It didn't start in 1920. Or with the drawing of lines and the signing of treaties. Partition goes far deeper than that and a long way further back. As far as memory.
To understand it you need to place fear at the heart of your calculations. Begin with the Plantation: the native's fear that his land is lost forever; the settler's fear of overthrow by the native, all framed by the wars of Reformation and Counter Reformation which reshaped the politics of Europe but left Ulster mired in a toxic lake.
Now with Sinn Fein's dramatic gains in the Assembly elections, the looming reality of Brexit and the resurgent claims of Scottish nationalism, the possibility of a united Ireland is being canvassed once more. This time it is not only nationalist true believers.
The thinking classes of Britain, for whom the counties across the water have always been a confounding and disturbed entity, are daring to dream openly of peacefully tiptoeing away. Not so quickly, ladies and gentlemen.
Wishful thinking has always abounded when it comes to the protestants of Ulster. The Dublin protestant Wolfe Tone saw in the comradeship of Belfast Presbyterians the possibility of a movement that would unite all Irishmen. He paid too little attention to the raw sectarian reality that dominated life in Ulster. When the order that guaranteed the position of the protestant working classes - in city and country - was threatened they responded with violence.
The Liberator, Daniel O'Connell, made the same mistake when he planned his great march into Ulster in January 1841. When the loyalists began to mobilise and riot, O'Connell abandoned the procession and slipped into Belfast under a false name. It was ignominious but instructive. O'Connell's unhappier legacy was the association of nationalism and Catholicism. He had made the priests his ward bosses in the campaign for Catholic Emancipation, a tactic that worked well but sent an unmistakable signal to the Ulster protestants about the Ireland they could expect if O'Connell managed to repeal the Act of Union. On his visit to Belfast he learned the depths of popular protestant opposition. It wasn't a lesson that stayed learned. In every generation nationalist politicians and revolutionaries found themselves surprised by what, in retrospect, was screamingly obvious.
Parnell paid little attention to the reality of life in Ulster - sharp edged, violence bubbling - until the Home Rule riots of 1886 made the truth impossible to ignore. Hundreds of Catholic workers were expelled from their shipyard jobs, others badly beaten and one drowned in the River Lagan by rioters.
When Northern protestants mobilised against Home Rule once more in 1912 they brought the gun back into Irish politics and threatened a treasonous rebellion against the Government of the Crown. It is endlessly worth remembering that they were openly encouraged in this by the leader of the Conservative Party, Bonar Law.
But advanced nationalists saw much to admire in this armed self-assertion. Pearse and his comrades saw the British Crown as the true enemy. The protestants, perhaps with a little armed nudging, would ultimately see the sense in a United Ireland and fall into line.
The Provos made the same basic error. Unlike Pearse and the Southern leaders of the rebellion and War of Independence, they had grown up with the sectarian reality. Nobody could accuse them of not understanding what loyalism was capable of when push came to shove.
They understood but still fought on, believing, into the 1980s, that a united Ireland could be won by the Armalite and the ballot box and that if British support was taken away, loyalist resistance would collapse.
The Brits would get tired of it all. The prods would eventually make a deal. And the South would collapse under the weight of corruption and cronyism, paving the way for an all-Ireland socialist (ish) republic, once the party had figured out what that involved for its different constituencies. It didn't work out like that.
It is significant to me that in the revived discussion of a united Ireland, the Shinners appear more circumspect. It is after all the party's raison d'etre. The dissidents who accused them of being 'Free Staters' were wrong. Successive Southern leaders were content to put unity on the longest of fingers, a rhetorical commitment that might take many generations to come to pass and by the luck of God wouldn't happen in their lifetime.
I don't believe the current Sinn Fein leadership abandoned that ideal for the cushy life of ministers in a devolved eternity. They are no longer willing to be cheerleaders for killing and maiming but they are true believers in an all-Ireland republic.
It is significant that their spokespeople have been careful not to drum up nationalist expectations and protestant fear. The next year could prove the most febrile in the constitutional history of these islands since 1920 and the Government of Ireland Act. Politicians in the North have an immense responsibility to be responsible.
Working class protestant fear has not gone away. It may be less obvious these days, certainly since the flags protests died away. But it would be a catastrophic mistake to think the gun has been banished from loyalism no matter what.
Thirty years of violence in the North made many of us in the South back away from the idea of a united Ireland, or at least the narrow vision that was expressed through the campaign of killing.
But there is a strong case to be made by the proponents of a united Ireland, an economic, social and cultural case in which politics might at last be dragged out of its long confessional nightmare.
The problem is in our minds. In the South we regarded the Northerners of both traditions as whiney moanies forever reciting how we wronged or threatened them. We flinched and turned away. That is no longer feasible.
If a united Ireland is to eventually come about the role of the Republic is to be imaginative and generous. How would the South recognise the British identity of most Northern protestants? Is it possible to give legislative guarantees to safeguard that identity?
You might argue that 'Britishness' in itself could be a problematic identity to legislate for if the Scots opt for independence. But up North I believe a certain idea of Britishness, rooted in distant history, is likely to endure for quite some time to come. It is an attachment which gave a minority on a hostile island a sense of being protected in a much larger imperial entity. We are back to fear. Erode that, however long it takes, and you can talk optimistically about a united Ireland.