Albert Einstein is famous for discovering the theory of relativity, among other world-changing insights. But the great physicist didn't concern himself just with the rules governing space and time. He was also interested in the rules of our internal state of mind. 

As Psychology Today has noted, Einstein spoke regularly about the search for happiness. "It's happiness we're after," he told an interviewer who asked him what humans desired most in 1931. "Will any student of history agree that the inhabitants of an American city are, on the whole, happier than those of a Greek or a Babylonian city of the past?" he mused at a symposium that same year. 

Clearly the great genius was as intrigued as the rest of us by the question of what constitutes a truly happy life. Did he find an equation that solved this eternal riddle? Yes, and he even scribbled it down in all of 17 words. 

17 words worth $1.56 million 

In 1922, one year after he won the Nobel Prize, Einstein traveled to Japan for a lecture series and found himself continually surrounded by curious admirers. Apparently even back then, he was turning over the question of happiness in his mind, because one day when a bellboy came to make a delivery to his hotel room, Einstein handed him a pair of signed notes rather than a tip. 

One of them read (in German): "A calm and modest life brings more happiness than the pursuit of success combined with constant restlessness."

It's unclear whether Einstein was passing a note to posterity or was simply caught without pocket change and knew his signature would be worth a lot more than any tip one day. But whatever Einstein's motivations, his gesture turned out to be incredibly generous. In 2017 the note, now owned by one of the bellboy's descendants, sold for $1.56 million at auction

Was Einstein right about happiness? 

Clearly Einstein had as good a grasp of the market for future memorabilia as he did for physics. But what about his happiness advice itself? Was Einstein onto something with his note advocating for contentment rather than never-ending striving? Unsurprisingly, the last century's best mind actually understood a few things about happiness way before modern psychological research confirmed them. 

The first is that the concept of happiness isn't at all self-defining. As another Nobel laureate, Daniel Kahneman, has pointed out, sometimes when people talk about happiness, they mean momentary pleasures like eating a slice of chocolate cake or cuddling a puppy. Other times they mean something more like life satisfaction or the sense of contentment you get from feeling you've achieved important things or lived in line with your values. 

These two types of happiness are often in conflict. Chasing big dreams and facing down tough ethical dilemmas often feels terrible in the moment. Petting puppies (or buying pretty things) feels nice but could leave you feeling empty if it's all you fill your life with.

Other psychologists add another wrinkle. There's also something called psychological richness, which is another form of happiness that comes from having rich and varied experiences. It's the joy of feeling you've seen and experienced a fair slice of what the wide, amazing planet has to offer.   

What all this implies is that there are multiple definitions of happiness and each of us must decide how much of each type we're chasing. Einstein clearly understood this when he acknowledged two possible paths -- toward contentment or success -- in his note.

Einstein and the hedonic treadmill 

Einstein spotted the choice we all have to make correctly. Did he recommend the right decision? That probably depends on an individual's character to some extent. It's hard to imagine someone wired like Elon Musk finding much happiness whiling away his time with "a calm and modest life." (Though, to be fair, sometimes the guy seems pretty tortured on his current trajectory too.) 

But that caveat aside, Einstein's secret of happiness actually aligns pretty closely with the modern psychological concept of the hedonic treadmill. 

"We work very hard to reach a goal, anticipating the happiness it will bring. Unfortunately, after a brief fix we quickly slide back to our baseline, ordinary way-of-being and start chasing the next thing we believe will almost certainly -- and finally -- make us happy," psychologist Frank T. McAndre has explained

Einstein clearly saw that when he linked the "pursuit of success" with "constant restlessness" in his note. If you're aiming for some external marker of success both Einstein and modern psychology agree you're never going to reach your destination no matter how frantically you run. After all, Einstein had clearly reached the pinnacle of outward scientific success, so he should have known. 

17 words isn't enough, but it's a good start

Happiness is a balancing act. Too much striving will likely leave you lonely and miserable. Not enough will leave you regretful. And much depends on how you mark your progress. Are you judging by internal measures like mastery and impact or external ones like glory and money? Even Einstein couldn't encapsulate everything everyone needs to know about happiness in 17 words. 

But his "theory of happiness" note still hit on something important that we can all benefit from bearing in mind. Happiness isn't a simple concept. Each of us needs to define it individually for ourselves (and those definitions may shift over time), but whatever definition you choose, be very wary of endless striving. All too often, chasing external success leaves us running miserably in place, getting no close to contentment. 

As the world's most successful scientist, Einstein knew that was true in 1922. It's equally true for all us non-geniuses now.