Until the French psychiatrist Jean-Étienne Dominique Esquirol gave them their current name, hallucinations – seeing or hearing things that are not there – were called “apparitions”. This captures their profoundly disturbing nature, an eeriness that is not fully reflected in the definition Oliver Sacks quotes from William James's classic The Principles of Psychology (1890): “An hallucination is a strictly sensational form of consciousness, as good and true a sensation as if there were a real object there. The object happens to be not there, that is all” (2012: X). Unlike mental images, hallucinations do not remain obediently inside your head – they are projected into the world that surrounds you – and they can surprise. Having a life of their own, which reinforces their compelling reality, they are intimate invasions that break into your chosen biography rather than belonging to it.

Their power to terrify may be, in part, due to their content: a giant spider irradiating venom meant just for you or something like W. H. Auden's “lean horror flapping and hopping” closing in on you “with inhuman swiftness” (The Sea and the Mirror). But even the most benign hallucination is deeply unsettling precisely because, as Sacks says, there is no “consensual validation”. Nobody else can see, hear, feel, smell or taste what you are experiencing. To be in the grip of such incorrigibly private experiences is to be sequestrated in the most profound solitude.