The Principal Doctrines (Kyriai doxai, sometimes Kyriai doxiai) are forty authoritative conclusions set up as official doctrines by the founders of Epicureanism: Epicurus of Samos, Metrodorus of Lampsacus, Hermarchus of Mitilene and Polyaenus of Lampsacus. The first four doctrines make up the Tetrapharmakos (Four Cures), which have sometimes been compared to Buddhism's Four Noble Truths.[citation needed]
Some of the Principal Doctrines ("PDs") are organized into groups and are meant to be studied together. PDs 10-13 discuss the Epicurean philosophy of science. PDs 18-21 explain the natural limits of desires and in time, and how the flesh is unable to learn these limits but the mind can. PDs 22-25 deal with the importance of the canon, or the Epicurean standard of truth. PDs 31-38 explain the Epicurean doctrines on justice based on mutual advantage and contractarianism.[1] PDs 39-40 call for an intimate society of friends.
Since most of Epicurus' 37 books "On Nature" are lost to us,[2] the Principal Doctrines are, together with Epicurus' Letters to Herodotus, Menoeceus, and Pythocles, the most authoritative writings in Epicureanism. The Principal Doctrines exemplify the Epicurean philosophers' practice of publishing summaries and outlines of their teachings for easy memorization. However, they are so concise and short that it's difficult to understand them in depth without the context of additional commentaries and writings by ancient sources or by modern Epicurean practitioners, whenever possible.
In his work "Alexander the Oracle Monger",[3] comedian Lucian of Samosata praised the PDs saying: "What blessings this book creates for its readers and what peace, tranquillity, and freedom it engenders in them, liberating them as it does from terrors and apparitions and portents, from vain hopes and extravagant cravings, developing in them intelligence and truth, and truly purifying their understanding, not with torches and squills and that sort of foolery, but with straight thinking, truthfulness and frankness."