Pardon the gendered language- it was written 35 years ago.
In biography, in other respects probably the most difficult form of historical writing, there is at least in principle a clear guide to what is relevant. Biography is about a man, and the ideal data is that which seems to take us deepest into his or her personality, like Florence Nightingale’s Notes from God, or Alfred Deakin’s prayers. Since he knows what is central, the author should know what is peripheral.
Keith Sinclair ‘On Writing Shist’ Historical Studies Vol 13, No 51, 1968 pp. 426-432