It's the death of thought and progress to connect exclusively with your own kind. And in a place like Northern Ireland, where sticking to tribal comfort zones led to disaster, it's a counsel of despair.
Unionists should admit that the 1916 rebellion was probably a direct result of the escalation of the anti-Home Rule campaign by the formation and arming of the Ulster Volunteers.
Nationalists should face the truth that deifying the tiny cabal who led the Easter Rising has left a toxic legacy that even now legitimises anti-democratic violence.
Which is why we should welcome and participate in events like the Enniskillen Xchange Summer School, which was held at the end of June.
Judging by the audience reactions, it fulfilled its ambition to help people "who want to bring about social change in Northern Ireland" by encouraging "thinkers and explainers" to have "alternative conversations and changed perspectives".
Those of us on the panel on 'Changing the Conversation on History' were briefed to remember that Irish history is more diverse than "orange and green". We certainly did.
Mark Leslie, who has organised exhibitions on Churchill, on Yeats and is now at work on the Easter Rising and on the Apprentice Boys, kept us laughing over the complicated and clashing religious and political loyalties of various members of his opinionated Anglo-Irish family, illustrated strikingly by a photograph of unionist and nationalist brothers having a fight on the steps of Monaghan's Castle Leslie.
His serious point was that Irishness is so layered, complicated and intertwined as to resemble a set of Russian dolls.
The writer Carlo Gebler, son of the very green Edna O'Brien, told us of the tragic life in Ireland of his Czech-Bohemian grandfather and of the maternal great-aunt who married a member of the Royal Irish Constabulary and was banned forever from the family farm by her IRA brother – who later shot her husband.
The music journalist and BBC Radio Ulster presenter Stuart Ballie tried to persuade us to blank out all history and instead find common cause in punk, blues and other non-Irish music.
And I suggested that our worst problem was the triumph, north and south, of violent hardliners – both green and orange – and that it behoved those of us unhappy that bigots and murderers attracted votes to challenge the myths, prejudices and propaganda of our own tribe.
Which is what was happening last Tuesday evening in London, courtesy of the Irish Ambassador, Dan Mulhall – a history graduate, who is marking significant anniversaries over the next few years by hosting informed discussions.
This time the subject was the 1914 Home Rule Act, which was never implemented because of the outbreak of the First World War and then the Easter Rising and consequent Anglo-Irish war.
As a great parliamentary happening, it had originally been scheduled to take place in the Speaker's House – a magnificent state apartment at the heart of the Palace of Westminster.
Unfortunately, the ambassador's brief for inclusivity clashed with the Speaker's refusal to entertain Sinn Fein MPs, on the grounds that they show disrespect to the Commons by refusing to take their seats, so the event was rearranged for the embassy.
There was plenty of civilised disagreement among the distinguished speakers – historians Lord (Paul) Bew (Queen's University Belfast), Professor Michael Laffan (University College Dublin) and Professor Richard Toye (Exeter) and ex-Taoiseach John Bruton – and hard questions were posed by chairman Fergal Keane and members of the audience.
Bew, who is no nationalist, spoke of the massive provocation caused by the Larne gunrunning, Bruton slammed the destruction of the constitutional tradition, Laffan talked of the lack of generosity displayed by both unionists and nationalists, and Richard Toye complained of the British government's cowardice in refusing to call unionists' bluff.
If only Peter Robinson and Martin McGuinness could think and talk as honestly about the dark sides of their tribal pasts that are the root cause of all the rows about parades and paraphernalia, they might make a bit of progress. But they won't, of course.
Most Northern Ireland voters have no right to complain about political paralysis: they vote for those who most loudly bang the tribal drums.