The author Neil Gaiman once said that if someone tells you your book isn't working, they're almost always right, but when they tell you why it isn't working, they're almost always wrong. The same surely holds true in politics.
There will be plenty of wise sages right now offering unsolicited advice to Mary Lou McDonald about why Sinn Fein did so badly at the local and European elections. She herself says: "I'm determined that we will learn the lessons."
But what are those lessons? Clearly something went terribly wrong, but it's actually quite hard to pinpoint what that was.
In fact, the most honest assessment of what happened may have come from Matt Carthy, who did retain his seat as MEP for Midlands North-West, and who, when asked for an explanation of the party's poor performance elsewhere, said: "I really, really don't know."
It was easy when Gerry Adams was leader. Every setback could be put down to the smell of sulphur which he carried over with him from the Troubles. These days, analysts will have to struggle harder to find reasons why so many of the party's former representatives at council and European level are now "resting" between engagements, as unemployed actors say.
It has even been suggested in some quarters the Greens' support for a carbon tax, while Sinn Fein is against one, was crucial to the result - which would be very convenient, if the Greens hadn't polled a measly 5.5pc in the locals. It's hard to argue on those figures that the country is crying out for a tax on greenhouse gases.
It shows how important early impressions can be. Initial exit polls showed the Greens polling magnificently and that story stuck, even as party leader Eamon Ryan's boast about having a 50/1 bet on Saoirse McHugh becoming MEP for Midlands North-West became a cautionary tale about the perils of reckless gambling.
Where it did occur, surely what the rise in Green support shows again is that it's crucial to be transfer-friendly. No one minds transferring to the Greens, because most people think, wrongly, that it's like voting for Santa Claus.
Sinn Fein remains transfer-toxic, partly because Mary Lou, who rode in as the great white hope to decontaminate their terrorist legacy, bottled the challenge but seemed to hope it wouldn't be noticed.
What can be put down as absolute fact is that, where Sinn Fein has wielded power in local councils, they failed to hold on to seats, because voters were able to see for themselves that they talk big, but rarely deliver. There's an emptiness at the heart of all that high-faluting rhetoric. Mary Lou epitomises that problem. If you want someone who can talk, she's your only woman, but what has she ever actually done? Her enduring preoccupation with the North is key.
People are worried about what will happen after Brexit, but they're not lying awake at night longing for a Border poll. That's an indulgence which doesn't answer their immediate economic concerns.
Insofar as the North has impacted on perceptions of Sinn Fein, it has been to recast it as wreckers since bringing down the power-sharing Assembly with, it's obvious now, no clue how to restore it. The party's strength in the North fools Sinn Fein into thinking of itself as bigger than it really is. Mary Lou talks about Sinn Fein as if it's comparable in size to Fine Gael and Fianna Fail.
The party also still has a quasi- militaristic view of campaigning picked up in Northern Ireland. That is, you take an area, set down a camp and then move on to conquer new territory. Sinn Fein forgot that territory won needs to be continually defended. Again, that's because it doesn't face the same level of competition in the North - though even there, without opposition, it is going backwards, dropping more than 33,000 votes in the European election - while in the Republic there's an abundance of rivals.
They don't do nearly so well in a crowded field. They don't think fast enough to keep up and imagine that, once they have voters, they'll stay loyal, as they've generally done in West Belfast. That doesn't happen any more, and certainly not south of the Border, where no seat is safe.
That they still haven't grasped how badly they were damaged by claims of bullying by former members, and by a litany of IRA abuse and rape scandals, was demonstrated again last week when Carthy declared these were "just media-generated issues". In politics, blaming the media is the last refuge of the loser.
Headlines don't rape children. It's republicans who did that.
One could go on forever listing reasons why Sinn Fein took such a hammering at the polls, and perhaps that's the point. It wasn't one thing. It was an omnishambles, in which everything went wrong. That's why they lost ground to all parties - Greens, FF, even Labour.
One thing which does bear singling out is that so many of those who left the party over bullying claims won their seats as independents this time round. That gets to the heart of what is wrong with the party. David Yamada is a professor of law at Suffolk University in Boston and a renowned authority on workplace bullying. He has identified common features among those who get this issue wrong.
The first is a lack of accountability. Bad organisations regard allegations of past misconduct as ancient history, with those who want to face up to them being accused of dwelling in or on the past, even if that past was relatively recent. Sinn Fein does that all the time.
The second is the turning of those who criticise the organisation into "unpersons". Peadar Toibin TD, who left the party to form Aontu after being disciplined for his pro-life stance on abortion, had an encounter last month with a Sinn Fein official in Leinster House, during which he was told to "f**k off out of this office before something happens". Toibin observed caustically: "21 years I gave these people."
It's a wonder he never noticed this nasty side to them before. Even so, it illustrates a point. Critics are ostracised, cast out, blacklisted.
The third common feature of bad organisations, according to Prof Yamada, is that they hold to the theory the good things they've done in some way excuse all the bad things. That was how republicans tried to cope when found guilty of covering up sexual abuse. They declared they may have made some mistakes but were on the side of justice in other areas, so why not let the bad stuff slide?
This retreat into "rose-coloured memories" is invariably fatal.
Carthy has promised the party will now have an "intensive discussion internally" and "ask big questions" about what went wrong but, ultimately, it could be the real reason Sinn Fein has gone into such sharp reverse is simply because it's no longer relevant.
Its representatives complain when they're subject to media attacks, but at least that proves they're worth attacking. Nowadays, the party feels a bit surplus to requirements. It's not 2014 any more. The age of protest has given way to a more nuanced economic analysis than it seems capable of providing. Insofar as discontent still exists, the Greens are riding the zeitgeist, while Leo Varadkar has the nationalist tub-thumping on Brexit covered. What Sinn Fein really needs is another recession to give it back momentum, and no one should make the mistake of imagining it is not callous or fanatical enough to wish for one.