Peter Hitchens likes to present himself as the Millwall FC of punditry; no one likes him, and he doesn't care. He knows his new book about drugs will go down badly among metropolitan liberals, and says he has "absolutely no chance whatsoever of influencing anything". Fleet Street's patron saint of lost causes, he writes: "Almost certainly the battle to halt the spread of mind-altering drugs is lost." And yet for someone confronting futility and derision, he appears remarkably cheerful. "Well, it's what I've always dreamed of – of being the kind of person who gets written about. I dreamed of being part of the exciting people who were in the arguments."

For the guileless pathos of that statement alone, I could forgive Hitchens almost anything. You might say there is much to forgive. Every week in the Mail on Sunday he delivers a thunderous sermon against the moral degradation of all things modern, and his latest book, The War We Never Fought, makes some jaw-dropping claims. He conflates the scourge of drugs with everything from lottery winners to Oxbridge graduates who haven't heard of Mr Micawber, and has a hilarious gift for the waspish afterthought, as in: "Teachers are no longer really teachers. If they acted as if they are, they would probably be prosecuted", or police officers no longer patrol the streets, because they're too busy "suing each other for racial or sexual harassment". I can think of no other contemporary writer who, when describing a member of the dreaded liberal elite, would add this detail in parenthesis: "She and her second husband (she is divorced)."

We disagree about almost everything, but I find him impossible to dislike. In person he is polite and engaged, and in print always a contrarian but never a controversialist, sincere in beliefs that are almost as unfashionable on the right as they are anathema to the left. He champions civil liberties but abhors libertarianism, would like to bring back hanging and see off pre-marital sex, and is in a perpetual state of lament for the passing of Christian values, which he dates back to the first world war. "If anybody else tells me that I think the 1950s were a golden age, I'll strangle them. I remember the 1950s – chilblains, Wall's ice cream, everybody smoked. I didn't like the 1950s." Hitchens's golden age was the late Victorian/early Edwardian era, a period of "How shall I say? Increasing self-imposed moral conduct."

The War We Never Fought makes a characteristically counter-intuitive argument. It is nonsense to say the war on drugs has failed, Hitchens contends, when in actual fact we have never even tried to wage it. Drug-taking was, in effect, decriminalised by the Misuse of Drugs Act 1971, ever since when the authorities have deployed the rhetoric of toughness to conceal the truth that we are free to take drugs with impunity, knowing our crime will probably be ignored, or at worst not punished but "treated". It suits the liberal elite to pretend the draconian rhetoric is true, because it justifies their claim that the law is unjustly repressive and should be abolished altogether. The solution to the alleged failure of the war is not to give up and give in, but to start fighting it.

The author's great merit is his honesty. "Drug-taking," Hitchens writes, "is the purest form of self-indulgence," for it severs the link between hard work and reward, making "deferred gratification appear a waste of time and a foolish rejection of readily available delight". He regards all forms of self-stupefecation as morally wrong, and unlike others who make the case against drugs on legal or medical grounds is quite candid: this is ultimately, he says, a moral argument. The downside is that once he has stated this position, there is not much more to say. It would barely sustain a column, let alone a book; you either agree with him, or you do not. So he is drawn into making all sorts of arguments based on health, science and judicial statistics, none of which stands up.

Hitchens thinks the legal drug classification system, under which cannabis is considered less dangerous than heroin or crack, is "scientifically meaningless". Has he ever seen someone trying to give up heroin? "No, I can't claim to know what it looks like, no." Has he ever seen anyone trying to give up cannabis? "No." Does he think the two are equivalent? "Well, since I haven't seen either of them, I don't know. But as I don't believe in addiction, it's of no interest to me."

Hitchens thinks there is no such thing as addiction? "No, it's just laughable. I believe in free will. People take drugs because they enjoy it." I agree that many people take drugs such as cannabis because they like it – but doesn't he wonder why those same people would never dream of touching heroin? Happy, successful, stable people seldom inject smack, whereas most junkies suffered catastrophic childhoods, often in care and often abused. Doesn't that tell us something critically important about the difference between drugs?

"Yeah, but ultimately, so what? All these people would be helped by a properly enforced law which punished them for doing it, because then fewer would do it, and they'd be rescued from it." If the horror of heroin addiction is insufficient to deter someone from shooting up, the prospect of being arrested is unlikely to put them off. But Hitchens places unfathomable faith in the power of the law to control human behaviour.

