Power of auto-suggestion known in the Middle Ages. The power of thought, of idea, is incommensurable, is immeasurable. The world is dominated by thought. The human being individually is also entirely governed by his own thoughts, good or bad. The powerful action of the mind over the body, which explains the effects of suggestion, was well known to the great thinkers of the Middle Ages, whose vigorous intelligence embraced the sum of human knowledge.

Chapter I. The reality of auto-suggestion. - Autosuggestion : My method (1923)