History suggests that the temptation to write off the Labour party’s prospects is both recurrent and unwise. In 1910, even Keir Hardie concluded that “the Labour party had ceased to count”. Yet less than a generation later, in 1924, Labour formed its first government. Decades later, some of Britain’s most distinguished political scientists wrote a book about the 1992 election whose doom-laden title asked: Labour’s Last Chance? Yet a mere five years later, in 1997, Labour won one of its largest electoral victories of all time.

None of this, however, should be read as complacency about Labour’s position today. Just because Labour’s decline in 2015 may not in fact be terminal, it does not follow that its decline is not real, as a report from the Labour-leaning Smith Institute set out in detail today. If that decline continues – and the Smith Institute sets out many reasons for thinking that it may – Labour’s prospects of competing as a party of government will steadily diminish too. There is absolutely no iron law that says Labour will always rebound. One day, Labour may indeed have had its last chance, and may cease to count.

This, more than anything else, is why the very real prospect of Jeremy Corbyn becoming the next leader poses such a profound moment of choice for the Labour party. Like many of those who support him, Corbyn calls himself a socialist. His socialism, though, is more a matter of faith than a viable programme. He is not, as his three opponents are, a reformist who aspires to govern and get re-elected. He is not interested in making detailed policy choices or pragmatic compromises. Corbyn’s position is essentially made up of attitudes and slogans, not least about the place of the trade unions, many of them proudly unchanged for almost 50 years.

There’s nothing particularly wrong with being a faith-based socialist – but please don’t confuse it with politics. The overwhelming historical experience in almost every country on Earth in almost all modern epochs is that not enough people will vote for you – and for very sound reasons. To some on the left, this refusal to compromise or to change in the light of experience can still be a badge of honour. I grew up in a leftwing culture of this kind and we all felt very principled and even, in a sense, blessed because of our embrace of the true faith. But in the end, as Eric Hobsbawm recognised so honestly in his later writings, it is in many respects a religious approach to politics. It has very little to do with building an alliance, winning an election, forming a government or actually changing things. It is politics as being, not politics as doing.

In an earlier column on this theme, I suggested that Labour currently faces a choice between purity, as represented by Corbyn, and power, as represented by the other three more reformist candidates in the race. A former Labour frontbencher took me to task, pointing out, as Tony Blair did this week, that it was possible both to win power and to be principled. The ex-MP was surely right. Nevertheless Labour is unusual as a party because of the historic tendency of some of its members and supporters to think that being in government is a regrettable diversion from having socialist values, a tendency turbo-charged in the modern era by the anger and indignation of social media. The Conservatives have no equivalent hang-ups of any kind.

The left is unusually bad at asking itself really difficult questions about its approach to politics at all, ever

The disabling effect of this misconceived purity v power mindset has been unexpectedly striking since Ed Miliband quit the leadership in May. Corbyn’s success reflects it. In May, many argued that Labour needed to have a serious debate about what had gone wrong in the general election and with the Miliband strategy. That was true then and is still true now.But there has in fact been very little such debate in the nearly three months since that defeat.

This is not because the leadership election has got in the way, as some feared. It is surely also because the left is unusually bad at asking itself really difficult questions about its approach to politics at all, ever. For too many on the left, to ask a question about, say, the size of the state or the success of the Tories is equivalent to displaying a lack of true faith in the ancient verities. When it is Blair who asks those questions, the reluctance to engage even becomes self-regardingly virtuous – presumably that’s why Corbyn didn’t even bother to respond with any seriousness to Blair’s charges this week.

Three decades ago, a few months after bitter internal warfare and a weak leader produced Labour’s worst general election performance of the modern era, and with the SDP-Liberal Alliance still snapping at its heels, Labour’s newly elected leader Neil Kinnock called on his deeply divided party to rediscover what he called its common sense and realism. In a speech to the conference in Brighton that had just elected him, he asked Labour supporters to remember how dreadful it felt to have been smashed in the 1983 general election. Never again should Labour experience that. Party unity was now paramount at all times, Kinnock warned. It worked, in the end, just.

It is easy to imagine Labour’s next new leader saying something of this sort when the party returns to Brighton again this autumn. But it is not so certain that the next leader’s calls for unity and common purpose will fall on receptive ears, especially if that leader is Corbyn, or indeed Liz Kendall. In 1983 there was a prevailing mood of fighting back from the existential brink. In 2015 the mood in today’s much smaller, less politically confident Labour party seems somehow more fatalistic.

Like the US, Britain once had a two-party system. That is long gone. Today, like many European countries, our party politics are fragmenting into a range of smaller groups. Like Germany, we may soon have an anti-EU left party controlled by old-fashioned unions, a green party, a liberal party and a residual social democratic party, all competing for many of the same voters.

Unlike Germany, we will also have separatist nationalist parties to complicate matters still more, and we will still have a first-past-the-post electoral system to empower the unified centre right. The upshot, almost inescapably, will be a long, perhaps a very long, period of Conservative rule. Labour’s only way of preventing that is by competing in the centre, with a modern reformist agenda that can challenge the centre right. But that, it seems, may no longer be Labour’s priority.