The parable of Pasok, the Panhellenic Socialist Movement, has not been told often enough. Greeks know the story: they wrote it. The rest of us, bewitched for months by the triumphs and travails of Alexis Tsipras, Syriza, bail-outs and Grexit dramas, should attend to an instructive tale.

In 1981, Pasok and its leader, Andreas Papandreou, won 48.1% of the votes in elections to the Hellenic Parliament. For the next 28 years the party led and followed the conservatives of New Democracy through the revolving door of power, never once managing less than 38% of votes cast. In 2009, George Papandreou, latest in the dynasty, scored 43.9% and formed a majority government.

This January, as Tsipras swept to victory, Pasok got 4.68%. It achieved fewer than 290,000 votes. It trailed behind the Communists, the Golden Dawn fascists, and ragtag groups of independents. The party whose founders helped to restore democracy after the Colonels, who gave the country a welfare state, free education, and a European standing, learned the first and last laws of electoral politics: history doesn't matter and there is no such thing as gratitude.

One of these days, Labour in Scotland might master those truths. Reminding teenagers that your political ancestors founded the NHS is like answering a question that hasn't been asked. Pasok, too, was once supposed to be impregnable in its “heartlands”. Pasok, too, was one of the fixed constellations in an unchanging binary system. Its members filled the ranks of the elite, part of what southern Europeans call “the caste”. So what happened?

Over time, the movement grew routinely corrupt. It exercised its patronage in all walks of life and ran the country badly. In 2009, finally, the latest Papandreou decided that he and Greece had no choice but to accept austerity. Sound familiar? In 2009, Pasok, the party that had founded the Greek welfare state, resolved to cut pensions. Gratitude for past attainments evaporated overnight.

Look for the left in the modern world and the words “anti-austerity” is the shorthand you encounter. Around it there is a tangle of other words and phrases: inequality, globalisation, corporate power, neoliberalism, capitalism, social justice, community, environmentalism, gender, exploitation, representation, elites, self-determination. There are sub-sets of concerns, from trade deals to war-making, from job security to fracking, food banks to housing. Resistance to austerity is the working definition, in this young century, of “left”.

It is not, not by a long chalk, a perfect definition. Pasok, like Scottish Labour, like the German SPD or the Spanish Socialists, was in disrepute long before the G20 decided in Toronto in June 2010 to embrace “growth-friendly fiscal consolidation”. Despite the fond beliefs of opponents, in any case, being of the left is more than a reflexive protest against whichever status quo happens to be on offer. Nevertheless, if the left has begun to succeed, that is because others and their “solutions” have failed.

Labour in Scotland was not swept towards oblivion in May simply because the SNP added opposition to austerity to its list of causes. The startling response to Jeremy Corbyn's leadership bid is more than just a silly season fad. Pasok was heading for ruin in Greece long before the troika staged its coup against Syriza. Podemos was born in Spain after almost three years of the indignados uprising against cronyism and inequality. Failure, and the public recognition of failure, made these things possible.

A devil's advocate could assess RISE, launched in Glasgow yesterday, on hard terms. A left alliance – Respect, Independence, Socialism, Environmentalism – forged by familiar partners, it could be dismissed easily enough. A Scottish Left Project drawing on the International Socialist Group together with the Scottish Socialist Party (SSP) and including veterans of the Radical Independence Campaign, all with the aim of fielding candidates at the next Scottish elections? What could possibly go wrong?

You could bear in mind, after all, that the independence campaign was lost. You could add that the SSP has not fared well lately: a total of 875 votes in the four constituencies it contested in May; just 8,272 votes for the eight regional candidates it put up in 2011. Besides, “left alliances” have a comically dismal history. Any notion that memories of Yes campaign unity might linger are already being dismissed, sometimes brutally, by SNP activists.

All of that is true; none of it is to the point. The impulse to find alternatives on the left is being felt everywhere. Corbyn, far less Podemos or the Occupy movement, is proof enough. The Islington North MP has been available for socialist vanguard duties for 32 years. Only now, this year, among people sickened by what their society has become and out of patience with Labour's eternally compromised usual suspects, does he strike a chord. He strikes terror, too, within Labour's ancien regime. They know what became of Pasok, even if most voters do not.

Corbyn complicates life for RISE. The SNP won 50% of the Scottish vote in May in part because of a declared opposition to austerity. Who, it is fair to ask, will hold the Nationalists to that? Corbyn's interest in Scotland's affairs is slight, but the Scottish branch of his party will ride on his coat-tails next year if that's what it takes. If the country is moving leftwards, there will be born-again Labour socialists on every ballot paper. And much good it will do them.

