I recently asked David Ford, the justice minister, if Alliance ever considered withdrawing from the Executive. He ruled it out, arguing that, if he resigned the justice portfolio, it would be "too much of a shock to the system".
He questioned whether agreement could be reached on anyone outside Alliance holding such a sensitive position.
Most ministries are allocated on the d'Hondt system, with parties picking their portfolios in turn. Justice, alone, is allocated by a unique cross-community vote, which must be backed by a majority of both unionists and nationalists.
It is always possible that, if Sinn Fein and the DUP didn't have Alliance to fall back on, they would work out another solution, but they currently show very little sign of that level of trust. So the chances are that, as Mr Ford suggests, the system would collapse.
The control of policing and justice is as politically sensitive as that; yet, without powers over it, any devolved Assembly would look fairly hollow.
Policing is also the point on which the dissidents can attack; they watch every wobble in the hope that one push from them can topple power-sharing.
They need policing to be a divisive issue, they need the police to be under pressure.
Their deadly intent was seen in recent arms finds, especially the van full of mortars ready for use against a police station in Londonderry.
If that and other planned attacks had succeeded, it would have been a terrible backdrop for the visit of the First and Deputy First Ministers to America next week.
They need to show unity there and they need to show it at home, too. It may be galling to Sinn Fein when Troubles-era republicans are picked up, or charged, but they should resist the accusation of political policing.
The most serious turbulence of recent weeks was after the arrest of Sean Kelly, the former Shankill bomber, on suspicion of a so-called 'punishment' shooting.
Then both the DUP and Sinn Fein weighed in, with talk of shoddy policing, dire consequences for the political system and so on.
Mr McGuinness urged Mr Robinson to "keep his nerve" and it is advice that could be as well directed to his own party. They went off the deep end about the police's inability to halt the loyalist Union flag protests.
Mr McGuinness said the violence at the protests "represented a serious challenge to the political process" and Mr Robinson countered that refusing bail to those arrested was destabilising. Hopefully, now that the loyalist protests are cooling down, we will see people such as Willie Frazer and Jamie Bryson let out on bail, with suitable conditions. Whether or not this happens, senior politicians should keep their emotions under control in the run-up to the marching season.
Talk of instability and of dangers to the political process not only spreads panic, it encourages those who wish to collapse the institutions.
Next week, Mr McGuinness and Mr Robinson will be promoting Northern Ireland in Brazil and North America.
The best message they can send to foreign investors is that they won't break ranks under pressure on the issue of law and order.