Is the law a matter of fact or opinion? Today, Brendan O' Neill followed George Galloway, John Pilger, and Tony Benn to become the latest non-lawyer to offer up his understanding of the law on rape. He's also the latest to get it wrong, and he probably won't be the last.

In absolute truth, the law on a particular issue at any given time can be fuzzy. Our legal system regularly relies on the higher courts to bring clarity where what exists is anything but: badly-written Acts of Parliament, a glut of past rulings recent and ancient, and a nebulous sense of how certain words and phrases would now be commonly understood. This is why controversial cases often appear to result in judges changing the law, when what they are seeking to do is just to tidy up an incomprehensible mess.

This is perhaps why so many were disappointed when the High Court were unable to rule that Tony Nicklinson could be helped to commit suicide: our law on administering death to others is comparatively clear-cut, and simply leaves no wriggle room until such a time as Parliament rewrites it. But set against other cases where the judges appear to have taken a more interventionist approach, such as the 1991 decision that rape could be committed within marriage despite the historical "marital rape exemption", it looked arbitrary and even cruel.

So, yes: at times the prevailing interpretation of a particular law can be contested; even practically provisional while a court judgment is pending. But these are the exceptions and - oh so crucially- the law on rape is not one of them.

O'Neill's post appears under the headline "It is Wrong to Say 'Sex Without Consent is Rape'. This is provocative of course - but it also happens to be true. As with the vast majority of criminal offences in this country, rape is not a crime of strict liability, which would mean the outward circumstances of sexual penetration and the complainant's lack of consent were enough to establish guilt. Instead, a guilty mind or, as per the Morgan case O' Neill cites, "intent" is indeed an essential component.

But O'Neill has fatally misunderstood what is meant by the word "intent" in context - despite the fact it is set out in the very Act of Parliament he quotes, the Sexual Offences Act 2003. Rape is made out where the accused "does not reasonable believe" the other person consents. In other words, if the accused truly believed that there was consent, but the court held that belief to be unreasonable (for example because it was based on what the complainant was wearing), the accused would still be found guilty. O'Neill's statement that "the man must know that there is no consent... must not believe that consent is present, and therefore must know that it is absent" is, then, straightforwardly untrue.

As the CPS indicates in the publicly-available legal guidance that O'Neill appears not to have bothered reading, this was a major and quite deliberate Parliamentary change to the law that existed before 2003, under which an honest belief in consent, however unreasonable, negated the offence. But frankly, we don't need to go all the way to the CPS for this: it is so settled a legal principle that a first-year law student could probably reel it off in an instant.

That O'Neill made a factual error is forgivable, though disappointing when helpful explanations of what the law really says are so freely available. But what's so infuriating as to be almost unforgivable is that he has wasted an opportunity to make both a valid point about the importance of a reasonable belief in consent, and a contribution to the vital discussion about what that means. Instead, he has added yet another voice to the pundits' chorus which evidently feels rape is somehow ineffable or abstract; impossible to define, let alone to prosecute properly.

I don't really mind that O'Neill thinks feminists are "undermining ... criminal intent" despite the fact that many of us are also lawyers, legal policy experts and legal academics, and both know and value the law rather better than he apparently does. I do mind that he and others are undermining the right of his readers not to be lied to about the law that's made in their name, and on which it is a statistical certainty that many of them will one day have to rely. People who write about the law should do their research - is that really so unreasonable?

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