual arousal in consumers that is bad in a certain way.” This definition is a little hard to swallow, since it takes our naive answer and embeds a controversial moral judgment in it. Many definitions seem to take this tack, since opponents of pornography have a much bigger stake in a definition.

According to the sociologist Jon Huer, pornography is essentially commercial. Pornographers harness our sexual desires for monetary gain, and this exploitation is the defining feature of pornography. However, although the relationship between pornography, commerce, and commodification is interesting, it cannot be the whole story because plenty of material we would consider pornographic is produced without a profit motive and available for free.

One other frequently used definition of pornography is “sexually explicit material that is obscene.” Philosopher Roger Scruton defines sexual obscenity as a “depersonalized” way of seeing human sexuality that reduces sexual conduct to animal function, placing the body above all else. In literary or pictorial representation, obscenity is a matter of perspective: while the “genuinely erotic” work of art gives you a first-person perspective of a romantic encounter, an obscene—and therefore pornographic—work takes the voyeuristic third-person perspective that emphasizes the body.

This definition has the advantage of pointing to specific features of a picture, film, or literary work that would mark it as pornographic. But it also comes with a whole pile of philosophical baggage you may or may not care to import.

This is as far as I can take you – you’ll have to go the rest of the way by yourself. While defining pornography is a hard task, we have at least managed to lay a few things bare. In the meantime, I’ll be in my bunk.