How is success defined? For many of us, we focus on what we can most easily measure: money. Money does matter, of course, at least until we reach a "comfortable standard.” We need a certain level of income to be able to meet our basic needs and remove acute financial stress. Researchers at Princeton compared Gallup data on the income and happiness of 500,000 American households. They found that after about $75,000, money had no impact on their mood. In a household with two earners, that equals under $40,000 per year (obviously higher in markets like New York City).

But this isn't news. By now we all know that the person who dies with the most toys isn't necessarily the most successful. Philosophers, clergy, and in more recent times, economists and psychologists, have written endless books on this topic. It remains one of the most popular topics, and a common theme is that money is not core to the meaning of life. So, what does matter?

The roots of the conversation appear to go back to Aristotle, who described mere happiness as a vulgar idea. He observed that while many behaviors might produce pleasure, they do not produce well-being, or what he called eudaimonia. Feeling good is a fleeting experience and is not enough to sustain a good life.

Dr. Martin E.P. Seligman, from the University of Pennsylvania, picked up where Aristotle left off with his theory of well-being. Seligman is one of the leaders in the new field of positive psychology and the author of the book Flourish. According to Seligman, what we should seek to achieve is well-being, not simply happiness, which he sees as being one-dimensional. Seligman breaks down well-being into five areas: positive emotion, engagement, relationships, meaning, and accomplishment, or PERMA.

Positive emotion is the basis for having a pleasant life, and includes things like warmth and pleasure -- things we associate with basic happiness. Borrowing from the Work of Mihaly Csikszentmihalyi, he also describes how you can be in a state of "flow,” when you lose a sense of self-consciousness and are operating with all your mental and emotional powers. This level of engagement, he shares, requires deploying your highest strengths and talents. The element of meaning typically goes beyond what he describes as the often solitary experiences of positive emotions and engagement. Meaning is about who we are and how we are in the world. It is about more than the here and now, and it is about more than just the self.

So, how does this all play out in life? Some of the most interesting writing and research in this area has focused on parenting. Being a parent -- and especially a single parent, it turns out -- is negatively correlated with a traditional sense of pleasure or happiness. Studies show that people with kids report far less pleasure in their lives than their childless counterparts. This is 100 percent consistent with my experience and those of every parent I know, especially parents of younger children. Parenting kicks your ass. But being a parent is also the most important part of my life, and I wouldn't trade my life for those of my childless friends (well, not for more than a week). Parenting is profoundly meaningful and rewarding; it is, however, rarely engaging or pleasurable.

In an attempt to understand the relationship between the five components of well-being, British academics Mathew P. White and Paul Dolan asked people to assess different activities based on the degree of pleasure and reward they received from doing them. The winner? Volunteerism. It was the highest rated activity for both pleasure (i.e., positive emotion and engagement) and reward (i.e., meaning).

Having worked in pro bono service for over a dozen years, this came as no surprise to me. Volunteering your time is profoundly rewarding and pleasurable. However, I also learned that pro bono, the donation of your highest strengths and talents, was more rewarding and pleasurable than traditional volunteerism, because it combines the elements of meaning and engagement. As John Gardner wrote, "True happiness involves the full use of one's power and talents."

A Taproot pro bono consultant sums up this ideal perfectly: "My passion for helping people is rivaled only by my passion for automating things with computers. I want to combine these two things.” He wants to combine meaning and engagement, and he is seeking to do pro bono service to achieve a sense of purpose.

When we combine meaning with engagement, we find the pinnacle state of purpose, where our well-being is highest and most sustained.

It turned out that pro bono service was a great supplement for people who weren't obtaining the level of meaning they needed in their lives.

Supplements are sometimes necessary, but more important is adjusting our lives to have more integrated ways to sustain ourselves. During the week, most of us spend at least 50 percent of our waking time at work. If we aren't getting our need for purpose met here, we are unlikely to have satisfying levels of purpose in our overall lives. Purpose enables us to thrive; we need it in the activity we spend most of our waking hours doing: working.

Work is core to our well-being. To illustrate this on a small scale, the General Society surveyed Americans and asked how many would quit their jobs if they suddenly came upon a fortune that guaranteed a life of luxury until their last day. Nearly three-quarters of Americans stated they would not quit their job. Americans who feel they are successful at work are twice as likely to say they are very happy overall as people who don't feel that way.