RISE, the Greens, Labour revived: sooner or later, the SNP will find itself with an opposition and a test for its credentials as an anti-austerity party. That matters, but it is only one part of a bigger picture. Across Europe and beyond, a tide is turning. The real lesson of the battle between the lenders' troika and Tsipras was not the humbling of Syriza, but the fact that the battle was fought at all. Greeks refused the logic of austerity, therefore they were crushed. They and millions more are unpersuaded still.

The meaning of “left” in the 21st century is hard to interrogate, in part because its meaning is still being defined. It's true, but not enough, to say that if capitalism was subjected to the Blairite “whatever works” test we would be trying to spot the joke. It's true, but neither new nor enough, to say that the world is ill-divided. The easy part of being left-wing is stating what's wrong. Identifying what would be right, and better, evolves in argument. The tricky part is that the argument never ends.

The left, resurgent or not, has any number of other problems. One might be the word itself. In the 21st century, in societies constructed around the illusion of choice, many fancy themselves as left “on some issues”, not so much, if at all, on others. In fragmented cultures caught up in shifting social media allegiances, in job insecurity and improvised bonds of friendship and family, communities of interest come and go. Class, in the old sense, is a contested notion. Newer versions – the 1%-99% divide named by Occupy – are rough and ready.

Marx will do, nevertheless, in one regard: ultimately, discontents are economic. Or rather, the causes of injustice can be found in the abuse of economic power by those who possess it and will not yield it. So corporations mock democracies and control governments. So workers are reduced to serfs, labouring and consuming, always disposable. So women are held in subordination. So the environment is looted. So wealth is drained to pay for the toys and follies of the 1%.

And so on. One part of the left's revival stems from 40 years of argument (and rhetoric) on such topics. While voting as though there is no alternative, people have heard plenty of alternative explanations. For every glib George Osborne there is someone like the Dundonian political scientist Mark Blyth, of Brown University on Rhode Island, writing Austerity: The History of a Dangerous Idea, or a Thomas Piketty explaining exactly what connects capital and inequality.

To be left-wing, young or old, is to grasp one thing first and above all: it's not just you. Yet that insight leads to another problem. In the 21st century, capitalism is more truly global than it has ever been. We have a word for that statement of the obvious. What we lack, if we dissent from the process, is an equivalent response. When the entire G20 decided in 2010 that austerity was its new happy thought, responses were local, in Greece, Scotland, or elsewhere. Those answers were necessary, but not sufficient.

Occupy Wall Street, when it first appeared in New York's Zuccotti Park in the autumn of 2011, pointed – and waggled other fingers – in one direction. To William Beveridge's five “Giant Evils” of want, disease, ignorance, squalor and idleness, a sixth was added. Inequality, the distribution of economic power, will be the left's global cause in this century. You might say there's nothing new in that, but you'd be mistaken. When a European democracy is swept into the gutter, as Greece was swept into the gutter, one side at least isn't kidding.

The SNP's election victory was anything but an accident. Corbyn's success is not accidental. Syriza, Podemos, RISE, Occupy and all the others are not happenstance. Chatter over “populism” and “electability” miss the point as spectacularly as Pasok missed the point. When Bernie Sanders, for decades the comedy “democratic socialist from Vermont”, is giving Hillary Clinton a fright in America's Democratic nomination race, something is stirring. People are listening. The least a defender of the existing order can admit is that people, the people, are deeply unhappy.

Aren't they always? But here's Sanders drawing crowds when he talks about income inequality, when he promises a solution to student debt, when he argues for tax reform and an assault on Wall Street's privileges. This sort of thing is not often part of American discourse, but in 2015 it's mainstream. And Sanders is another who believes he can build a winning electoral coalition amid the discontents of a fragmented society.

The left now is a phenomenon marked by unity in ramshackle diversity. Cross-generational, cross-cultural, informed by the past or discovering old truths anew, it takes that single word, austerity, and explains a multitude of things to multitudes, inequality above all. It sits badly with the fine theories. Its inconsistencies are endless. It can be mocked, if that's your taste. It persists.

After all, its enemies have made themselves as obvious as the injustices they inflict. As Pasok, Labour and the rest fall away, arguments clarify. If you're not of the left, where are you in this life?