**How do we imagine the unimaginable?**If we're asked to think of an object - say, a yellow tulip - a picture immediately forms in our mind's eye. But what if we try to imagine a concept such as the square root of a negative number?

It sounds impossible, yet as this enchanting, quizzical and evocative book shows, the art of mathematical imagining is not as mysterious as it seems. Drawing on poetry, literature and philosophy, Barry Mazur shows how we can all make the leap of imagination in order to start visualizing the enigmatic 'imaginary numbers' that first baffled mathematicians in the sixteenth century - and are now seen as the bedrock of the universe.