It's certainly true that there's a lot of bad advice floating about. This is often the result of a line being taken out of context and widely quoted until it becomes a so-called 'golden rule', and I think that’s what has happened with the ‘write what you know’ dictum.
At it’s core, it is a sensible (some would say self-evident) guideline. You need to know about something in order to write it convincingly and with confidence.
However, the problem lies in the interpretation of ‘what you know’ as being ‘what you’ve directly experienced’ or ‘stuff you know without doing any research’, which is clearly bonkers.
If celebrated writers had followed that rule, we’d have no Middle Earth, no dystopian futures, no Alice in Wonderland, and no The Time Traveller’s Wife. Not to mention the fact that every crime writer would have to pen their novels from behind bars…
Then, there is the sort of knowledge you get from life. Drawing on experiences and emotions can add depth and verisimilitude to your work. Don't be afraid to use what you know, what you have experienced, albeit in a wildly fictionalised way.
To be perfectly honest, it's difficult to avoid using this kind of knowlege in our writing. Our work is, after all, a reflection of our own personal take on the world – something that is forged through our experiences.
I think that writing what we know is what we do when we write freely and with as little fear as possible. Not in terms of exact details or 'real' experiences as with memoir, but in terms of emotional truth.
Perhaps we can rephrase the advice 'be brave enough to write what you know.'
And it does require bravery. As Neil Gaiman said:
"The moment that you feel that just possibly you are walking down the street naked...that's the moment you may be starting to get it right."
What do you think?