The Principal Doctrines is a collection of forty of the most important articles of Epicurus’s teaching, presumably extracted by a disciple from the master’s voluminous works. It was widely known in ancient times and was preserved by Diogenes Laërtius (probably third century b.c.e.) in his Peri bin dogmatn kai apophthegmatn tn en philosophia eudokimsantn (third century c.e.; The Lives, Opinions, and Remarkable Sayings of the Most Famous Ancient Philosophers, 1688). Together with the Letter to Menoeceus, also found in Diogenes’ works, it constitutes our only firsthand source for the ethical teachings of Epicurus. The most important supplementary source is Lucretius’s poem, De rerum natura (c. 60 b.c.e.; On the Nature of Things, 1862).