In his retirement, Henry Marsh is visited by all the patients he used to treat. But his doorbell never rings. They are spectres, or as Marsh might more accurately describe it, synaptic impulses. “There are thousands of them,” says the former neurosurgeon and bestselling author. “I must have forgotten quite a lot. They’re all there. On the whole they’re benign ghosts.” The visits began after Marsh, who is 72, was diagnosed with advanced prostate cancer in early 2020. He wondered whether, “if I remembered them all, and they forgave me, then I would survive”. Marsh succumbed to other forms of...