"Willpower is a myth. The problem with trying to use willpower to achieve and sustain a behavioral change is that it is fueled by emotion. And as we all know, our emotions are, at best, fickle. They come and go. When your emotions start running down -- and they will -- even your best-laid plans will fall flat."
Source : "Essai sur le Mérite de la Vertu". A translation and adaptation of "Inquiry concerning Virtue or Merit" by Anthony Ashley-Cooper, 3rd Earl of Shaftesbury, 1745.