Bill Gates shares his biggest regret and a book recommendation with new grads

Bill Gates left Harvard to start his own company, but he says it took him years to learn some key things that are more obvious now.

Even the world's richest man has a few regrets. Microsoft co-founder Bill Gates says he has one big one.

"When I left school, I knew little about the world's worst inequities. Took me decades to learn," Gates wrote on Twitter Monday.

Gates kicked off what feels an awful lot like a guy trying out some lines from an upcoming commencement speech in tweet form with this classic clip from "The Graduate:"

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Dustin Hoffman's character receives a one-word piece of career advice: "Plastics."

Today, Gates says grads should be less interested in that material, known to cause a big mess in the ocean.

"(Artificial intelligence), energy, and biosciences are promising fields where you can make a huge impact. It's what I would do if starting out today," Gates tweeted.

But the billionaire would clearly rather see young people tackling that persistent issue of inequality that it took him so long to catch on to.

"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."

Gates also has a few other pearls of wisdom:

"Intelligence takes many different forms. It is not one-dimensional. And not as important as I used to think."

Gates is particularly optimistic, a sentiment that hasn't been much of a theme, at least in geopolitics, recently. But don't fret, because he's got a book recommendation to put you in the right frame of mind: "The Better Angels of Our Nature: Why Violence Has Declined" by Stephen Pinker."(Pinker) shows how the world is getting better. Sounds crazy, but it's true. This is the most peaceful time in human history," Gates writes. "That matters because if you think the world is getting better, 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."

Gates says that last bit is the core of his worldview that drives his work, and it's also a hell of a lot more interesting than plastics.

See Gates' favorite books of last year: