Zero. What exactly is it?
It's a similar quandry to the idea of space - can the absence of something actually be something in and of itself? It was a problem that the Ancient Greeks grappled with, leading to philosophical and even religious arguments about the nature of zero.
But, as this fascinating video from The Royal Institution shows, it wasn't just the Ancient Greeks who thought long and hard about it, as other civilizations, such as the Babylonians, and the early inhabitants of India and China sought to understand this mysterious concept before it spread to mainland Europe and the Romans, where it met some opposition.
Watch the video below, narrated by Hannah Fry, and trace the fantastical history of, well, nothing.