Vertigo is to dizziness what a migraine is to a headache, or the flu to a cold in the head; you don’t really grasp the difference until you’ve had the nastier one. True vertigo pitches you into a chaotic blackness in which you lose your bearings utterly; no relief is to be had from sitting or lying down, because the chair, the bed, the floor all fall away from you as well. Disorientation on the flat is bad enough, but in three dimensions it is terrifying, like Satan’s journey through the realm of Chaos in Paradise Lost where he meets ‘a vast vacuitie: all unawares / Fluttering his pennons vain plumb down he drops / Ten thousand fadom deep, and to this hour / Down had been falling …’