If authors only wrote what they knew about

then very little would get written at all. We would not have any science fiction or history or novels set in far-off Ruritania. All books would need to be written from just one point of view.
"Write what you know about" is advice often given to young writers, new writers, would-be writers. It is advice repeated over and over again.
But, on the Awfully Big Blog Adventure, Sue Purkiss asked the question about writing something from someone else's point of view. Is it true? If we write about an actual event in history how do we know if we are writing something that is true? Is it right to write about people who are still alive? How accurate do we have to make recent history?
Joan Aiken solved the problem with an "alternate history", one in which the Hanoverian succession does not take place. It is a neat solution that gave her all sorts ready made background without having too worry too much about getting the details right. If something did not happen in the real world then it might easily happen in hers. Diana Wynne Jones does a parallel universe in a slightly different way but again there is a ready made background there that she can tweak to her satisfaction.
It is different if you want to write straightforward historical novels. Cynthia Harnett spent years researching hers, so did Rosemary Sutcliff.
These people are writers for children. It might be thought that children will be less demanding and worry less about the accuracy of detail. I would not be too sure. There is almost certain to be a child around who will tell you "that is wrong because..." and the quote you chapter and verse out of an academic text or from a visit to the site in question.
Writing recent history is even more demanding. I once went to a children's literature conference where Jill Paton Walsh and Michelle Magorian were condemned for setting books in WWII when they had not experienced it themselves. When I asked what was wrong with doing this I was told that it was just not done. Apparently it was too recent in time and they "should not" have been writing about something they did not know. I hasten to add that the people saying this were librarians, not writers.
It may well be that adults will read "The Dolphin Crossing" or "Fireweed", "Back Home" or "Goodnight Mr Tom" and say "It was not like that." Possibly however what they need to say is "It was not like that for me."
Sue Purkiss had some anecdotes from her father who was a POW in WWII. When she did more research she found that his experience (and perhaps what he was prepared to remember and pass on) was different from that of other POWs. Of course it was. We all see things in unique ways. It does not make her father's experience or recollection any less valid.
History is what happened just a moment ago. If we were not to write any history at all we would have to write in the present tense and not even speculate about the future because the future is defined by the past. It would be an impossible task.
As for the other question that Sue Purkiss poses - should we be writing about people who are still alive? - I think my answer is "Why not?" It is not whether we write about them but what we write about them that matters. It is surely wrong to do as the editor of a woman's gossip magazine once told me, "Publish it now and find out if it is true later".
Writing about people who are still alive is something I feel needs to proceed with caution. I would not make any of our living Prime Ministers a major character in a book. I might give one of them a minor role. I would give Churchill or Menzies a greater role but I would give Thatcher or Blair, Fraser or Keating a much smaller role. At the same time it makes no sense at all to avoid all mention of public figures. What we need to avoid is speculation and libel.
And then, I might just invent a fictional Prime Minister instead. It might be easier.