He also places great importance on the function of the law to express society's disapproval. How can we expect parents to stop their children taking cannabis, he argues, when the drug's status as "soft" – and by implication therefore harmless – is enshrined in law? But society disapproves of lots of things, without making them illegal; he isn't calling for marital infidelity to be outlawed, say, so why drugs? "It's a question of practicality. How would you enforce such a law?" That's precisely the argument made against drug prohibition. "But who says it's unenforceable? No one has tried. Possession of drugs is an objective act, possible to prove in a court of law." But so is extra-marital sex, when it results in pregnancy; with DNA testing, every adulterous parent could be brought bang to rights. "Well, I suppose so, but I'm not in favour of that. I don't think that it's a sensible or proper use of policing." Which is exactly what people say about the law against cannabis.

In an ideal world, Hitchens would outlaw alcohol and tobacco too, but as it is he takes the eminently pragmatic view that both drugs are too entrenched in our culture to be banned. He applauds our laws controlling their sale for achieving valuable harm reduction – yet when the same principle of harm reduction is applied to other drugs, he is incensed. "Have you looked at the Talk to Frank website? Have you looked at the thing? Financed by you and me!"

Although he does drink, he hasn't been drunk since he was 15. On the other hand, he needs a strong caffeine hit every morning before he can start writing, and drinks coffee while we talk. If a stimulant that "severs the link between hard work and reward" is immoral, why is that OK? "Caffeine? Come on. It's just not a serious point. I mean, come on. Caffeine!" But that's exactly what people say about cannabis. "Well, it may be what they would say about it, but it wouldn't be true. There is no case of anybody taking all their clothes off and swimming naked in an English river after drinking coffee."

If his argument is fundamentally about morals, and he believes all drugs are by definition as bad as each other, their impact on a user's behaviour is logically neither here nor there. But Hitchens cites swimming in a freezing cold river because this was one of the things a friend's son did while in the grip of a psychosis triggered, Hitchens believes, by smoking cannabis. Hitchens is not faking his anguish at the boy's fate; his eyes well with tears as he talks about him. But it's here that his whole argument becomes increasingly inconsistent.

Hitchens doesn't believe in addiction, because it cannot be objectively proven by any scientific test. For the same reason, he refuses to recognise other "modern" medical conditions such as ADHD or dyslexia, which he dismisses as "disreputable, unscientific rubbish". But no objective test exists for schizophrenia and psychosis either, their diagnoses depending upon definitions so subjective that they change according to the whim of whichever doctors are put in charge of making them up. And yet, time and again, Hitchens cites their grave risk as the reason why cannabis should be illegal.

"To say that I can't give you an objective description is not axiomatically to say that they don't exist," he protests – and he may well be right. But that's what people say about dyslexia. "Well, it may be, but they are wrong."

Hitchens quotes with approval Toby Young's claim that cannabis did him far more harm than cocaine – but then dismisses as delusional Young's insistence that cocaine did him no harm. If I were to tell him that I'd taken lots of drugs in my youth, and suffered irreparable mental damage, what would he say? "I would say: 'What a pity.'" And if I told him they had done me no harm at all? "I would say: 'How can you know?'" Leaving aside the self-evident bias in these responses, by his own logic I might have been much less happy, successful or intelligent had I never taken drugs. "I accept the possibility," he concedes reluctantly. "But it's bull."

"Those who do not wish to listen to the informed and cogent warnings of leading scientists," he writes, "will find excuses not to do so." But isn't that exactly what he does, whenever any leading scientist – Professor David Nutt, say – presents data he doesn't like? "I do not think that Professor Nutt's statements on the dangers of drugs are cogent," Hitchens retorts testily, rather proving the point. He accuses Nutt of "mixing the subjective with the objective", but I think a psychologist would probably call this projection. Where is the reputable scientific justification for his insistence that cannabis sends people mad, and is "one of the most dangerous drugs known to man"?

Peter Hitchens and his older brother Christopher (left) deep in discussion in 1999. Photograph: Sam Pearce/Camera Press

"I'm a propagandist," he shrugs.

