The common wisdom is that depression starts in the mind with distorted thinking. That leads to “psychosomatic” symptoms like headaches, stomachaches, or fatigue. Now, models like the Polyvagal Theory suggest that we’ve got it backward. It’s the body that detects danger and initiates a defense strategy meant to help us survive. That biological strategy is called immobilization, and it manifests in the mind and the body with a set of symptoms we call depression.