Self-control
Part of speech: noun
Definition: The ability to control one's desires and impulses; willpower.
Willpower
Part of speech: noun
Definition: The unwavering strength of will to carry out one's wishes.
Example sentence: You have to believe in a placebo or it won't work, but if it works, it's obviously working in some indirect way, through feedback in the immune system, let us say, or in the willpower of the patient to take a more strenuous exercise in their own therapy.