[3.23.1] haec est ergo aquarum, ut videtur, divisio: A further way to classify waters, now by their age (instead of e.g. by temperature or weight 3.2.2 or whether they are standing or flowing 3.3.1). The ergo indicates that this divisio is springing from the previous paragraph and should be logically tied to it. This would seem to be the final divisio of waters as subsequent chapters investigate hot springs, mirabilia aquarum, and the flood. Seneca does not use ut videtur elsewhere (and it is hard to explain why he suggests some reluctance to go along with this divisio of his own creation), but will discuss other divisiones (of lightning, 2.47.1-2.50.1) in the Naturales Quaestiones.
quaedam post illum: A major crux that has caused divergent readings. This follows Hine 1996, but see Hine 1980: 10-11 for elucidation, and Vottero 1989: 157-58 for an alternative construction. The text should indicate both waters that were coterminous with the creation of the world and those that appeared later. Cordoñer Merino 1979 had supplied simul cum mundo, quaedam for a similar reason. The later waters then are further divided.
ex his posterioribus caelestes, quas nubila excutiunt: Hine’s conjecture creates a structure to the sentence that would not be apparent at first, but helps to explain the subsequent ex terrenis. Seneca spends the fragmentary NQ 4b explaining certain celestial waters such as dew, frost, hail, clouds, and rain (cf. Williams 2012: 136n.3). Technically, according to Seneca, nubila are part of the sublimia, not caelestia (NQ 2.1.1-2). Elsewhere in the treatise, excutere is used especially of lightning shaken out from clouds (NQ 2.22.3, 2.24.2, 2.30.1).
ut ita dicam supernatantes: The collocation ut ita dicam is common in Seneca’s prose (37 times) and used to defend a novel image or word-choice – in this case, supernatare “to float on the surface”, which is its sole appearance in Seneca’s corpus.
quae in summa humo repunt: Further describing those waters in personified language as repere is usually applied to snakes or animals (Pliny Nat. 9.73, 29.136), but now signifying their slow movement over the ground (s.v. OLD 3b). Seneca elsewhere uses it of comets (NQ 7.21.2), fevers (Ep. 95.17), and of someone crawling (Ben. 5.24.2, Dial 7.26.8).
aliae abditae, quarum reddita est ratio: Although he has not used the adjective abditus to describe the waters per se (cf. 3.14.3 supra), they will be commonly described in this way later in the work, cf. NQ 3.28.4, 3.30.3, 6.7.5.
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