Bookshelves creak with memoirs and biographies of prime ministers. Even the dullest occupant of Number 10 becomes the subject of many studies. There are far fewer books, however, on the nature of the office and its freakish demands. This is an unfortunate imbalance. Each incumbent fascinates, but what about the qualifications for leading the UK? Why are they so rarely met? We need some answers, and Anthony Seldon is one of the few prime ministerial biographers to seek to provide them.

He does so insightfully and mischievously. Early on in The Impossible Office? the similarities and differences of the two ‘bookend prime ministers’, Robert Walpole and Boris Johnson, are explored in the form of an imaginary conversation. More than three hundred years separate their premierships. The dialogue shows that in some respects, the responsibilities and challenges are unrecognisably different. In others, nothing has changed, to revive a famous phrase deployed calamitously by Theresa May.

Both Walpole and Johnson went to Eton and Oxbridge, a familiar route for quite a few of the prime ministers over the centuries. Both ruled with constraints, though the limits on Walpole’s powers were different from those on Johnson’s. In the imaginary dialogue, Walpole is taken aback that Johnson has