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In Seneca’s essay on tranquility, he uses the Greek word euthymia
Stephen Hanselman • The Daily Stoic: 366 Meditations on Wisdom, Perseverance, and the Art of Living

How to stay calm (without any hacks)
This is all you need. https://t.co/RgVVVwBlpx
Tranquility may be my new favorite emotion. Why? Check out the research definition: “Tranquility is associated with the absence of demand” and “no pressure to do anything.”
Brené Brown • Atlas of the Heart: Mapping Meaningful Connection and the Language of Human Experience
Rather, Stoic tranquility was a psychological state marked by the absence of negative emotions, such as grief, anger, and anxiety, and the presence of positive emotions, such as joy.