There's a thought-provoking exchange in Julian Mitchell's play Another Country, enjoying a revival at London's Trafalgar Studios.
It is set in an English public school during the 1930s and in one scene Vaughan Cunningham (described as a "literary intellectual") challenges a schoolboy called Judd about his zealous communism. "You, none of you allows any room for doubt," he observes.
"For me," he adds, "doubt is the basis of all moral life." And if you "take away doubt and claim absolute authority, whatever name you give it, you diminish the humanity of man. You diminish mankind."
This got me thinking, inevitably, of the independence debate, for doubt rarely intrudes into the claims and counterclaims of Nationalists and Unionists. Now arguing that this diminishes mankind might be a bit much, but it certainly diminishes healthy political discourse.
For the art of the possible, as Bismarck described it, is nothing without the occasional doubt, which the Oxford English Dictionary defines as "a feeling of uncertainty or lack of conviction"; only uncertainty and lack of conviction are today considered by politicians to be unacceptable displays of weakness.
Of course, a distinction must be drawn between the public and private sphere. In private, both sides of the constitutional debate have, historically, acknowledged doubts through shifts in policy. Devolution (and possibly more of it), for example, was an implicit admission that the Union as was needed a tweak, while the squaring of circles involved in so-called "indy-lite" betrayed a realisation that full independence could not possibly cure all ills.
Publicly, however, such doubts are rarely conceded: the offer of "more powers" (for example the surprise transfer of the power to set the Discretionary Housing Benefit cap) is pitched as pragmatic Unionism while retaining the Queen, sterling and UK-wide energy market is presented as pragmatic Nationalism. Proponents of both tend to claim absolute consistency.
Even so, I've always been struck by the lack of doubt possessed by many advocates of independence. Take my father, a member of the SNP for half a century (he joined long before it was fashionable); I've never once heard him concede there might be any downside to independence, as if nothing had changed economically, geopolitically or constitutionally since the early-1960s. For him, it's almost an article of faith.
Similarly, nothing is capable of knocking the faith of today's Yes campaigners that independence remains the best option in 2014. They had, for example, no doubt as to its benefits before the 2008 crash and a near identical lack of doubt after it. Taking a longer time frame, many present SNP ministers were convinced in 1975 that membership of the-then EEC would be bad for Scotland, just as nearly 40 years later they have no doubt EU membership would be good.
There are many Unionists with a similarly unshakable conviction that Scotland is better off as part of the Union, but generally it's more a fact of life than a cause.
Even my own constitutional views have never been 100% fixed; tomorrow I'll publish a short tract advocating a federal UK, but I was full of doubt in writing it. Which brings me to the First Minister, who last week demonstrated not a trace of self-doubt during the row as a consequence of comments praising (with qualifications) Vladimir Putin. Once the furore began, he contrived the usual "what I actually said was" defence.
In short, Mr Salmond praised Mr Putin for restoring "a substantial part of Russian pride" which, he added, "must be a good thing". It is impossible to separate restoration of pride from the means - mostly unpleasant - used to that end, helpfully displayed during last week's May Day rally in Moscow, an event fuelled by restored national pride in the wake of the annexation of Crimea.
The real insight into the First Minister's character came in his response to demands that he apologise. This is something he is constitutionally incapable of doing. Instead he dug in his heels, rewrote history and turned on his opponents, which unhelpfully included those well-known bastions of Unionism, Amnesty International and Scotland's Ukrainian community.
Anyone else would just have fessed up and said, "sorry, it was a stupid thing to say, I meant no offence", and drawn a line under it. Such a gesture would have made Mr Salmond appear human and fallible but he simply couldn't bring himself to admit having made a (relatively minor) mistake. Instead he made matters worse, needlessly so.
It isn't the first time. In fact, his lengthy interview with GQ magazine highlights several past errors for which there isn't a flicker of contrition. Alastair Campbell asked if he stood by his "unpardonable folly" remark about Nato bombing of Kosovo ("Yes, I do"), getting too close to Rupert Murdoch (not at all, "a remarkable man") and the existence of EU legal advice ("I was investigated by a [former] permanent secretary and cleared").
How often do you say, asked Mr Campbell at one point, "I am right and you're wrong"? "Hardly ever" was the First Minister's response, although one assumes he was talking about his non-public persona.
More troublingly, this lack of doubt trickles down from Bute House to the wider SNP and Yes campaign creating, as the journalist David Pratt put it a few days ago: "a nationalist constituency ... that cannot admit when errors of judgment have been made".
This exists, sadly, as much online as in people, as Vaughan Cunningham puts it in Another Country, "so sure of the absolute rightness of their position" that they feel "no need to put their names" to vehement, often visceral, criticism.
Some think nothing of blaming the Union for child poverty in one breath, and in the next defending the First Minister's praise of Mr Putin as a piffling distraction.
But then that's one of the problems with lacking doubt: one ends up defending the indefensible, no matter how many intellectual contortions are necessary.
The popular, pro-independence website Wings over Scotland, for example, scrutinizes Unionist claims - at meticulous and often eloquent length - but never applies a penetrating eye to its own side. Over the past few days the Bath-based blogger has been prominent in the defence of Mr Salmond.
Compare and contrast with the CBI, whose recent car-crash over its affiliation with Better Together at least concluded with its chief executive Jon Cridland admitting to "an honest mistake".
If the First Minister had only done the same in relation to his comments about Mr Putin then, perhaps, by conceding a flaw rather than claiming absolute authority, his cause might have been enhanced.