" When conventional medicine fails, when we must confront pain and death, of course we are open to other prospects for hope.

And, after all, some illnesses are psychogenic. Many can be at least ameliorated by a positive cast of mind. Placebos are dummy drugs, often sugar pills. Drug companies routinely compare the effectiveness of their drugs against placebos given to patients with the same disease who had no way to tell the difference between the drug and the placebo. Placebos can be astonishingly effective, especially for colds, anxiety, depression, pain, and symptoms that are plausibly generated by the mind. Conceivably, endorphins -the small brain proteins with morphine-like effects - can be elicited by belief. A placebo works only if the patient believes it’s an effective medicine. Within strict limits, hope, it seems, can be transformed into biochemistry. "

― Carl Sagan , The Demon-Haunted World: Science as a Candle in the Dark