Just as people used to ask me how I accomplish so much, recently a few people have asked me how I stay so serene. Although this question is harder to answer, I'll give it a try.
At root, I suspect that innately I am blessed with what theorists of the Five-Factor Model of personality call the trait of emotional stability. There might not be not much you can do to cultivate inner peace if you happen to be on the other end of the spectrum (typically labeled neuroticism).
That said, here are some speculations.
First, I'm convinced that having a balanced "life portfolio" helps keep me grounded. As I sketched in two blog posts a few years ago, I see the areas of life as health and vitality, work and achievement, love and caring, character and personality, wisdom and spirituality, and beauty and aesthetics. I'm extremely fortunate to be in good health, to be somewhat successful financially and in my career, to have a great partnership with my wife, to be emotionally stable, to benefit from deep study of many wisdom traditions, and to appreciate the many beautiful aspects of life all around me.
Second, I've discovered some practices that keep me calm in a world that seems increasingly agitated and angry: I avoid the news, I ignore politics, I dropped out of social media, I follow a low-volatility investment strategy, I read quite a bit of history (life was much harder 500 or 1000 or 2000 years ago!), etc.
Third, I consciously cultivate a philosophical perspective on life. This should come as no surprise, since I majored in philosophy. :-) However, I do find that immersing myself in a wide range of "Minerval arts" enables me to be more reflective, to trim the worst excesses of my actions and emotions, and to understand various outlooks on life so that I can more easily find common ground with other people. Furthermore, taking the long view on human existence and my own life promotes the realization that most of the things people get worked up about are ultimately insignificant.
Fourth, over time I have come to adopt a less dogmatic attitude about human affairs. Part of this is just having lived for some decades, and part of it comes from my study of Taoism, Buddhism, and Pyrrhonism. All of these schools of thought encourage you to let go of cravings and attachments and ambitions, which together contribute to avoiding cycles of hope and disappointment, possession and loss, and pride and deflation. I plan to explore these ideas in depth by writing a novel about the ancient Greek skeptic Pyrrho a few years from now (tentatively entitled Gods Among Men).
I'm not sure how well my personal experience with cultivating serenity applies to other people, but perhaps this post will point you in some fruitful directions.
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