In Ali Smith’s novel How to be Both, teenage girl George – recently motherless – becomes obsessed with a pornographic clip. She spends almost all her free time watching it, and watching it, and watching it. It features a very young woman, perhaps young enough to be a girl herself, although of course George knows nothing about who she is or how she came to be in this film. Understandably, George’s father is concerned when he finds out what his daughter is doing. He wants to know why, and so she tells him:

“This really happened, George said. To this girl. And anyone can watch it just, like, happening, any time he or she likes. And it happens for the first time, over and over again, every time someone who hasn’t seen it before clicks on it and watches it. So I want to watch it for a completely different reason. Because my completely different watching of it goes some way to acknowledging all of that to this girl. Do you still not understand?”

Most people, of course, do not watch pornography for the same high-minded reasons as George. Most of them watch it to get off, and most of them are men – pornography is produced by and for men, an orgiastic confirmation of the most brutal sexual and racial stereotypes. At this point, it’s habitual for pornography defenders to step in and muddy the waters. Not all porn is like that, you will be told, and anyway how can you define porn, and even if you could, how would you prove that pornography actually caused harm?

One thing at a time. There is actually a perfectly good and workable definition of pornography – it’s from Dworkin and MacKinnon’s Antipornography Civil Rights Ordinance. This is it: “Pornography is the graphic sexually explicit subordination of women through pictures and/or words.” They also specify that in porn, women will be dehumanised as sexual objects, or shown to enjoy pain and humiliation, or to take pleasure in being raped, or shown tied or mutilated or injured, or presented in sexually submissive poses, or reduced to body parts.

The difference between porn and not-porn, which is so often presented as an intractable question of taste beyond which the discussion cannot proceed, is clearly described here as political rather than aesthetic. There will be cases that test the boundaries or demand deeper consideration than others, but for the most part, everything that you think is probably porn would count as porn under the Ordinance. (Which is not to say the Ordinance, were it enforced, would ban it: the purpose of the Ordinance is not censorship, but to allow women harmed through the production or use of pornography to sue the makers for damages.)

I imagine the 19,000 images possessed by Nathan Matthews and Shauna Hoare, the killers of Becky Watts, would pass the Ordinance definition. They preferred images of teenagers, young women in school uniform, threesomes; most of the material was legal, but one of their files was a video of a woman being raped. (“And anyone can watch it just, like, happening,” says George in How to be Both, “and it happens for the first time, over and over again.”) I say “they”, but it is pretty clear whose sexual tastes this collection reflects. The schoolgirl fetish is Matthews’: Hoare was the schoolgirl herself when Matthews first picked her up, a child of 14 or 15 who had been in and out of care.

He was seven years older, and confirmed his control over her in all the usual ways that men do: isolated her from her family, stopped her going to college, attacked and strangled her, told her she was fat, withheld food and cigarettes, and when all that failed to keep her in line, threatened to harm himself. The evidence presented in court showed Hoare was a collaborator in the fantasies of kidnap and rape the two concocted, but she was exactly that: a collaborator, an occupied population choosing between resistance and compliance with the occupier.

It is not a question of whether pornography “caused” Matthews and Hoare to commit their crime. What matters is this: in a world sodden with violence against women, pornography is one more form of it. Matthews and Hoare apparently made no distinction between legal images and the video of the rape. All served the same need to see women (in Hoare’s case, other women besides herself) subordinated and dehumanised. Pornography is the propaganda of gender. Through it, men and women alike learn what women are supposed to be for: something to fuck, something to use, something to hurt if you’d like to, and something to dispose of when you’re finished. Matthews and Hoare dismembered Becky Watts with a circular saw.

Mark Bridger watched images of child abuse and murder before he murdered April Jones. Stuart Hazell watched images of child abuse and searched for incest porn before he murdered Tia Sharpe, the granddaughter of his partner. Vincent Tabak watched pornographic videos of blonde women being strangled before he strangled blonde Joanna Yeates. A 13-year-old boy raped his eight-year-old sister after watching pornography. Jamie Reynolds used violent pornography with images of nooses before he murdered Georgia Williams by hanging. First the theory, then the practice.

And this pattern does not apply only to confirmed criminals and obvious monsters. A 2014 BMJ study of teenagers found an increasing prevalence of anal sex, which the participants explained they had learned about from porn. There was little thought that the girls would enjoy or even consent to it – boys “accidentally” penetrating the wrong orifice was presented as normal, and girls expected anal sex to be painful. It was pornsex: the subjugation and humiliation of women to serve male desires. And this is how porn operates: first through the eyes, and then in the mind, and then back through the body, against other bodies. Humans are creatures of culture, and the culture we have made for sex is one where women are destroyed. Do you still not understand?