Generally, the reason for such seasonal back-and-forth movement is to seek resources that aren't available within a single area year-round.

Mr Israel argues that for as long as historians treat the two wings of the Enlightenment as a single movement, they have misunderstood the phenomenon.

That said, the single market is one of the EU's big achievements, and its "four freedoms" of movement-of people, goods, services and capital-are worth defending.