When Clarke was diagnosed with Polymyositis he was depressed, angry, mad at the world but also helpless. Because the disease had progressed very quickly it was difficult to understand what was happening and as his body was declining, his mind was driving him downward as well.
At the time Clarke didn't really know how much mindset can affect physical health but an experience he had when he thought he was going to die changed things and he realised he had something to live for. This change in mindset led to a change in physiology and Clarke now feels that mindset was the key to this and, we can do anything we want if we focus on what we say to ourselves and what we believe.