An attack on democracy
And so an exercise in democracy - indeed a discourse about competing interpretations of democracy - has been interrupted by an attack upon democracy.
The day had started in decidedly normal fashion. Nicola Sturgeon visited a nursery in Edinburgh to meet happy, smiling children.
She had two purposes. To announce extra funding for child support. And to demonstrate that she remained resolutely focused upon the day job.
There was even time, at that stage, for a little drollery. Invited to choose a toy as a mascot for the debate, the FM opted for a shark.
At Westminster, meanwhile, questions to the prime minister followed a fixed, familiar pattern. Labour's Jeremy Corbyn asked about education spending. The PM replied robustly.
The SNP's Angus Robertson challenged the PM over Scotland's relations with the EU. He accused her of dodging the issue. Theresa May insisted that Scotland remained fully in her mind, firmly within the UK.
Back at Holyrood, Nicola Sturgeon and her deputy John Swinney arrived for day two of the debate about whether powers should be transferred to the Scottish Parliament in order to hold a referendum.
But then the news from Westminster began to arrive. Like a slow-acting poison, it steadily infected the continuing debate.
MSPs were, inevitably, distracted from their own day job as they contemplated the assault upon their fellow Parliamentarians at Westminster.
Some were initially reluctant to suspend. Some indeed persisted in that view, arguing that closing down a debate was giving way to terrorism.
But most I spoke to took a contrary view. They argued that it would be wrong to persist with a Holyrood debate while the UK Parliament was in lockdown.
Some argued, further, that it would be particularly inappropriate to persist in a debate about the UK's role in Scotland at a moment of crisis for the Palace of Westminster. To be clear, that is not universally shared.
Either way, it was agreed by the parliamentary bureau - and announced by the Presiding Officer, Ken Macintosh - that the debate would be suspended.
To be clear once more, debate will be resumed here and in Westminster. I watched the shocking scenes at Westminster, where I used to work decades ago.
I observed the uncertainty at Holyrood, together with the safety-first heightened police presence. Democracy does indeed require persistent vigilance.