Memory is a double-edged sword in the human consciousness. We relish, and even idolise, memories of the past, but we often overlook memory’s enduring partner: forgetfulness. Our memories are rife with deliberate amnesia, and history, at best, is a selective remembrance. Historians edit and repackage the past, sometimes invidiously omitting those “inconvenient truths” which could upset their preferred interpretations. Memory’s imperfect fabric is the platform from which Tom Holland plunges into the story of the rise of Islam in his latest book, In the Shadow of the Sword.