“We are what we repeatedly do. Excellence then, is not an act, but a habit.”
In fact this quote is from Will Durant’s The Story of Philosophy: The Lives and Opinions of the World’s Greatest Philosophers published in 1926. It is Durant’s summation of Aristotle’s thoughts in Aristotle’s work Nichomachean Ethics Ethics, Book II, 4; Book I, 7.
“Excellence is an art won by training and habituation: we do not act rightly because we have virtue or excellence, but we rather have these because we have acted rightly; ‘these virtues are formed in man by his doing the actions’; we are what we repeatedly do. Excellence, then, is not an act but a habit: ‘the good of man is a working of the soul in the way of excellence in a complete life… for as it is not one swallow or one fine day that makes a spring, so it is not one day or a short time that makes a man blessed and happy”