How much should a political party set out who it would and wouldn’t work with after an election? And is too strict a statement ultimately disrespecting the wishes of the voters?

I was prompted to wonder this following a packed public meeting in Lewes last week at which I shared a platform with Caroline Lucas, Sue Goss and the Guardian columnist Rafael Behr. We were discussing how Labour, Liberal Democrats and Greens could work together to offer a progressive alliance as a genuine compassionate alternative to the Conservatives.

Right at the end of the meeting, Caroline Lucas chastised Tim Farron for failing to rule out categorically that the Lib Dems might go into coalition again with the Conservatives. I have an awful lot of time for Caroline, believing her to be one of the outstanding politicians of our age, so it always slightly unsettles me when we disagree, but I felt the need to defend Farron’s statement.

You could feel the room go cold as I did so. People were muttering in open rebellion. Later, one person tweeted that my comment, allied to the Lib Dems’ record on tuition fees, showed that you couldn’t trust the Lib Dems, while another emailed saying I had spoiled an emerging consensus on cross-party cooperation.

There’s nothing like that to make you really examine your own pronouncements.

My biggest concern, as someone who believes in politics (with all its faults) as a good guarantor of the people holding the executive to account, is that we should establish our integrity. From that can flow trust. It’s a very hard task, because it means setting out policies that emphasise differences, knowing they will be misrepresented, misreported and exaggerated, but we still have to be clear what we stand for.

Then we put what we stand for – in the form of policies, with red lines clearly marked – to the electorate. We then assess the results, and if it’s not clear that one party can form a government, some sort of coalition or minority government has to emerge. That, to me, is how democracy should work.

But what happens if the electorate votes in such a way that the only viable majority is between two parties who never envisaged working together, as happened in 2010? That’s where you have to trust your policies, and if you can get your red lines through, you can form a coalition, or at least a basis for supporting a minority government. The alternative – a minority government with no real mandate – doesn’t offer much stability. Or we could have a new election, though that might elicit the same result and we’d be back to square one.

I personally don’t want another coalition with the Tories, and I believe most Lib Dems feel the same. I definitely don’t want a coalition with any party unless there’s a cast-iron guarantee of a proportional voting system, and that would kill any Tory-LibDem coalition stone dead. But if you say ‘In no circumstances would we go into coalition with X party’ (the Tories or anyone else), then you’re effectively saying there are circumstances in which you won’t listen to the voters. To me that’s fundamentally wrong.

So the ethical way to proceed is to set out your stall based on what you believe, and if a coalition between the Lib Dems and the Tories becomes impossible simply because the two platforms are too far apart to get a viable coalition agreement, so be it. But I still think the principle is right and one we have to be aware of if we’re to keep our integrity.

And really, every party has to work by that principle. Look at Germany. There there’s a Grand Coalition where the SPD (the equivalent of Labour) is in coalition with the CDU/CSU (equivalent of the Tories). And if the extreme right AfD polls well in this autumn’s elections, another Grand Coalition might be the only way of preventing extremists getting into government. Does this mean the SPD has betrayed its supporters by going into coalition with the CDU/CSU? Maybe some think so, but based on the result delivered by the voters, the SPD felt that the best thing for the country and for getting some (though not all) of its own policies enacted was to go into a Grand Coalition. And at state level in Germany, there have been all sorts of mixed governments, including ‘black-green’ administrations in which CDU and Greens go into coalition – yes really!

So I don’t see Tim Farron saying he cannot absolutely rule out a future Tory-LibDem coalition as scuppering the progressive alliance idea. If anything it actually strengthens it. If Lib Dems, Labour and Greens put forward honest manifestoes at the next election, it will be clear that (a) they dovetail neatly around each other, (b) none of them fits with what the Tories will want to do, and (c) none of them can be manipulated to fit around Tory plans.

And it also emphasises the fact that we take our cue from the voters, not from our own parties. Paying more attention to our party loyalties than to the voters is what got politics into a mess in the first place, and led to us totally underestimating the sense of despair that resulted in the Brexit vote.