The German philosopher Immanuel Kant, tired of arguments that went round in circles, published a short book in 1783 setting out some ground rules, Prolegomena to Every Future Metaphysics.
It is not always clear whether the ideal of a united Ireland belongs more to the realm of metaphysics or to practical medium-term politics. Whichever is the case, a prolegomena is needed to cut some of the dead wood out of the argument.
First, it is legitimate to seek a united Ireland peacefully and by agreement. A letter to the editor (Dick Keane, September 29th) argues that we must formally renounce the very notion of a united Ireland for the sake of peace.
This suggestion would require repudiation of the Good Friday agreement, which is the basis of the peace that has been established with some difficulty. It explicitly underlines the legitimacy of seeking a united Ireland instead of the Union.
The central point of the peace process was to provide a political alternative to violence. It would be indefensible to have brought about peace by offering that alternative, which has no guarantee of success, and then to block it off.
The second, and obsolete, proposition, coming from another direction, is that the British government becomes a persuader for a united Ireland. That point has been argued, negotiated and settled. Since the Downing Street Declaration of December 1993, the British government has contracted to be a vigorous persuader for an agreed Ireland, which need not preclude assisting when there is consent to a united Ireland.
As the declaration pace John Hume put it: "Both governments accept that Irish unity would be achieved only by those who favour this outcome, persuading those who do not, peacefully and without coercion or violence."
It is a telling admission of weakness and lack of persuasiveness with unionists, after 30 years' attempted use of force, that the two governments are called in by Sinn Féin, the British government to persuade unionists, the Government to produce a green paper on Irish unity.
As a historical footnote, previous British efforts to persuade unionists to consider a united Ireland, in November 1921 and 1940, failed dismally. The question of unity is premature at this point, as the first priority is to have the agreement working fully.
The most widely accepted and recognised rule is the principle of consent, whereby constitutional change in the status of Northern Ireland requires the consent of a majority of the people of Northern Ireland. Bringing about a united Ireland, however, requires the concurrent consent of the people in both parts of Ireland. The support of a majority in the South can no more be taken for granted than in the case of a united Cyprus.
The principle of consent is attacked from two sides. Some dissident republicans justify continuing armed struggle in the absence of a single all-Ireland vote. Ruairí Ó Brádaigh, president of Republican Sinn Féin, claims "the historic Irish nation" has a right to self-determination, but fails to spell out what electorate, if any, that would involve.
The 32-County Sovereignty Movement, operating looking-glass logic, and for which the "Real IRA" is the law, declares the Good Friday agreement and all its works illegal. Only the IRSP, even though critical of the agreement, has been politically consistent since 1998 in calling for an end to all republican armed campaigns, because they confuse the working class.
Some unionist voices have expressed doubts about the validity of a narrow majority for change, implying that the rules may need to be altered, lest the principle of consent should ever work against them.
As a matter of record, the loyalist parties supported the agreement, including full decommissioning and the principle of consent. They have no more right to prevent by force a united Ireland from coming about democratically, than republicans have to bring it about by force undemocratically.
Border polls are not designed to be held for propaganda purposes, unionist or republican, but only if change seems likely to result.Preferably, the people North and South should be asked to vote on a fully worked-out and prenegotiated agreement, rather than an abstract proposition. Otherwise, a Border poll resulting in a vote for change would have to be followed later by a concurrent voting decision North and South on the detailed shape of a united Ireland.
The Good Friday agreement is a peace settlement and a political settlement in its own right. It would be wholly illogical to try to bypass it in favour of something more ambitious, without first having developed greater trust and co-operation. North-South co-operation can be expanded under the terms of the agreement, but only if the Assembly is sitting and the executive re-established.
In the absence of consent to ending partition, the agreement mitigates the separation between North and South. However, certain economic benefits, like participation in the euro, the low rate of corporation tax or Southern levels of infrastructural investment are unlikely to be available only to Northern Ireland from London, because of likely knock-on demands from Scotland and Wales and other UK regions.
One of Kant's essays was entitled "What does it mean, to reorientate one's thinking?" The renunciation of coercion on the republican side requires a major reorientation of thinking. To attempt to bring about a united Ireland, even could it be done, without active participation from at least some members of the unionist community in Northern Ireland would be a sorry exercise.
Unionists for their part face the challenge of showing that their preferred course, continuance of the Union, is capable of integrating Northern society on a basis of equality, fairness and decency, of overcoming appalling manifestations of intercommunal and sectarian hatred, and reversing the ghettoisation and separate development, that is now regarded as dangerously undesirable in Britain.