There is something deliciously morally simple about nuclear weapons. So destructive, so indiscriminate, so gruesome are they that their use is one of the few things about which we can say simply, "it is a categorical and indefensible evil." Under no circumstances should nuclear weapons ever be used.
The next step should then be easy. If there is truly no moral justification for using nuclear weapons, we will not use them, and thus must decommission them.
Alas, it is not so easy, as friends of the late Sir Michael Quinlan can testify. Quinlan was a devout and courageous Catholic, nicknamed "the Jesuit", who spent much of his 40-year career working for the Ministry of Defence and was a major architect of Britain's nuclear deterrence policy. Although a courteous and instinctively diplomatic man, he was an outspoken critic of the second Iraq war and of Tony Blair's "sofa" government.
He had an unparalleled reputation in the field of nuclear deterrence, briefing successive prime ministers, and speaking and writing influentially about the ethics of war.
Those who engaged with him from unilateralist foundations quickly found themselves wrong-footed by his military and diplomatic sophistication, and his ethical acuity. Morally repugnant as nuclear weapons are, the fundamental question was how to avoid their use rather than how to feel morally unpolluted by possessing them. Much as it sticks in the unilateralist throat, the history of the cold war shows that deterrence worked.
Quinlan was less sanguine about the post-cold war world, recognising that it was a messier and less predictable place, devoid of the certainties that made cold war deterrence a success. And yet, when asked by Stephen Sackur on the BBC's "Hardtalk", a few years before he died, he still argued that Britain should retain its nuclear arsenal when it became obsolete 15 years hence.
This did not make him a belligerent. Although his recently-published book Thinking About Nuclear Weapons argues that you couldn't have a deterrent without willingness to use the weapons "if necessary", it also dedicates a chapter to discussing the possibilities for abolishing nuclear weapons entirely.
Quinlan refused to engage in counterfactuals. The world had nuclear weapons. The task was to negotiate them into silence and if that involved retaining some, so be it.
It's a conclusion that smacks of doublethink – we need them in order not to use them – and is instinctively unpalatable to many people, not least many of Quinlan's fellow Catholics.
Yet, Quinlan was not two-faced, let alone naive. He had decades of experience and history on his side and operated in a complex, compromised real world that was devoid of easy or agreeable answers.
Those who advocate unilateral disarmament are clearly singing the song of angels but, as Quinlan recognised, the world may be too out of tune to hear it.