On the Tranquility of the Soul is a Stoic philosophical dialogue by Seneca addressed to his friend Serenus, who seeks to overcome persistent inner turmoil. The work explores ways to achieve inner peace (tranquillitas or euthymia) by identifying and healing the causes of anxiety, such as misdirected desires, vain ambition and dispersion of the mind.
Seneca, playing the role of the doctor of the soul, offers therapy based on reason and self-control. He advises detaching the mind from external goods, accepting destiny and cultivating virtue. The text rejects both the total retreat of the world and the sterile agitation of public life, advocating rather a fair balance where the individual chooses his commitments with moderation, finding serenity not in the avoidance of evils, but in the transformation of one's state of mind in the face of the inevitable.