In a referendum 40 years ago today, Britain went to the polls and voted 'Yes', by a 2-1 margin, to the question "Do you think the UK should stay in the European Community (Common Market)?"
Sometime in the next two years, we'll do it again. Europe has changed a lot in that time, expanding its membership and its remit, but the question remains essentially the same: "Should the United Kingdom remain a member of the European Union?"
How Britain will answer remains wide open. New research by Survation for British Future, published today, shows that over 70% of voters still haven't fully decided where they stand in the EU debate.
Most have a preference but remain uncertain: they are the 'Leaners', with 31% saying they are "leaning towards voting to stay in the EU" and 28% "leaning towards voting to leave the EU". Both say they want to know what the conditions are before making up their minds. 13% say they don't know.
These are the people who will actually decide whether or not Britain stays in Europe.
As the debate on EU membership looks set to dominate political debate over the coming months, we will hear much from the most passionate advocates on both sides: those who get up in the morning thinking how they can help get Britain out; and those who spent the night before dreaming of ever-closer union. Both groups remain in a minority: just 12% say they are 'definitely out' and 16% 'definitely in'.
But will their arguments reach and persuade the 'Leaners' and the 'Don't Knows'? It doesn't look like it at the moment.
The new research suggests that some of the most prominent advocates in both the 'pro' and 'anti' camps in the EU debate may be harming their own cause. Neither Europhile Tony Blair nor Eurosceptic Nigel Farage is trusted by voters when they talk about Britain's EU membership.
Farage remains a polarizing figure, with a Marmite effect on the public. Those who want 'out' would have him on their toast every morning, with three-quarters putting their trust in the Ukip leader. But he makes most Britons screw their face up: 54% say they don't trust Nigel Farage on the EU debate, with 36% saying they trust him on Europe.
And nobody, it seems, wants a serving of Tony Blair with their breakfast - whether that's a Full English or pain au chocolat. The former Labour PM, who has been a cheerleader for Britain's EU membership, is the least-trusted politician of all on Europe, whether people are 'in' or 'out', 'decided' or 'undecided' alike. Nearly six in ten (59%) people distrust the former Prime Minister when he talks about whether Britain should remain in the EU, with less than a third (28%) saying they trust him on the issue.
Both 'in' and 'out' are singing their songs for Europe, but so far only to their own respective choirs. To win, they will need to reach outside their core support and persuade the undecided majority - the 'Don't Knows' and the 'Leaners' - to buy the record.
'In' will never win with a message that only appeals to graduates, internationalists and the lucky few for whom regular weekends in Paris and Berlin are the norm. Contrasting a failing Britain with a thriving and modern Europe, at risk of leaving us behind, may have worked in 1975 but really doesn't ring true today.
Similarly the 'out' camp will never secure 51% of the vote if their appeal is only to those who are old enough to have voted back in 1975, or if they sound like they would like to take Britain back to a sepia-toned heyday twenty years before that.
Whether the UK remains part of the European Union is one of the biggest questions that this country faces. The debate is far from decided. Either side could win the referendum. Yet at the moment, the more pertinent question seems to be who will lose it.