The heart is seen as a symbol for love, but it was also used as a symbol of sincerity. When I write from the heart, I try to be as sincere as I can manage. It is writing with your heart on the page, laid bare and bleeding. You can’t fake it.
Sincerity takes many forms. Terry Pratchett loved to write, and his form of sincerity was to allow that love to show through his work. Angela Carter’s feminism coloured her adult fairy tales and turned old tales into something new and wonderful … without being preachy. When they tell you to write what you know, they should add that you should write what you love.
This doesn’t mean you should physically flay yourself and leave blood on the page, as Hemingway and Gallico seem to suggest. (You can if you like that kind of drama.) What they are really suggesting is that you share something of yourself with your audience, to make a connection with your readers. Sharing your soul can be hard. It leaves you open to rejection and pain.
And yet … the more you share, the more you will find to share.