"She doesn't have an original idea in her head," said the man in the tweed jacket, with a contemptuous laugh.

There was only one person he could be referring to. Not since Maggie Thatcher has there been a more powerful or controversial woman in the public eye. Whatever you think of her, Nicola Sturgeon is the Queen of She, the only person anyone is likely to be referring to this week when you overhear a discussion about an unnamed 'her'. Some of us find the First Minister impressive, regardless of the way we vote; others see not a sophisticated politician, but a gorgon itching to cut the heads off unionists and venture capitalists, who must be put down at every chance.

I would have liked to take Sturgeon's detractor to task for his slur, but not only was I eavesdropping, tucked in my corner of a cafe, but I didn't know him or his cronies. Nor is it ever a good idea to launch into verbal combat with strangers. Feeling cowardly, I left without a word, consoling myself that soon she would prove him wrong.

The situation is less simple when you can't avoid an argument. There are occasions when you are obliged to express your views despite suspecting that your companions come from a hostile camp. At dinner last week with friends from London, it was hard not to feel defensive when, after being asked how we'd voted in the referendum and why, my enthusiasm for independence was challenged, point by point. Fortunately for the mood of the evening, one of the few things age has taught me is sometimes to stop talking: not to respond to what feels like provocation nor to raise one's voice to a pitch that only bats can hear.

It is the civilised option, but it is also the most difficult, because as recent years of coalition and SNP government have shown, politics is a touchy subject, and growing more so. If you take any interest whatsoever in how this country is run, then it is almost impossible to be relaxed or urbane given the issues at stake.

With one close friend who is a staunch Tory, I long ago realised that the relationship mattered far more than trying to berate or convert her. Possibly she was thinking exactly the same thing, although being more phlegmatic she was more likely untroubled by my left-leaning sympathies.

Back then, as the poll tax riots raged, it was hard to imagine the temperature getting any hotter. But with the prospect of the Union's collapse, what was previously a pan of boiling sugar seems to have turned to molten magma, so thick are the clouds of steam it's generating. Despite the referendum, it feels as if every political conversation is a rehearsal for another one. Not only does it grow wearisome, but at such times I can't help noticing that while SNP voters are often castigated as belligerent, it is we who are always asked to explain ourselves, not the other way round.

I won't be the only one who has felt an ideological frost settling over a few relationships of late. With friends, it is possible to survive a mini-ice age, knowing that it will eventually thaw. When it comes to family, however, cooling is not an option. In mine, there are almost as many shades of political allegiance as there are colours in a black eye. While it is tempting to go into their midst whirling political opinions as if they were a lasso, I have taken the Trappist oath. Barring the occasional sharp intake of breath, or the need to go outside and talk to the wheelie bins, that's as far as dissent among relatives has gone.

In such times that is little short of miraculous. Yet increasingly I recognise that how we vote is as much a matter of conviction as of rigorous analysis. Like religion, politics is about faith. And like religion, it is so profoundly personal that except where it strays into misinformation or defamation, it should be respected, not challenged. Just as it is bad manners for a Kirk minister to try to convert someone who is minding their own business, so in politics those from one side of the fence should leave those on the other in peace.

I'm not saying it's easy to stay quiet, when you itch to enlighten someone you think is misguided. Tempering belief with a touch of sang froid, knowing when to engage and when to say nothing, is a high wire act. It requires natural skill and years of practice. Just look at David Cameron and Nicola Sturgeon. Both of them seem to have mastered it - and see where it's got them.