Appreciation of allusions in Milton's poems require familiarity with classical Greco-Roman mythology and epics; to borrow the words of an American contemporary Thomas Bulfinch: "Milton abounds in .. allusions" to classical mythology, and especially "scattered profusely" throughout Milton's "Paradise Lost". Keightley was one annotator who meticulously tracked Milton's mythological sources. Some of Keightley's flawed commentary have been pointed out. He argued that Milton erred when he spoke of "Titan, Heaven's first-born," there being no single divine being named Titan, only a race of titans. Though that may be so according to the genealogy laid out by Hesiod's "Theogony", it has been pointed out that Milton could well have used alternate sources, such as Lactantius's "Divinae Institutiones" ("Divine Institutes"), which quotes Ennius to the effect that Uranus had two sons, Titan and Saturn.