[Jan. 31, 2011. My latest and final version of this post appears in the "What is a central purpose in life?" section of the Appendix to my latest book, The Power and the Glory, described here: reasonversusmysticism.com]
In the first scene of Ayn Rand's novel, The Fountainhead, the protagonist, Howard Roark, stands on the edge of a cliff, preparing to dive into the lake below. As he looks at the world around him, elements of his central purpose in life (CPL) come naturally to mind. He sees granite, to be cut into walls. He sees a tree, to be split into rafters. He sees iron ore, to be formed into girders. In fulfilling his central purpose in life, he will take these elements and build homes and businesses. Howard Roark's CPL is to bring into reality his form-and-function designs of buildings.
Ayn Rand's CPL was the "presentation of an ideal man" in various forms of literary art--a prose-poem, short stories, plays, movie scripts, and novels. At age 50, two-thirds of the way through her life, she fulfilled that purpose to the maximum degree with the publication of Atlas Shrugged. In that novel, the protagonist is John Galt, a complete portrayal of her ideal man as well as the basic elements of "the philosophy that made him possible." At that point she re-examined her CPL. She decided to devote the remainder of her life to defending that ideal by further developing and then speaking and writing about the underlying philosophy, a philosophy of reason. In my view, the essential elements of her CPL--portraying the ideal man--remained the same, and only particular activities changed. She continued to portray the ideal man but, as she had all along, by functioning as an ideal man in terms of the philosophical values and virtues of her own philosophy.
For a third example, consider a particular student of intellectual history. He defines his CPL as telling success stories from history. By doing so, he brings together tasks such as research, outlining, writing, and editing. He moves from project to project, from book to book, or article to article, but his CPL subsumes all of them.
THE NATURE OF A CPL. First, a CPL is a statement of action. It takes this form: "My central purpose in life is to ...," followed by an action word (for example, "design," "portray," "tell," or "write") that names a form of production. What is being produced? Something of high value to the producer and, ideally, to others in society who might wish to buy the product.
Second, a CPL is an abstraction, one that subsumes and integrates the many particular productive tasks in which a man engages to support his ultimate purpose in life, happiness. A simple metaphor for the relationship between a man's ultimate purpose and his CPL is a great tent, which is a life of happiness. Its main (but not only) tent-pole is his CPL. Other high purposes holding up the "tent" might be social relationships and a favorite leisure activity. Howard Roark loves designing and constructing buildings. He loves Dominique Francon and befriends Mike the construction worker. When time permits, he enjoys swimming.
Third, while the need for a CPL is philosophical--applying to everyone, everywhere, at all times in history--a particular person's formulation of his CPL is a personal statement, that is, it is tailored to (1) who he is as an individual, (2) what he wants to do, and (3) the world in which he lives. (A CPL is objective when it is drawn logically from all three facts, though the initial trail leading to its formulation might be a strong, persistent, positive emotional response to certain sets of activities that he later learns to state abstractly.)
Fourth, and ideally, a CPL should provide an income, thus paying one's way through life, at some level. Carpenters, architects, and some fiction writers earn an income from implementing their personal CPLs and selling their products--their labor, designs, or novels. However, for some CPLs, such as writing poetry, there may be too much competition or too little demand to provide a sufficient and steady income (at least during the first decades). In such cases, the passionate valuer might hold an intermittent, part-time, or full-time "day-job" to earn enough money to meet his minimal needs, while devoting as many hours as he can to the work he loves. A poet, for example, might work as a warehouseman because: he can mentally walk away from the job at the end of each workday and then devote his attention to his poems; he can work limited, predictable hours, freeing time for his art; and his warehouse skills are easily transferable elsewhere if he decides to move. On the other hand, a man who loves reading and critiquing poetry, but not writing it, would not need a "day job," but could develop a financially rewarding CPL as a full-time university teacher of poetry.
