Hence, since such a course is of service to all who take up natural science, I, who devote to the subject my continuous energy and reap the calm enjoyment of a life like this" ~ Epicurus,
So sounds like "calmness" coming through the practice of studying natural science?
And at the end of the letter to Herodotus, Epicurus writes:
"It is of such a sort that those who are already tolerably, or even perfectly, well acquainted with the details can, by analysis of what they know into such elementary perceptions as these, best prosecute their researches in physical science as a whole ; while those, on the other hand, who are not altogether entitled to rank as mature students can in silent fashion and as quick as thought run over the doctrines most important for their peace of mind (γαληνισμὸν galenismon - related to the same word he uses to describe his own calm coming from the study of nature)."