Man is most nearly himself when he achieves the seriousness of a child at play.

Although widely attributed to Heraclitus, this wording is best treated as a modern paraphrase rather than a securely attested fragment. The thought links two ideas often associated with Heraclitus: (1) play as a serious, world-shaping activity (famously, the image of time or the cosmos as a child at play) and (2) authenticity as alignment with the fundamental character of reality. Read this way, the line suggests that people are most fully themselves when they engage in an activity with wholehearted absorption—free, inventive, and unselfconscious—yet with real stakes for the participant, as in a child’s play. It elevates play from frivolity to a mode of truthful being.