And when they ask us how dangerous it was . . .

As every biographer knows, all evidence is suspect. Probably the diary comes nearer to the truth than any other source: it is subjective and no doubt biased but a least it usually reflects what the author really thought at the time. Letters are second-best. They too are contemporary but they contain what the writer wanted someone else to think, not necessarily what he or she thought themselves. Most problematical of all is oral testimony.

Memory plays fearful tricks. With the late Tom Harrisson I once conducted an experiment. From the diaries kept by Mass Observation volunteers during the second world war we picked a few which contained particularly vivid Blitz experiences. We then wrote to the authors, asking them, without referring to anyone or any document, to tell the story again. Half a dozen obliged. Any resemblance between the original story and the version recounted 30 years later was almost entirely coincidental.