We are given to thinking of a habit as simply a recurrent external mode of action, like smoking or swearing, being neat or negligent in clothes and person, taking exercise, or playing games. But habit reaches even more significantly down into the very structure of the self; it signifies a building up and solidifying of certain desires; an increased sensitiveness and responsiveness to certain stimuli, a confirmed or an impaired capacity to attend to and think about certain things. Habit covers in other words the very make-up of desire, intent, choice, disposition which gives an act its voluntary quality. And this aspect of habit is much more important than that which is suggested merely by the tendency to repeated outer action, for the significance of the latter lies in the permanence of the personal disposition which is the real cause of the outer acts and of their resemblance to one another. Acts are not linked up together to form conduct in and of themselves, but because of their common relation to an enduring and single condition—the self or character as the abiding unity in which different acts leave their lasting traces. If one surrenders to a momentary impulse, the significant thing is not the particular act which follows, but the strengthening of the power of that impulse—this strengthening is the reality of that which we call habit. In giving way, the person in so far commits himself not just to that isolated act but to a course of action, to a line of behaviour.
— John Dewey in Theory of the Moral Life (1932), emphasis mine
The title of this post is a quotation often and mistakenly attributed to Aristotle. In fact, it was written by the American historian Will Durant (1885-1981). It is followed by the sentence, “Excellence, then, is not an act, but a habit.”