Classical myth has it that the Sibyl offered nine books of prophecies to Tarquin the Proud, the last King of Rome.

Tarquin thought her asking price too exorbitant, and despite the invaluable wisdom contained in the books, tried to bargain with her. She threw three of the books on the fire, and offered him the remaining six for exactly the same price. Obviously, he refused – why pay for six the same amount that he’d just refused for nine?

The Sibyl burned another three and repeated her offer. Conscious that he’d just thrown away some of the most valuable books in existence, and that this was his last chance to get even a fraction of what he wanted, Tarquin relented. He swallowed his pride and bought the last three books for the original price.

This expert lesson in sunk cost theory was attempted by Theresa May earlier this week.