Professor of the Practice of Public Leadership at Harvard Kennedy School and Arthur C. Patterson faculty member at Harvard Business School.

When I ask my Harvard students, "what is happiness?", usually start talking about feelings. And feelings are related to happiness. But happiness as we understand it in the modern world of neuroscience and social science is basically a combination of three phenomena. The first is enjoyment. The second is satisfaction. And the third is purpose. People who are truly happy - and I mean those people who have these things in balance and abundance - are the people who will report high levels of self-reported happiness. And so, enjoyment means pleasure plus elevation. So, it's not just pure pleasure, it's actually being able to enjoy things in a way that you understand what you're enjoying, which is important. Satisfaction is the reward for a job well done or a goal accomplished. And purpose, which is perhaps the most paradoxical of all, comes from really understanding the coherence and meaning of your life. I suspect that most people who know this will know that it actually requires sacrifice, even pain and trouble in our lives. So, the great irony is that in order to have happiness, you need purpose, and in order to have purpose, you need sacrifice and pain. And that actually leads to some unhappiness. So, when people go through their lives trying to avoid unhappiness, what they do without realising it is that they end up avoiding a lot of their own happiness.