Let's say I'm a type A personality (http://en.wikipedia.org/wiki/Type_A_and_Type_B_personality_theory). This has been clearly associated with an increased incidence of heart disease. So we get confused and say that this way of thinking causes heart disease. But if we carefully untangle the threads, it's the associated behaviors that cause the illness. If I learn to relax in the face of my triggers, loosen up when things feel crazy, etc, I won't start the cascade of physiological events that lead to heart problems. So yes, sometimes we can create profound effects on our health by changing our thinking (eg deciding to relax if we're hyper-tense). But this is not the same as having control over illnesses.