“[Happiness is] The exercise of vital powers, along lines of excellence, in a life affording them scope.”

Source: Ultramarathon Man: Confessions of an All-Night Runner

Thoughts on Man's Purpose in Life (1974)
Context: Happiness comes from the full use of one's power to achieve excellence. Life is potentially an empty hole, and there are few more satisfying ways of filling it than achieving and exercising excellence.
This principle of excellence is on which Americans seem to be losing, and at a time when the Nation stands in need of it. A lack of excellence implies mediocrity. And in a society that is willing to accept a standard of mediocrity, the opportunities for personal failure are boundless. Mediocrity can destroy us as surely as perils far more famous.
It is important that we distinguish between what it means to fail at a task and what it means to be mediocre. There is all the difference in the world between the life lived with dignity and style which ends in failure, and one which achieves power and glory, yet is dull, unoriginal, unreflective, and mediocre. In a real sense, what matters is not so much whether we make a lot of money or hold a prestigious job; what matter is that we seek out others with knowledge and enthusiasm — that we become people who can enjoy our own company.

"Remarks to Student Participants in the White House Seminar in Government (334)" (27 August 1963) http://www.jfklibrary.org/Research/Research-Aids/Ready-Reference/JFK-Quotations.aspx
1963

As quoted in De Natura Deorum by Cicero, ii. 8.; iii. 9.

Source: "What I Believe" (1930), pp. 14-15

Source: The Ordeal of Change (1963), Ch. 13: "Scribe, Writer, and Rebel"
Context: It has been often stated that a social order is likely to be stable so long as it gives scope to talent. Actually, it is the ability to give scope to the untalented that is most vital in maintaining social stability. For not only are the untalented more numerous but, since they cannot transmute their grievances into a creative effort, their disaffection will be more pronounced and explosive. Thus the most troublesome problem which confronts social engineering is how to provide for the untalented and, what is equally important, how to provide against them. For there is a tendency in the untalented to divert their energies from their own development into the management, manipulation, and probably frustration of others. They want to police, instruct, guide, and meddle. In an adequate social order, the untalented should be able to acquire a sense of usefulness and of growth without interfering with the development of talent around them. This requires, first, an abundance of opportunities for purposeful action and self advancement. Secondly, a wide diffusion of technical and social skills so that people will be able to work and manage their affairs with a minimum of tutelage. The scribe mentality is best neutralized by canalizing energies into purposeful and useful pursuits, and by raising the cultural level of the whole population so as to blur the dividing line between the educated and the uneducated. If such an arrangement lacks provisions for the encouragement of the talented it yet has the merit of not interfering with them.

Source: Why Men Are the Way They Are (1988), p. 103.

"De la Ligne" in La Difficulté d’Etre [The Difficulty of Being] (1947)
Context: What is line? It is life. A line must live at each point along its course in such a way that the artist’s presence makes itself felt above that of the model... With the writer, line takes precedence over form and content. It runs through the words he assembles. It strikes a continuous note unperceived by ear or eye. It is, in a way, the soul’s style, and if the line ceases to have a life of its own, if it only describes an arabesque, the soul is missing and the writing dies.

Source: Disturbing the Peace (1986), Ch. 5