Epicurus ' only surviving complete works are three letters , which are to be found in book X of Diogenes Laertius ' Lives of Eminent Philosophers , and two groups of quotes : the Principal Doctrines , reported as well in Diogenes ' book X , and the Vatican Sayings , preserved in a manuscript from the Vatican Library . The only surviving complete works by Epicurus are three letters , which are to be found in book X of Diogenes Laertius 's Lives of Eminent Philosophers , and two groups of quotes : the Principal Doctrines , reported as well in Diogenes 's book X , and the Vatican Sayings , preserved in a manuscript from the Vatican Library .