Billionaire investor Thomas Kaplan shares 3 tips for a rich and happy life

• Thomas Kaplan says being grateful and playing to your strengths are key to a rich and happy life.
• "Don't be afraid to say, 'I don't know,'" the billionaire investor said in an interview.
• Kaplan sold a Rembrandt for almost $18 million this week to support his "big cat" conservation work.

Gratitude, humility, and conviction are key ingredients for a happy and successful life, Thomas Kaplan says.

The billionaire investor, who owns the world's largest private collection of artworks by Rembrandt van Rijn, spoke to Business Insider ahead of the auction of "Young Lion Resting" at Sotheby's New York on Wednesday.

The Rembrandt drawing, which Kaplan co-owned, sold for nearly $18 million — with the proceeds going to Panthera, the "big cat" conservation nonprofit he cofounded.

Kaplan said that protecting endangered felines has been his greatest passion since he was six years old. He dreamed of studying big cats as a felid zoologist, only to realize a few years later that he lacked the flair for science required.

"But I made a vow to myself — I think I was about 12 — if I ever make enough money, I'm going to come back to the subject, and I will support and enable all of those conservationists who had the aptitude that I didn't have," Kaplan said.

He went on to make a fortune in the energy and precious metal industries. True to his word, he cofounded Panthera in 2006 with leading cat conservationist the late Alan Rabinowitz, and the pair built the organization into a world leader in the field.

"I'm very mission-driven, and I will do anything to move the needle on those things that move me," Kaplan said.

Be grateful and play to your strengths

Kaplan shared three pieces of life advice with Business Insider.

Asked how he kept his three children grounded in an ultrawealthy household, he said they were simply "not motivated by money."

"No bling, no social media, no cars or watches or any accoutrements of wealth. None," he added.

Kaplan said he drilled into them the importance of giving back and being thankful, as he believes the "basis for all happiness is living a life of gratitude."

He's made sure they "understand if you are an ungrateful person, you will be miserable and deserve to be."

Kaplan also underscored the need to recognize the limits of your knowledge and resist speculating beyond what you deeply understand. Warren Buffett would describe that as staying within one's "circle of competence."

"Don't be afraid to say, 'I don't know,'" Kaplan said. "Credibility is hard won, easily lost."

The NovaGold Resources chairman said that people respect his expertise in certain subjects, and he never wants to "debase that currency by being flippant."

"Be disciplined enough not to feel that because you're asked something, you have to answer it," he said.

Kaplan's final piece of advice was related: When you speak up about a subject you know intimately, or invest in an asset you understand deeply, do so with conviction.

"If you're going to go down, go down for something that wasn't flippant, but was thought through," he said.

"When you have a relative advantage, push the pedal to the metal," Kaplan added. "If you don't have a relative advantage, don't play."