When I was a teenager, I read an essay, "The Energies of Men" by William James (1842 - 1910). In it, Mr. James, a well-known and respected psychologist and philosopher of the late 19th and early 20th centuries, put forward the idea that health and physical condition could be influenced to some great extent by attitude.

His idea was that most of us had reserves of which we were not aware, but which were there, waiting below the surface of our existence, to answer our call when needed or wanted. As he states:

"... in exceptional cases we may find, beyond the very extremity of fatigue-distress, amounts of ease and power that we never dreamed ourselves to own,--sources of strength habitually not taxed at all, because habitually we never push through the obstruction, never pass those early critical points."

...Read the whole article here.