Johann Hari: What sort of freedom do you believe in?

In a lush patch of Yorkshire, something strange is happening. The quaint practice of a British by-election has thrown up a serious philosophical debate about what it means to be free. Sure, to get to it, you have to jostle past the silicone implants of the Miss Great Britain Party, David Icke's seven-foot lizards plotting world domination, and the Westminster correspondent-flock wondering what it all means back in SW1. You have to burrow deep, and listen hard. But if you do, you can begin to see what liberty will look like in a techno-charged 21st century.

Like opposing Robert Mugabe and cuddling puppies, everybody in Britain is theoretically in favour of freedom. But the battle in Haltemprice and Howden is a slap-in-the-face reminder that we fundamentally disagree about what freedom means – so we are increasingly shouting at each other across a chasm of miscomprehension. The philosopher Isaiah Berlin said there was an irreconcilable divide between those who believe in "negative liberty" and those who believe in "positive liberty." He was right. The divide survives.

David Davis has been an eloquent spokesman for negative liberty – the belief that you are free when you are left alone by the state. He argues that the largest threat to your freedom is from the state, and you become freer and freer as its frontiers are pushed back. He instinctively sees increased government action – through CCTV cameras, or the DNA database, or taxation to redistribute wealth to the poor – as an assault on your freedom.

Jill Saward has been an equally eloquent spokeswoman for positive liberty. Her own story helps us to understand why. When she was 21 years old, she was watching television in her father's vicarage in Ealing when a gang broke in. As they beat her father and boyfriend, they raped her, and assaulted her with the handle of a knife. At her trial, the judge said she had suffered "no great trauma".

She believes there are two primary threats to your liberty. There's the state – and then there's other people like you. Sometimes, you need one to protect you from the other. Isaiah Berlin said "liberty is the absence of obstacles to the fulfilment of your desires." While the state can put obstacles in your way, it can also clear obstacles – like poverty or rape – from your path.

The issue of rape exposes the canyon between these two competing visions of freedom. Saward believes it is the biggest abuse of civil liberties happening in Britain today that 50,000 women are raped every year, and fewer than 1 per cent of the men responsible are punished. What does the state do, domestically, that compares? But if, like Davis, you believe the state is the prime enemy of freedom, you are left powerless to do much about it except offer pieties. Hansard shows that in his four years as shadow home secretary, Davis has never raised rape at the Dispatch Box. It's not that he doesn't care; it's that his anti-state philosophy paralyses any real solutions.

Saward's does not. She believes the state should use CCTV and the DNA database to enhance your freedom. It is CCTV that caught Steve Wright, the Ipswich serial killer, for example, preventing him from raping and murdering even more women. It is the DNA database that has – in its short life so far – caught 114 murderers and 184 sex offenders. All the victims they would have raped and murdered are obviously more free as a result. "Part of British liberty," she says, "is to expect law enforcement agencies to use every tool at their disposal to catch people responsible for attacks."

Davis, by contrast, wants to scrap most cameras and most of the DNA database. He includes them in his "long list of repressive measures" that are causing "the erosion of British liberty." Again: it is not that he doesn't care about preventing rape. But he prioritises something else – the pain some people apparently feel at having a speck of their DNA stored in a database, or knowing they are seen by a CCTV camera – above rape. Where he sees Big Brother putting obstacles in your way, Saward sees Big Sister clearing them away.

It's become as fashionable as a Jean-Paul Gaultier handbag to say that the old left/right divide is dead, and from its grave has risen a new divide between libertarians and authoritarians. But this ignores the reality that the left and right have these clashing, conflicting visions of freedom. Apply this to the economy and it becomes obvious. Will a poor family be more free if they lose £4,000 a year in tax credits, as Davis demands? Is an elderly woman more free when the state ramps up charges for Meals-on-Wheels, as Conservative councils across the country have just done?

Yet well-meaning liberals are increasingly reinforcing the we're-living-in-1984, whack-back-the-state paranoia of the right. Tony Benn, Nick Clegg and Shami Chakrabarti, for example, have endorsed Davis. I understand the appeal of negative liberty and its tunes. You have a one-size-fits-all solution; you can feel you are standing up to a looming tyranny, even as you leave more people vulnerable to the tyranny of rape; you win easy applause. But it does not, in the end, produce more freedom out here in the real world.

No. For that, you have to believe in positive liberty – which requires a mixed menu of state action and state inaction. It is hard; it is complex, because you have to weigh the mix differently according to every situation. But only it produces real freedom.

Of course, there are times when these warring defenders of freedom can unite. Locking up a person for 42 days without charge is unethical, and it makes us less safe. Remember when we introduced internment in Northern Ireland, and IRA recruitment rates shot up? If this was a referendum, I would be on Davis's side against the Sun campaign braying that Magna Carta is for jihadis and pussies.

But this by-election has cut deeper than the 42-day groove. Davis has made it about whether his constituents endorse a vision of negative freedom that is deeply conservative and anti-state. Its logic demands the dismantling of family credit and CCTV alongside 42 days – and if we followed his advice, more women would lose their liberty in the most foul of circumstances.

The campaign has also deliberately glossed over the fact that Davis himself has, throughout his career, contradicted his own philosophy. He inexplicably thinks the state should kill its own citizens if they commit certain crimes, opposes basic legal equality for gay people, and wants to ramp up the "war on drugs". So if you vote for him, you endorse both a faulty theory of liberty, and a man who diverges from it into ugly abuses.

We should, however, be glad about one thing. In the villages of Yorkshire, one of the great questions of our time has been sharpened. Do you want a state that leaves you alone, or a state that intervenes to make you free?

j.hari@independent.co.uk