One of Harvard’s most famous professors to ever live was William James. Having studied medicine and biology, he was first appointed as an instructor in physiology. From there, he went on to teach anatomy, before he established himself as a preeminent psychologist and philosopher.

In the world of philosophy, William James is most famous for championing the theory of pragmatism; the idea that truth isn’t about hard logic or metaphysics, but that truth is what works in the real world. It’s perhaps no surprise, then, that he dedicated much of his life to the study of psychology, a field he laid the groundwork for in his book The Principles of Psychology.

Among many other things, he was the first psychologist to claim that our core traits don’t change much across time. In The Principles, he wrote, “In most of us, by the age of thirty, the character has set like plaster, and will never soften again.” Much subsequent evidence has shown this observation to be largely true. Genes play a huge role, and the environment, too, makes its case, and after the first few decades of life, people stop changing their inner core in meaningful ways. Whether or not change is possible is one thing, but it does seem true that most people are happy to stick with what they have grown comfortable with.

There are two things we can be sure of in life: for one, change itself, and secondly, uncertainty. When we are born, we don’t know much. In fact, we are incredibly needy, and without parents and caretakers, babies would have no chance of survival. Over time, however, we learn, and we develop our mental model of the world. The cultural stories and the physical incentives around us mold our minds and our behaviors until we form an initial concept of self. We learn to avoid danger, seek rewards, and see ourselves through the eyes of others.

For the most part, growing up, we know that we don’t know much. Our bodies are also changing so fast that change itself is evident. And we are okay with it, mostly because we still have other people who protect us from facing the downsides that uncertainty and change offer. At some point, however, when we leave the safety of our parents and our caretakers’ nests, we are thrown into the world, and suddenly we actually have to face uncertainty and change ourselves. This is a time of maturity.

Humans innately crave stability and security, and when we are faced with making sense of a complex, changing world, one which our parents can no longer protect us from, the only source of stability grows from within — our inner self and its behaviors become a center of safety. After decades of experience, they have learned how to weather the trials and tribulations of life, and so, they decide that it’s time to settle down and continue to use these same habit patterns for whatever the future has to offer us.

When we are young and growing rapidly, most of our time is spent in exploration mode. We seek new adventures. We spend a lot of time doing leisure activities, playing around to see what we like and what we don’t like. Once the threshold of maturity is crossed, however, once we have learned our preferences, once we have come to terms with ourselves, we start to move into exploitation mode. This is a stage that we tend to stay in for the next few decades of life. Rather than exploring new habits and new likes and dislikes, we settle down and we double down on what we already have.

So far, so good. In this sense, there is nothing to fear about James’ claim, and for most of us, this is as it should be. This is the basic definition of what it means to grow into an adult. If you have done the work to know who you are and what you want, it makes sense to just zone in and focus.

Around the time of maturity, there is, however, something else that sneaks up on us that isn’t quite as beneficial for us in the long run. That something is seriousness. Seriousness often looks and feels and behaves like maturity and adulthood, which is why it is so dangerous, but it’s an entirely different beast, one that inhibits growth rather than supports it.

Maturity says: I am ready to take responsibility for myself. It’s the part of our character that has figured out how to manage our emotional landscape and can reconcile it with the world as it pursues what it wants. Seriousness says: I know what is best. It’s the part of our character that thinks that it has the world figured out, and then attempts to control it based on a limited perspective rather than working with it as it moves along. Maturity is about having developed the habit patterns that support your growth as a person in a complex world in your chosen direction. Seriousness is about trying to enforce your belief systems onto other people because you know best.

You can be mature while still retaining your ability to keep an open mind. Seriousness, by default, is rigid and closed-minded. The former has personal preferences, while the latter dresses judgments up as facts.

When we are young, much of what we learn is learned through play. Evolutionarily, we play because it teaches us about our physical surroundings, our social norms, and our cognitive capacities in a low-cost way. Although the point of play isn’t to learn, learning is a natural result of play. It’s a state in which our mind is open to the stimuli of the world — a non-judgmental kind of flow and engagement. There is no right or wrong way to play — it’s a constant negotiation with the present moment.

Seriousness is the precise opposite of this. It’s a pure state of judgment confined by preexisting boundaries of what the wearer of the mask assumes is right and wrong, without any consideration of the fact that it may be lacking complete information in an incomprehensible world. The base of seriousness is the dread of uncertainty. Deep down it knows that it doesn’t know much and is terrified of it. Rather than continuing the state of play into mature adulthood, it instead doubles down on what brings it comfort.

Ultimately, seriousness is no way to live if growth and challenge and truth is what one is after. Maturity may harden some of our habits, but it can generally be fluid enough to respond to things, good and bad, in ways that are diverse, keeping itself open to learning new things. William James, even as he wrote those words about character hardening knew this, because even in his own life, he did some of his best work in his later years. He only did so, however, because he never fell into the trap of closing his mind.

One of my favorite quotes of his comes from The Varieties of Religious Experience, where he wrote:

“Good-humor is a philosophic state of mind; it seems to say to Nature that we take her no more seriously than she takes us. I maintain that one should always talk of philosophy with a smile.”

In doing this, in taking this approach himself, James’ chose to play, just like we played when we learned how to walk, how to talk, and how to socialize. The common tragedy of adulthood isn’t necessarily that we forget to play — it’s perhaps the most natural thing that we can do as human beings. Rather, it’s that we convince ourselves that it just isn’t a thing we are supposed to do as we age. We stop looking at work as play. We stop treating our interactions with friends and loved ones as play. We stop making time to play.

The cost of all this is that our mind becomes rigid. It forgets how to open up to new information, new stimuli. But the beauty is that it can learn to do so at any time, as long as it is willing to face the underlying uncertainty. The road leading there is precisely where the gift waits.

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