So…on the heels of the closing plenary, I’m speechless. But I’ll try.

My biggest takeaway this week about social entrepreneurs is that they have the DNA of a pitbull.

These change agents have a clear vision of a better world, and they are dogged about seeing that vision through to completion. Their sense of urgency is palpable at every moment. They’re always on.

They’ll readily talk your ear off about interventions that need to be scaled to solve the world’s most difficult problems, while flashing you a warm smile that belies the raging flames of their inner intensity. To borrow a phrase from John Elkington’s new book, they are “unreasonable people” with great power.

And they’re not just pitbulls. They’re principled pitbulls.

The hard work of social change, as J.B. Schramm used to tell me, is a marathon, not a sprint. Burnout is all too commonplace. Naysayers abound. Resources are scarce. Competition is fierce. Impact can be hard to see.  But social entrepreneurs have a bottomless reservoir of faith, of dedication and hope, of deep-seated belief in humanity and possibility.

They’re woven with a strong moral fiber, and they ground themselves in their values—putting service to other people (including Mother Earth) at the center of their life’s work.

No one typified this better for me than President Jimmy Carter (below, on stage with Sally Osberg, Jeff Skoll and 2008 Skoll Awardee Paul Farmer), who spoke to us from the pulpit of the Sheldonian Theatre. Watch the video of his speech on Social Edge immediately.

This is a man who has unwaveringly committed his life to improving the lives of others—brokering peace, eradicating disease, changing the dialogue of governments and the tenor of the citizenry. He has done so even when it was politically unpopular, by anchoring himself in his own well-formed value system. Look up the word “conviction” in the dictionary and there’s President Carter.

We could all learn something from this peanut farmer from Georgia, whose life, as with Gandhi, has been his message.

He and Al Gore were overflowing with passion and commitment, which struck me as an almost youthful exuberance. It got me thinking about how spending your brief time on this planet helping people can actually lead to longer, healthier earthly existence. Service to others is life-giving.

It's like vegetables for the soul.

Jimmy Carter is 83, but he speaks as if he’s half that age, and he walks with a hop in his step. He jet-sets from one developing country to another (he was on his way to Africa after swooping in for a few hours to address us in Oxford). He is as committed and energetic as ever.

And, to my point, a cool random factoid: President Carter’s mother joined the Peace Corps at age 70!

In a mentoring session I attended, a seasoned social entrepreneur said that the generation currently leading organizations actually need young people as their mentors. Be that as it may, take it from this 27-year old naive idealist: People like Jimmy Carter are an inspiration to social changers young and old.

I, for one, will be sure to eat my veggies...