Habit is often defined as a tendency to repeat an action that's already been performed many times. But this definition, which goes back to Aristotle, is subject to several objections. First, if an action is simply continued over an extended period, it can become habitual without being repeated. Even with this correction, however, Aristotle's definition still might be criticized. It's true that a habit grows stronger with repetition, but the self has a tendency to reproduce an action after performing it just once. Continuity or repetition develops but doesn't constitute this initial seed. So to study habit in itself, and to really understand it, we'll have to take a fresh approach and examine habit in its normal state, as it develops after the single performance of an action.

Looked at this way, habit has two characteristics. First, it's a faculty of preservation – it ensures the survival of our past actions. The second characteristic is that the action preserved tends to reproduce itself, so that later it seems to appear out of nowhere.

So habit is the faculty that preserves our past actions as well as the force that tends to reproduce them.

We might also say that habit has almost all the characteristics of instinct, but to a lesser degree. First, instinct is unconscious, while habits become more unconscious the stronger they are, so that an extremely strong habit can make us act almost as unconsciously as does instinct.