Bill Gates has some advice for new college graduates.

The billionaire Microsoft (MSFT) co-founder turned philanthropist said artificial intelligence, energy and biosciences are "promising" fields where new graduates can make "a huge impact."

"It's what I would do if starting out today," Gates said in a series of tweets on Monday.

Gates, who dropped out of Harvard in 1975, shared on Twitter and via a post on Mic lessons he wished he had known when leaving college. For example: Intelligence takes many different forms and isn't as important as Gates previously thought.

4/ E.g. Intelligence takes many different forms. It is not one-dimensional. And not as important as I used to think. — Bill Gates (@BillGates) May 15, 2017

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"When I left school, I knew little about the world's worst inequities. Took me decades to learn," he also tweeted.

Gates said he didn't see true inequality until his late 30s, when he and wife Melinda Gates took their first trip to Africa. "We were shocked by what we saw. When we came back, we began learning more. It blew our minds that millions of children there were dying from diseases that no one in rich countries even worried about," he said in the Mic post.

College grads know more than Gates did when he was their age, he added. "You can start fighting inequity, whether down the street or around the world, sooner," Gates tweeted.

Gates advised graduates to surround themselves with people who challenge, teach and push them "to be your best self," saying his wife Melinda does that for him.

.@melindagates 8/ Like @WarrenBuffett I measure my happiness by whether people close to me are happy and love me, & by the difference I make for others. — Bill Gates (@BillGates) May 15, 2017

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Gates also offered a book recommendation -- "The Better Angels of Our Nature: Why Violence Has Declined" by Steven Pinker -- saying it was the most inspiring book he's ever read. Pinker argues that the world is improving. "Sounds crazy, but it's true. This is the most peaceful time in human history," Gates said.

.@melindagates @WarrenBuffett @sapinker 11/ That matters because if you think the world is getting better, you want to spread the progress to more people and places. — Bill Gates (@BillGates) May 15, 2017

"This doesn't mean you ignore the serious problems we face. It just means you believe they can be solved, and you're moved to act on that belief," Gates said, adding this is the core of his worldview.

"This is an amazing time to be alive. I hope you make the most of it," Gates said.