One of the last times I saw my grandmother in her nursing home she chatted cheerfully about her son, who was away studying at university. She spoke with complete conviction and considerable pride, despite seeming also to recognise that her only son - my father - sitting right beside me, was not far off retirement age. The impossibility of her tale caused her no apparent distress or confusion. Her story was lucid and complex. It was as though a perfectly plausible anecdote had been plucked from several decades earlier and woven into the void of her recent memory.