The Moral Letters to Lucilius (Latin: Ad Lucilium epistulae morales) were composed by the Roman philosopher Lucius Annaeus Seneca ('The Younger'), during his retirement from the Imperial court in 62-65 AD, a period which ended with his suicide on the orders of the emperor Nero.
The 120 letters, collected in 20 books , were addressed to Seneca's friend Lucilius Junior, then the procurator of Sicily. They amount to informal moral essays, covering subjects such as the nature of the good, happiness and the right attitude to death. It is among the most popular of Seneca's works and as such an important source on Roman stoicism. It's adoption by early Christians contributed to its influence in the Middle Ages and Renaissance.
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Free online texts
Internet Archive: Ad Lucilium Epistulae Morales. Vol. I | Vol. II | Vol. III. Public domain Loeb edition in Latin and English. PDF and other formats.
Latin Library: Epistulae Morales Ad Lucilium - HTML format.
Loebulus. L075 - Seneca -- Ad Lucilium Epistulae Morales I: Letters 1-65. PDF of public domain Loeb edition in Latin and English.
Loebulus. L076 - Seneca -- Ad Lucilium Epistulae Morales II: Letters 66-92. PDF of public domain Loeb edition in Latin and English.
Perseus: Ad Lucilium Epistulae Morales. HTML and XML formats.
Wikisource: Latin - Epistulae Morales ad Lucilium. English - Moral Letters to Lucilius. HTML and other formats.
BBC In Our Time: Seneca the Younger - Radio discussion with Melvyn Bragg, Mary Beard, Catherine Edwards and Alessandro Schiesaro.
History of Philosophy without any gaps: Anger Management - Seneca, podcast by Peter Adamson.
How to be a Stoic: Seneca to Lucilius - Philosophy as a Guide to Life.
Internet Encyclopedia of Philosophy: Lucius Annaeus Seneca.
Librivox: Moral Letters - public domain audiobook.
Stanford Encyclopedia of Philosophy: Seneca.
Stoics.com: Why Seneca's Moral Essays and Moral Epistles?
Wikipedia: Epistulae Morales ad Lucilium.
The Great Conversation: Further reading at Tom's Learning Notes
Cicero: On Duties.
Marcus Aurelius: The Meditations.
Francis Bacon: Essays.
Latin Resources: Online materials for learning Latin.