The Commentarius (Commentary) is the second volume of Karl (Carl) Lachmann's first edition of Lucretius, De rerum natura (Lucretius wrote in the Late Republican Rome, at the middle of 1st century b.C., the most important Latin work about Epicurus' Perì phýseōs and among the most important ones, with the Georgics of Virgil, in Latin didactic poetry). Lachmann was one of the leading German classicists (and as a distinguished Germanist made relevant editions, too, of many Middle High German poets: see also Karl Lachmann. Eine Biographie, by Martin Julius Hertz, 1851). The two books were the principal occupation of his live from 1845 and are the founding work of 19th century classical philology and modern textual criticism; while drawing the stemma of the older extant MSS of Lucretius with a rigidly scientific recensio Lachmann succeeded in tracing back the lost archetype of the De rerum natura (written during the Carolingian Renaissance) and developed the method which still today is named from him (Lachmann's method, or Lachmannsche Methode: see Sebastiano Timpanaro, La genesi del metodo del Lachmann, Firenze 1963, and many other revised editions; English ed. The genesis of Lachmann's method, Chicago 2005; French ed. La Genèse de la méthode de Lachmann, Paris 2016).
A second, revised edition (both the text of Lucretius and the Commentary) was published in 1855.