Much discussion, including my most recent post on Brexit, argues that a second referendum is unlikely on political grounds. But that is very different from the question of whether there should be a second referendum. That is what this post is about.
A few days ago the Guardian published a piece by Simon Jenkins arguing that there should not be a second referendum. The argument is essentially this. The people have already voted to leave, and “effectively” delegated to government/parliament the choice of how we do it. All we should be talking about now is how to get the best deal. He writes:
“There are no rules for referendums because they do not feature in any constitutional edict, but convention dictates a second one would be justified only if the circumstances of the first have radically changed. The Danes (in 1993) and the Irish (in 2009) held second referendums, after renegotiations, to stay in the EU. Circumstances do not include second thoughts.”
There are so many problems with this argument that it is difficult to know where to start. Let me begin at the end. Now occasionally second thoughts arise through deep contemplation about your original decision, but most of the time second thoughts arise because circumstances change. The information people have about what leaving means has changed. Democracy is all about letting people change their minds because circumstances have changed. 
Jenkins implies that holding a second referendum after a renegotiation is fine. What on earth does he think has been going on for the last year and a half. The conceptual difference between a negotiation and a renegotiation is zero. People may well have voted Leave because they believed, as they were encouraged to do, that the UK could get all the benefits of the single market after leaving. Finding out this isn’t possible is not some whimsical second thought, but exactly equivalent to an agreement being renegotiated as far as voters are concerned.
Jenkins does concede that “there might, just, be an argument for a referendum after a final deal, purely to confirm it.” You have a choice, as long as you say yes. This accepts the absurd framing of the government that the referendum fixes the decision to leave for all time, whatever the circumstances. There are no conventions that say that. The obvious choice to put to people once the deal is done is whether they still want to leave, now that the best feasible terms for leaving as approved by parliament are known.
The argument that this must be the convention because we have not had a referendum since 1975 is nonsense. The first point to make about 1975 is that we had already joined, so the negotiation had already been done. Second, there was a sort of understanding among politicians that we should only have another referendum if the nature of the EU changed, but (a) there was no referendum after the Single Market was established, and (b) we had a referendum in 2016 years after any conceivable change. So as conventions go this one has been honoured in the breach, for the straightforward reason that it makes little sense. Information about the benefits of being in a club of nations changes for many reasons besides treaty change. I think it is best to forget about conventions and just use common sense. 
The vote in 2016 was pretty close, and was based on no clear understanding of what leaving would actually involve. So common sense says that the democratic thing to do is revisit the question once the terms are known. People should be allowed to change their minds. If people are not allowed to do so, and must be bound by that one vote for a generation, then we are no longer a democracy.
If you disagree with that, imagine the possibility that the UK ends up effectively agreeing to stay in the single market and customs union, accepting to all intents and purposes free movement and the authority of the European Court. Given the Irish border issue, and the economic advantages for the UK in staying in the single market for services, it is quite possible this is where we will end up. If this had been the choice in 2016, do you really think voters would have chosen it? Of course not, because we give up a say over the rules we have to obey.  That possibility shows that the case for a second referendum is unanswerable. 
A popular reposte to the case for a second referendum is why not make it the best out of three. An alternative way of putting it is that the elite wants to keep holding a referendum until voters give an answer they like. But this is nonsense, because in reality what is being argued by the Brexiter elite is not to hold a second referendum because it would probably give the wrong answer from their point of view. Clearly we will only have the chance of one more referendum before we leave: what can be wrong about giving people a second say on a decision that will profoundly influence their lives?
Unfortunately, because of the transition period that both sides are so keen on, we will not be able to have a second referendum on the final deal, to confirm or otherwise the first, because the final deal will be done after we have formally left. There is no inevitability about that, because extending the 2 year period for Article 50 is far easier than any transition period. But, as I have argued before, this transition is a democratic affront, precisely because it prevents a second referendum on a final deal. The only democratic response to that affront is to hold one anyway before we leave, on the basis that we now know a lot more about the final deal than we did in 2016.
If the polls were showing a larger majority in favour of leaving, then there might be a case that a second referendum was unnecessary, but of course polls are now consistently showing a majority to Remain. Insisting that despite this the 2016 referendum, which was based on almost zero knowledge of what leaving would mean, and where the majority was only 52%, denies any chance of a second referendum is, quite simply, a profoundly anti-democratic act.
 We have fixed terms for governments for continuity, but Brexit is different: an irrevocable decision which will be with us forever. That makes it imperative that we allow people to change their minds before we leave.
 The underlying reason why it is a waste of time talking about conventions is that conventions are simply a cover for political expedience. In the UK at least, we have had referendums when the government is hopelessly divided about what to do.
 Jenkins thinks this possibility is fine because “any trader can influence rules”. Not if they are outside the EU. The naivety here about the influence of the UK outside the EU is worthy of any Brexiter. I have argued that a Norway type deal for the UK is unlikely to be sustainable in the long run precisely because our influence on the EU and world stage generally once we leave will be much diminished.
 Some people object to a second referendum on the grounds that referendums are generally a bad idea. That is not a debate I want to address here, so it is best to take the statement that the case for a second referendum is unanswerable as contingent on parliament approving any final deal because they do not think they can override the 2016 referendum.