WHAT A CPL IS NOT. First, a central purpose in life is not primarily a statement of being, but a statement of doing. Of course, for convenience in communicating with others, a CPL might initially be stated most concisely as being a certain kind of producer: an architect (who designs and constructs buildings), a soldier (who destroys his country's enemies), a carpenter (who manipulates wood to fit human needs), or a doctor (who treats illnesses). However, a full statement of a CPL should identify the essential (causal) productive activity: portray, tell, write, construct, and so forth.
Second, a CPL is not merely a career, which is an ever-more demanding progression of jobs (or projects). A physician might progress from general science student, to medical student, to intern, to physician in private practice or a staff position in research or administration. That progression of jobs is a career. Each job, in turn, includes a combination of particular tasks such as diagnosis, treatment, report writing, and leadership. As an abstraction, a CPL subsumes the particulars of career, jobs, and tasks but it is not any one of them. As a broad abstraction, a CPL subsumes a vast number of details in a variety of circumstances and potentially over the span of one's life. In this sense, having a clear, concise, objective CPL is a tool of integration--that is, of finding "the one in the many."
Third, a CPL is not a statement of philosophical values or virtues. "Act rationally," "integrate logically," and "solve 'mysteries' in life" are not proper statements of a CPL, but identifications of what everyone needs to do in order to achieve his CPL and other purposes in life. A CPL is a personal, possibly unique, statement of productive purpose, a purpose that need not be attractive to anyone else.
CRITERIA FOR FORMULATING A CPL. What criteria should a CPL meet if it is properly formulated? First, it must be objective, that is, drawn logically from the facts of who you are, the nature of the productive activity, and the nature of the world in which you live. Second, as an abstraction, it must be broad enough to subsume a variety of circumstances, activities, jobs, and perhaps even more than one career. Third, it must be an active statement that you can expand or shrink as you need from moment to moment. A long-form statement should be specific enough to help you start planning the main steps of your career. A short-form statement of the same CPL will help you recall it quickly and thereby help you "refuel" and keep yourself on track in moments of turmoil, fatigue, or temporary loss. Fourth, it must be ambitious enough to make you "stretch" to fulfill it, but feasible enough to offer a probability of success. (If you do not succeed at your CPL, you will not be happy, though you might still earn second prize, which is the great satisfaction that arises from knowing you did everything you could have done.)
A central purpose in life is not all of life, but it is the core of life.
Author, The Power and the Glory: The Key Ideas and Crusading Lives of Eight Debaters of Reason vs. Faith
 For Ayn Rand's statements of her central purpose in life: The Romantic Manifesto, pp. 161 and 163 (hardback), in Ch. 10, "The Goal of My Writing." Ayn Rand also used the terms "portrayal" and "projection," apparently as synonyms of "presentation." In the title of the chapter she speaks of a "goal," and in the main text she speaks of "purpose," "first cause," "prime mover," "motive," and "ultimate literary goal." For me, here is an unresolved problem: What is the distinction between the ideas named by the words "goal," "value," and "purpose"? Do they all refer to essentially the same thing--something we want that requires action to achieve--but perhaps as seen from different perspectives? Or do they refer to essentially different things? I do not yet have an answer. I welcome one.  For the comment about Atlas Shrugged completing her ambitious literary quest, see her biographer's description, from which I have quoted: Jeff Britting, Ayn Rand, pp. 87-88 and 91. (Available from The Ayn Rand Bookstore.)  For the change in Rand's CPL: Britting, Ayn Rand, p. 92.  For Ayn Rand's distinction between happiness and satisfaction: Mary Ann and Charles Sures, Facets of Ayn Rand, pp. 74-76. (Available from The Ayn Rand Bookstore.)  For Ayn Rand's comments on CPL and subjects related to a CPL, see the entries for Purpose, Productivity, Career, Ultimate Value, and Happiness in The Ayn Rand Lexicon. (Available from The Ayn Rand Bookstore.)