Brexit supporters won't say what the alternative to EU membership is. Will it be a free trade agreement? Or the Swiss model? Or perhaps the Norwegian one?
There are two reasons for this reticence. One is they can't agree. This is because what is good politically, as it allows more policy independence, is also bad economically, as it reduces access to the European market. Those campaigning for Britain to leave the EU would make this trade off in different ways.
The other reason is Leave campaigners don't want to distract attention from the deceptively simple in-out question. As a consequence, we should expect no clarity before 23 June.
Who then, would eventually decide what 'out' means? Will it be voters? Members of parliament? Or perhaps just David Cameron? The answer is none of the above. It would be the members of the Conservative Party.
If David Cameron loses the election he will be called upon to resign and find it hard to resist. He would have lost the confidence of his party and as the person who must lead the negotiation that must follow to take the UK out of the EU and establish a new relationship.
His resignation would trigger a Conservative leadership contest, with the winner becoming the next prime minister. Under the rules of the contest Conservative MPs must vote to produce a short list or two or more candidates, with the ultimate choice left to party members across the country.
There can be little doubt that following a vote for Brexit, the central issue in that campaign would be Britain's future relationship with the EU.
For any candidate to be credible they must make it clear where they stand. What's more, if they are to have a chance, that vision must be one that has a strong appeal to Conservative Party members.
Whatever you might think of Conservative Party members one thing is clear - they are not representative of the country as a whole, either demographically, geographically, or politically. They are ... conservative.
The polling evidence suggests they are likely to prioritise sovereignty and policy independence over economics and access to the European market.
This should be of concern to many more business leaders than those who have contributed to the public debate on Brexit so far to explain their company's interests in the issues.
While we may not know what 'out' means before 23 June, we can be pretty sure who would eventually provide the answer.