It seems a simple and obvious fact that the sectarian legacy in Northern Ireland is historic. That is acknowledged in the foundational documents of the peace process.
That's a crucial part of the analysis of conflict here, because, in some measure, it absolves people in the present of at least some of the responsibility; they are acting out the momentum of past events for which they cannot be blamed.
The way then to bring peace, our peacemakers concluded, was to identify the historic problems, the unfinished business of the past, and put it right.
Now, I don't disagree with that and I'm sure not a lot of people do. The impact of the past on the present is identifiable at every stage of the Troubles.
In fact, two books written in the early stages of the Troubles illustrated the connection. One was Andrew Boyd's Holy War in Belfast, which culled old News Letter reports to describe the sectarian rioting in the city in the 1880s.
One of the irritants then was the use of Scottish Army regiments against Catholic rioters. An echo of the same kind of concern expressed itself in the recent rumour that members of the Garda Siochana were being used to police the flags protests in Belfast.
So plausible did one Executive minister, Edwin Poots, find this charge that he checked it first before going on Facebook to assure paranoid Protestants everywhere that it wasn't. The other book was ATQ Stewart's classic The Narrow Ground. Stewart asked a simple question: how did the riots of the 20th century in Belfast come to be so similar to the riots of the 19th?
He said that the eruption of rioting was like the past itself crashing up through the cobbles and history repeating itself.
Well, one explanation was that the same concerns were still there; the same old, unresolved questions about identity and the constitution.
Another was that people carried the lessons from one period to another; old guys taught young guys how to make petrol bombs, told them where to get guns and how to shoot; taught them to make barricades and how to exploit occasions, like parades and funerals.
That passing-down of skills and grievances is still obvious. The last generation of violence is not so far behind us that there aren't plenty of people around still who know what it takes to organise a riot, or train a few assassins.
But is it true that history itself is a player; that the unfinished business of the past is the problem? Surely one problem with that theory - axiomatic as it seems to be - is that most people know very little history.
After the BBC Politics Show on Sunday, we chatted in the studio about the coming visit of Liam Neeson. Ukip's David McNarry - a perfectly genial and decent bloke - said that he had really enjoyed Neeson in the film Michael Collins. And it seemed to me a hopeful thing that a unionist would have been interested in republican history.
"How accurate it is, of course, I couldn't say." So he doesn't know his republican history. That's no slight on him; I'm sure a lot of republicans don't, either.
Neil Jordan, the director, was accused of tinkering with history for the film anyway, describing Collins as the man who tried to take the gun out of Irish politics, a phrase that had, in fact, been coined 70 years after Collins had died, to honour Gerry Adams.
A friend pointed out the other day that lines of William Drennan, the founder of the Royal Belfast Academical Institution ('Inst'), had been cited in tribute to Dolours Price, the former IRA bomber who died last week. You have to wonder how many students at Inst today would accept the relevance.
Dolours herself had spoken of how she was inspired to republican ardour by her aunt, injured moving a bomb, before reading any republican history. And many republicans didn't like the new history books, anyway, accusing them of being "revisionist", when they argued that the issues that had driven violence in the past had varied.
Dolours also told me that her sister, Marian, when officer commanding the women prisoners in Armagh jail, would discipline those who came back from a visit with a love-bite, thereby undermining the dignity of the cause.
That story alone tells you about a puritanical streak that suffused republicanism in the 1970s that would look ridiculous today.
In the 1980s, Belfast republicanism looked more like its 1930s' forebear than like the culture of today, with its Catholic iconography on some murals.
I remember rallies that were ended early to allow people to go to Mass. Maybe that would still happen in parts of Tyrone; I doubt it would happen in Belfast. And how typical of a traditionalist loyalist legacy is the current wave of protests over the Union flag?
Well, we have not seen any hymn-singing at the roadblocks, such as would have been normal when Ian Paisley snr was leading protests.
There has been virtually no theological input into the discussions; no claims that the Catholic Church is manipulating anyone.
The first thing that would strike a time-traveller from the 1970s is how secular these protests are; suggesting that what is still with us is just division - and that division survives both political reform and huge cultural change.
We cannot predict what Protestants and Catholics will be fighting about in a hundred years from now.
Only that, on past form, they will be fighting about something.