The citations of authorities in the _De naturis rerum_ are of much interest. A number of references to the law books of Justinian show Neckam’s knowledge of the Roman law,[568] and, as we should expect after hearing of his commentary upon Ovid’s _Metamorphoses_, allusions to that work, the _Fasti_, and the _Ars amandi_ are frequent. Claudian is once quoted for two solid pages and considerable use is made of other Latin poets such as Vergil, Lucan, Martial, and Juvenal. Neckam believed that the diligent investigator could find much that was useful in the inventions of the poets and that beneath their fables moral instruction sometimes lay hid.[569] Neckam quotes Plato, perhaps indirectly, and repeats in different words the fable of the crow and fox, as given in Apuleius.[570] The church fathers are of course utilized--Augustine, Jerome, Gregory, Basil, and a more recent theologian like Anselm, archbishop of Canterbury; and familiarity is shown with the early medieval standard authorities, such as Boethius, Cassiodorus, Bede, and Isidore. Of writers who may be regarded as dealing more particularly with natural science there are mentioned Pliny and Solinus on animals--but he seems to use Pliny very little and Solinus a great deal, Macer and Dioscorides on the properties and effects of herbs,[571] while works in the domain of astronomy or astrology are attributed to Julius (perhaps really Firmicus) and Augustus Caesar as well as to Ptolemy.[572] [Sidenote: His knowledge of Aristotle.]