Migrant Mother was made while Lange was travelling home to San Francisco in the rain. She passed a sign that read 'Pea-Pickers' Camp' and drove for anoher twenty miles, before something compelled her to turn back and visit the camp.
From the Genius of Photography: How Photography Has Changed our Lives, a remarkable BBC title that accompanies the television series of the same name. 
The book is a treasure, a gift from @bobcatrock, which I've only just recently had a chance to dive into and devour. (Thank you, friend: it's amazing.)
Reading its pages has made me acutely aware of why I care about photography.
If I have a theory of photography it resembles the philosophy of the Tao: Nothing is fixed, all things change and there is value in studying this slice of time before we move on to the next.
I've art directed hundreds of hours of commercial studio photography where someone else worked the shot and captured the magic. I've marveled at the photographers' skill and talent, and been grateful that they managed to capture the image I needed to tell my story and sell my goods.
But the photographs that arrest me with wonder are the ones where chance conspires with geometry and light and the odd artifact of this brief now. They're unstaged and elusive, and they're true.
If I have any rules for my own photography they are these:
Work with (only) this.