The Principal Doctrines 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 Tetrapharmakon (Four Cures), which have sometimes been compared to Buddhism's Four Noble Truths. They are often cited as "PDs" in English.

Since most of Epicurus' 37 books "On Nature" are lost to us, 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.

(From Goodreads)