The phenomenon is easy to describe, but hard to explain. Suddenly, after years of pessimistic predictions of ever-deepening voter apathy, political engagement is back. It might not be engagement of a kind always welcomed or appreciated by self-selected insiders, but it is real. Involvement in politics is no longer written off as pointless.

The independence referendum is one obvious case in point. A faintly-staggering 84.59 per cent turnout was followed by an equally remarkable upsurge in SNP membership. Scots on both sides voted in overwhelming numbers. Many who voted Yes then turned their enthusiasm into active support for a party.

Less discussed has been the pattern of general elections. Turnout, it transpires, has risen steadily since the 59.4 per cent nadir of 2001, the lowest such figure since 1918 (when the aftermath of a cataclysmic war at least provided an excuse). In fact, May 2015 saw the highest participation rate in a general election this century.

Then there is Jeremy Corbyn’s elevation as leader of the Labour Party. No one predicted his success, of course, but nor did anyone foresee thousands rushing to join the party or register as supporters simply because of one outsider’s apparently quixotic campaign. Labour says 30,000 new members have joined in the few days since Mr Corbyn was chosen. Patently, these people are galvanised and inspired. They certainly do not believe they are wasting their time or their money.

Perhaps most encouraging of all, regardless of your views, is the evidence that the 16 and 17 year olds who won the right to vote in the referendum have lost none of their enthusiasm since. Contempt for political elites does not discourage them. The cliche that parties “are all the same” does not deter them. These are believers to democracy, recruits to an idea.

Why is all this going on? Ironically it may be that disillusionment with politics has produced its own, political reaction. As with Scotland’s 16 and 17 year olds, one factor in these upsurges is clearly generational. Fresh eyes are being brought, at last, to old arguments.

There is a sense, too, of movements beyond the party-political boundaries. The word “insurgency” is often used, but it would be more exact to say that this new politics involves a willingness to ignore existing structures. Social media might have something to do with it. Party professionals – and more than a few journalists – might not care for the gatecrashers, but when issues come to matter more than allegiances, that is surely no bad thing.

A campaign group against fracking emerging within the Scottish National Party provides one illustration. This is not a simple case of the SNP versus the Conservative Government. It does not require agreement over a fraught issue to be appreciated. It simply counts as still another example of the politically engaged finding new ways to pursue causes.

Only the hidebound or the foolish would dismiss any of this as mere populism. Formal party politics has fragmented in the 21st century. The share of the vote achieved by once dominant parties has been shrinking steadily. To many enthused by the possibilities, that looks less like a problem than an opportunity.