On the morning after his first address to conference as Labour leader, Jeremy Corbyn was pressed by Sarah Montague on the question of whether, as prime minister, he would ever use the nuclear button. “No” said Corbyn. “I am opposed to the use of nuclear weapons. I am opposed to the holding of nuclear weapons.”

This puts Labour’s leader at odds with his own party’s policy (the position in which Jeremy Corbyn has, historically, felt most comfortable). It also, of course, undermines the rationale for holding nuclear weapons, which is to deter others from using them on you, in case you use them back (which is why Corbyn’s own shadow defence secretary immediately deemed his answer unhelpful).

In a sense, none of this matters, since Corbyn is never going to be in a position to take such a decision, partly because of unserious answers like this. There is a respectable case that Trident shouldn’t be renewed – it’s just that he didn’t make it, and wouldn’t know where to begin. This exemplifies a problem that extends beyond the issue of national security, and beyond Corbyn: the way in which false dichotomies of perfect versus good shut down serious thought.

Corbyn told Humphreys: “I am opposed to the use of nuclear weapons. I am opposed to the holding of nuclear weapons. I want to see a nuclear-free world. I believe it is possible.” Like many of Corbyn’s statements, much of this is impossible to disagree with. Even the most hawkish American neo-cons do not pretend that using nuclear weapons is a good idea – it’s more that they argue that holding them, and signalling your willingness to use them, is the best way to stop any being used.

And who wouldn’t want to see a nuclear-free world? President Obama shares Corbyn’s aspiration, though not the belief that it will come about if America, or its allies, unilaterally disarm. But the unpleasant truth is that there will never be a nuclear-free world, for the simple reason that knowledge, once acquired, cannot be unlearnt. Even if all the nuclear powers got together and agreed to dispose of their nuclear arsenals, they would still be nuclear powers, just latent ones, with the expertise and facility to quickly re-arm. The game theorist and arms control specialist Thomas Schelling has questioned whether a nuclear-free world is even desirable:

“In summary, a “world without nuclear weapons” would be a world in which the United States, Russia, Israel, China and half a dozen or a dozen other countries would have hair-trigger mobilization plans to rebuild nuclear weapons and mobilize or commandeer delivery systems, and would have prepared targets to pre-empt other nations’ nuclear facilities, all in a high-alert status, with practice drills and secure emergency communications. Every crisis would be a nuclear crisis, any war could become a nuclear war. The urge to pre-empt would dominate; whoever gets the first few weapons will coerce or pre-empt. It would be a nervous world.”

The nuclear question – perhaps the most serious question of our age - does not yield easily to idealism. To grapple with it, you have to put to one side your wish for a world in which nuclear weapons don’t exist, and think hard about the one in which they do.

In a paper from 1969, the American economist Harold Demsetz distinguished between two approaches to public policy: the “nirvana” approach, and the “comparative institution” approach. The former presents the choice as between an ideal norm and the imperfect existing arrangement; the latter as between alternative, real world arrangements, imperfect and less imperfect.

This is colloquially known as the “nirvana fallacy”: the tendency to assume that there is a perfect solution to a problem. A politician who uses the nirvana fallacy gains an easy rhetorical advantage. He can paint inspiring pictures of his perfect world, and attack the existing state of affairs for not living up to it. He can accuse anyone who doesn’t accept its plausibility as cynical, lacking in vision, or principle.

But this advantage comes at a cost, because the nirvana fallacy makes you stupid. It stops you from doing the hard, gritty thinking about how to improve the world we have, since, faced with a series of complex, imperfect options, you overleap them to reach the sunlit uplands of an ideal scenario. Soon, you forget how to think about the real world at all.

The left is particularly susceptible to this problem. Should we intervene in Syria? No, because we want a peaceful Middle East. Fine. That saves you the onerous work of confronting the truth that Syria is on fire, that hundreds of thousands have died there, and that many of the survivors are now pouring into Europe, and what the hell are we going to do about it?

Should we be making hard choices about public spending? No, because we want a high-growth economy in which only the rich pay more tax. Should we reform the way in which the NHS allocates resources, or schools are run? No, because we want a country in which everyone, regardless of background, receives the best healthcare and education, for free. Thank you for the applause, comrades.

Ideals are necessary, but so are plans, and the most admirable idealists are also cold-eyed realists. Abraham Lincoln didn’t think it was enough, as some of the abolitionists of the north did, merely to shame the slavery-supporting politicians of the south. He trimmed and hedged and compromised his way towards abolition. Martin Luther King was not the airy figure of myth, but a highly astute politician and campaigner who out-thought and out-manoeuvred his opponents. He had a dream, but he wasn’t content to live inside it.

Perhaps it is unrealistic to expect Jeremy Corbyn to disavow the nirvana fallacy; after all, he owes his current position to it. Many of those who voted for him find it almost impossible to grasp that the choice is not between an imperfect Labour government, and an ideal one, but between an imperfect Labour government, and a Tory one. They revile the Blair government, but don’t stop to think what the country would be like today if the Tories had won in 1997, and kept winning.

I want to see a Labour Party which gets beyond nirvana fallacies, and engages with the world as it is. I believe it is possible.