"New college grads often ask me for career advice," begins a series of tweets from Bill Gates. " [Artificial intelligence], energy, and biosciences are promising fields where you can make a huge impact."

"If I were starting out today and looking for the same kind of opportunity to make a big impact in the world, I would consider three fields," Gates writes on his blog, which expanded upon his tweets. "One is artificial intelligence. We have only begun to tap into all the ways it will make people's lives more productive and creative. The second is energy, because making it clean, affordable, and reliable will be essential for fighting poverty and climate change. The third is the biosciences, which are ripe with opportunities to help people live longer, healthier lives."

Gates, who dropped out of Harvard in 1975, then shared things he wish he had known when leaving college: "Looking back on when I left college, there are some things I wish I had known. Intelligence takes many different forms. It is not one-dimensional. And not as important as I used to think. I also have one big regret: When I left school, I knew little about the world's worst inequities. Took me decades to learn.

"You know more than I did when I was your age. You can start fighting inequity, whether down the street or around the world, sooner. Meanwhile, surround yourself with people who challenge you, teach you, and push you to be your best self. As Melinda Gates does for me. Like Warren Buffett, I measure my happiness by whether people close to me are happy and love me, and by the difference I make for others."

Gate's estimated wealth is 65 billion dollars, equal to the annual GDP of Ecuador, but he says that "money has no utility to me beyond a certain point." That's why he and his wife Melinda have so far given away $28 billion through their foundation, with more than $8 billion going to help the poorest individuals.

Gates then recommends to young graduates the most inspiring book he's ever read. "If I could give each of you a graduation present, it would be this [book]. Pinker shows how the world is getting better. Sounds crazy, but it's true. This is the most peaceful time in human history."

The Better Angels of Our Nature: Why Violence Has Declined, by Harvard psychologist Steven A. Pinker, argues that our era is more peaceful than any previous period of human existence. Mark Zuckerberg named it one of his "Book of the Year" selections. After Gates tweeted about the book, it quickly climbed 608,850 percent to reach the No. 2 spot on the overall Amazon best-seller book list.

"If you think the world is getting better," according to Gates, "you want to spread the progress to more people and places. It doesn't mean you ignore the serious problems we face. It just means you believe they can be solved."