A break in party unity
Politics, especially party politics, involves compromise. Few individuals will agree with all elements of their party's programme.
They subsume their doubts about one aspect or another in pursuit of a wider, collective aim.
That is particularly true of the SNP, which is an independence movement as well as a party.
However, sometimes an issue looms so large that an individual member can no longer observe the compromise involved in supporting a party.
That was true for some Tories over the EU. Much earlier it was true over issues like Irish Home Rule and the Corn Laws.
Now John Finnie and Jean Urquhart have concluded that they can no longer remain as Nationalist MSPs in the aftermath of their party's decision to support Nato membership.
They continue to support independence - but sans Nato.
In essence, they are arguing that they are no longer able, on principle, to support the overall SNP pitch because they regard it as contaminated by the new posture on defence.
Alex Salmond has regretted their decision and looked forward to working with them in the 2014 referendum.
But not, it would seem, as party colleagues. Privately, Mr Salmond will be aggrieved at a break in the unity he has sought to project - just as he has sought to depict his rivals as divided.
Privately, he will be of the view that the decision by his two colleagues weakens, at least in the short-term, the campaign for independence.
Frankly, much of the argument for Nato within the SNP rested on a pragmatic calculation that most voters favoured membership.
Mr Finnie and Ms Urquhart have concluded that this is a pragmatic compromise too far.
They will argue that their decision is driven by honourable principle - which many, notably in the Greens, will applaud.
This will subside to some extent as other issues arise, but right now Mr Salmond may feel like offering advice to his colleagues, borrowed from an earlier party leader: "Damn your principles. Stick to your party."