Texting while driving. Bullying people on social media. Buying into the latest conspiracy theory. Passing off AI-generated work as your own.

That may seem like a random list of 21st-century vices. But I’d argue they’re all examples of the loss of one particular virtue: sophrosyne.

An ancient Greek concept, sophrosyne – pronounced “suh-fros-uh-nee” – is what we might call “sound-mindedness” today. It’s a constellation of characteristics, including moderation, reflectiveness and self-knowledge. They’re found in the kind of person who can respect and trust herself, and be respected and trusted by others.

As a philosopher and philosophical counselor, I research the connection between virtue and happiness. In particular, I’ve noticed a connection between sophrosyne and eudaimonia, the Greek philosophical concept for happiness, or living well.

For the Greeks, sophrosyne represented excellence of character, moderation and self-control. It was connected to phronesis, or practical wisdom, and stood in marked contrast with hubris: excessive pride, dangerous overconfidence and lack of self-insight. Heraclitus, a philosopher who lived around 500 B.C.E., taught that sophrosyne was the most important virtue of all.

Plato, who taught a century later, discussed sophrosyne as the ability to know oneself – and to know when you don’t know something. In “Republic,” he likened sophrosyne to a harmony or friendship between the three parts of the soul: reason, spirit and bodily........

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