A man is pointing a revolver at you. You know that at least four of the chambers have bullets in them, maybe five, and possibly six. Do you make a lunge at him? Of course not. There's a high probability that you'll end up in a pool of blood on the floor.
Deterrence is about probabilities. If there's at least a good chance that someone will kill you, you don't take the risk. But nuclear deterrence in the UK has instead always been based on certainties. It's been a tenet for decades that our enemies must be certain that the country could instantly hit back with a nuclear strike at any time.
This certainty has proved expensive. We're told that we need four Trident submarines, armed with nuclear warheads, so that even if two are in dock, being refitted, there are still two at sea, ready to strike, the third as a back-up for the fourth should anything go wrong. At £80bn or so for the belt and braces, that's a price that even Jermyn Street would be embarrassed to charge.
Let's leave aside for now whether we need a nuclear deterrent at all. I happen to believe we do. But do we need a perfect deterrent, or one which is good enough? Given how little money we have these days, surely the latter will do.
What deterrence needs is ambiguity. We don't know much about North Korea's nuclear capability, but we're certainly not going to risk nuclear annihilation by taking them on, even if the risk were 50 per cent, 25 per cent or just 10 per cent. That's why Israel is so sensitive about its nuclear secrets being revealed. The less other countries know about your nuclear capability, the more effective will be its deterrence.
The trouble with Britain's is that, when it's not at sea, it's highly visible. Every time a Trident submarine comes back to dock, the local residents around Faslane know about it. So if all the Trident subs were there at once, it would theoretically be possible for an adversary to launch a surprise attack and destroy them all. That was the worry during the Cold War, which led to the Continuous At Sea Deterrence (CASD) policy that we've adopted ever since.
Even in those days, the chances of the Soviet Union launching a first strike against Britain were vanishingly small. Now they're imperceptible. Our main enemy now is not even a state – it's organisations such as al-Qa'ida, whose foot soldiers are British citizens with rucksacks. If they use weapons of mass destruction on our soil, we're hardly going to launch a nuclear attack in response. Where would we send it?
Our nuclear deterrent is only of use against state enemies. And if tensions were to rise against – say – Russia or China, we would have plenty of warning. This is the premise of a paper written by Professor Malcolm Chalmers for the Royal United Services Institute, published last week. He argues that we don't prepare our conventional forces for a surprise attack by another state against the UK, so we shouldn't do the same for our nuclear forces. It's our insistence on CASD for Trident's replacement that is making it so expensive. Instead, he says, we could reduce the number of submarines from four to three or two.
Chalmers has been a special adviser to two foreign secretaries. He is not alone in arguing that the thinking on Trident's replacement needs to be more flexible. Yes, one of Tony Blair's last acts in 2007 was to push through the House of Commons a vote on a like-for-like replacement of Trident. But that was when there was money to spend. Now Lord (Des) Browne, who was then Defence Secretary, and helped to win the vote, has, on the Labour Uncut website, said that Trident should be rethought as part of the Government's Strategic Defence and Security Review.
"I know how much the world has changed since we made our original renewal decision," he says. "There is no way of examining the necessary trade-offs between nuclear and conventional capability in this defence review if Trident is left out of the process." He is right. Now that George Osborne has told the Defence Secretary, Liam Fox, that all the costs of replacing Trident must be met out of the Ministry of Defence's budget, the knock-on effects on the rest of the armed forces of buying the premium version will be horrendous.
Fox has a reputation at the MoD for being a bit of a "boy's toys" man. But massive aircraft carriers and top-of-the-range nuclear submarines aren't much use for fighting the Taliban in Helmand or Iraqi insurgents in Basra. The top brass are trying to wean him off his predeliction for what they call "highly expensive platforms". As one puts it: "You have to be quite sensitive because he's a small man with a big ego. You have to persuade him that what you're saying is what he's thinking even if he didn't think it three days ago."
Fox has already acknowledged that the defence budget faces "brutal" cuts and that the Government will have to act "ruthlessly and without sentiment". He is, after all, a deficit hawk as well as a military hawk. But that was before Osborne told him last week that the Treasury wasn't prepared to pay the capital costs of Trident.
This was a huge slap on the wrist. Osborne was angry that Fox had implied the Trident costs were off the table when it came to negotiating cuts in the department's budget. And Fox is not popular anyway with Numbers 10 and 11. They didn't like the way he told the newspapers he was going to sack Sir Jock Stirrup, Chief of the Defence Staff, before he told Downing Street. And they suspect he is trying to set himself up as the leader of the disaffected Right inside the Cabinet. For that reason, he can't do what Defence Secretaries traditionally do, which is to appeal over the head of the Chancellor direct to the PM.
So what are the chances of the Government opting for a cheaper replacement for Trident? It's interesting that Nick Harvey, Fox's Liberal Democrat deputy, is being described as "Cameron's man at the MoD". It was Harvey who co-authored with Sir Menzies Campbell a report for the Lib Dems before the election called "Policy Options for the Future of the UK's Nuclear Weapons". This argued strongly for Trident to be included in the defence review and for more examination of cheaper, "good-enough" deterrence.
Nick Clegg agrees. Writing on his website, he says: "This report destroys the myth perpetuated by Labour and the Conservatives that Britain has to choose between full-scale Trident or unilateralism. Excluding Trident from the defence review is both absurd and irresponsible."
It does seem extraordinary that the most strategic of Britain's weapons isn't included in a strategic defence review. However, Fox is at least committed by the coalition agreement to review the value for money of the Trident replacement. In an ideal world, the Defence Secretary would not dream of allowing Britain's nuclear deterrent to be anything other than perfect and certain. But he is presiding over a department which is virtually broke. It can't finance much of the procurement to which it is already committed and the Treasury is asking for deep cuts on top of that.
Add together a Defence Secretary who is politically weak, a military leadership that isn't convinced of the value of a full like-for-like replacement, a Lib Dem leadership that wants a cheaper option, and a Chancellor who isn't in a mood to be generous. What do you get? More lateral thinking on Britain's nuclear deterrent than anyone is yet prepared to admit.