Round and round we go, not getting very far. Hitchens maintains that it's practically impossible to get locked up for possession, and that even dealers are unlucky to wind up in jail. That might come as a surprise to 16% of the prison population. Between 1998 and 2006, the number of people sent to prison for drug offences increased by 91%, whereas the increase for other offenders was just 53%, according to Alex Stevens, professor of criminal justice and author of Drugs, Crime and Public Health. Doesn't that sound like a war being waged on drugs?

"Criminal statistics are easy to misread," cautions Hitchens, and he is right. But in the absence of any definitive factual consensus, his argument looks more like an article of faith than an empirical thesis.

The war on drugs is a lot like abortion, both debates being framed in legal and medical terms, but really motivated by something much more primitive. Hitchens invokes morality, and deserves credit for coming closer to the truth than most, but ultimately I think it boils down not to morals so much as emotions. His position is just as inconsistent, subjective and contradictory as everyone else's, but makes perfect sense as an expression of how he feels about the world.

It was Hitchens's defining misfortune, almost 61 years ago, to be born the younger brother of the more famous writer Christopher. "When you're a small child, and you have a brother, you want to catch up with them. I just wanted to be as big as, be as strong as, all the things a younger brother feels." Hitchens owed his first job – in 1973, on the Socialist Worker – to Christopher's connections, and was a loyal young apprentice to his brother's revolutionary leftwing politics. But he could never compete with his mercurial sibling's legendary charisma, leaving a rightward march back towards their parents' parochial, blimpish politics as the only available alternative. In 1977, he joined the Daily Express, where he toiled away as a worthy if unglamorous reporter until Richard Desmond's arrival propelled him – a conscientious objector to pornography – into the arms of the Mail on Sunday and a weekly column where he could be as unlike his brother as humanly possible.

If Christopher was louche, hedonistic and iconoclastic, Hitchens would be fastidious, puritanical and Christian. Why else would his latest book condemn rock music as tantamount to a narcotic, "sometimes a stimulant and sometimes a depressant, but always influential over the moods of its listeners," when he must know the same could be said of opera or Elgar? Only his heart, not his head, could write that Toby Young was "lucky" not to wind up in a locked psychiatric ward after smoking cannabis, for even the most alarmist interpretation of the medical data does not declare psychosis the most likely consequence. For Hitchens, drugs are really a metaphor for what he calls the "cultural revolution" of the 60s, which swept aside what was left of the England he now longs for.

Christopher died of cancer last year, after a lifetime of heavy drinking and smoking, but even in death his memory still tends to eclipse his younger brother's presence. "This is an amazing and unique experience to have an interview in which nobody has brought up my brother until I do it!" he says, but his laughter sounds more peevish than amused. Christopher had published his memoir only a year before his death. "One of the problems of having Christopher as a brother is that you felt that the writing about our family was writing about his family. He forgot he was also writing about mine." Hitchens would much rather the world did not know that their father drank to excess.

The two brothers were never close in adulthood, for along with their politics, their lifestyles also parted company; unlike the twice-married, atheist Christopher, Hitchens is a devout church-going Anglican and lives with his wife and three children in Oxford. But his politics today share something important with the communism of his old revolutionary self. Just as Marxism only makes sense if you believe basic human impulses can be eradicated in the interest of a greater good, the same is true of Hitchens's faith in the power of the law to defeat a basic human instinct for intoxification. "I'm not a utopian! I'm not a perfectionist!" he protests, but perhaps a touch too much. As with most of his political positions, his argument for a drugs prohibition makes sense – as long as you have never met an actual human being.

I ask him if he has ever broken the law himself. "Oh yes," he replies, as quick as a flash, he often broke it during revolutionary protests in his youth. Why didn't the threat of penalty deter him? He thinks for a moment. "When I was in my law-breaking phase I was the kind of person who should have been locked up. I was the kind of person who should be locked up, and possibly strung up. It would have been the only language I understood."

I ask why he thinks people always say he is humourless, when it seems to me that while he is certainly irony-free, he can be very funny. "Hmm, probably unintentionally," he murmurs. "I have no sense of humour. You know that."

I don't think he believes that for a minute. "Oh, I'm a horrible, brutal, sexist, racist, homophobic monster." That's not a self-appraisal but a self-defence mechanism, isn't it?

"Well, there you are then. Alternatively, none of that is true."