The identity of the Diogenes in the Aëtian passage in which he, along with Cleanthes and Oenopides, is said to identify the deity with the world-soul (1.7.8 Mansfeld–Runia,Footnote ¹ henceforth MR) has long been controversial. With a few exceptions, this fundamental theological tenet is ascribed to Diogenes of Apollonia. This paper endorses the opposing thesis—that the Diogenes in question is the Stoic philosopher—and paves the way for a reconsideration of other Aëtian evidence, such as Stoic doxai, in which the name Diogenes appears without the ethnicon.

In Book 1 of Aëtius’ compendium, chapter 7, devoted to the nature of God (Τίς ὁ θεός, Who the Deity is), is introduced by an extended paragraph, transmitted by Stobaeus (= Stob.) and absent from Ps.-Plutarch (= [Plut.]),Footnote ² on the history of ancient atheism and its representatives in both poetry and philosophy. In this list, Diagoras, Theodorus, Euhemerus, Callimachus, Euripides, Plato and Anaxagoras are cited in that order.Footnote ³ Immediately after §1, Aëtius describes the various conceptions of the deity that ancient thinkers, from the Milesians to Epicurus, espoused (§§2–25, in Stob. and [Plut.]). The doxographer here follows a sequence whose rationale is the same as that of chapter 1.3, On Principles, What They Are (Περὶ τῶν ἀρχῶν, τί εἰσιν): the number of items (in our case that of divinities rather than of principles) and the subsequent contrast between monists and pluralists.Footnote ⁴ However, Aët. 1.7.2–25 MR combines the treatment of school successions and single philosophers, and for the most part does not match the order of the analogous lists in Cicero's On the Nature of Gods Footnote ⁵ and in Philodemus’ On Piety, although in all three lists the beginning and the end, viz. Thales and Epicurus, are the same. Let us compare synoptically the sequence of philosophers provided by these three sources; I highlight in bold the cases in which the name Diogenes appears.

Cicero's and Philodemus’ parallel passages differ from each other to the extent that they demonstrate that neither depends on the other, nor do they share a unique common source.Footnote ⁸ Proof that Cicero and Philodemus did not draw on each other's work includes (a) striking doxographical lacunae, such as the lack in Cicero of any reference to Heraclitus, whose theology is instead the subject of an important snippet in Philodemus’ extant text; (b) the dissimilar style, much more polemical in Cicero than in Philodemus; (c) the often substantial difference in the philosophical content of the doxai of the authors cited in the two catalogues. Their varying treatment of Diogenes of Apollonia illustrates these discrepancies well. Cicero briefly expounds the theology of this Presocratic philosopher between his discussions of Democritus and Plato; Philodemus, on the other hand, mentions Diogenes of Apollonia between Heraclitus and the unknown (Presocratic) philosopher whose theology is described in an unfortunately lost column of P.Herc. 1428. However, in addition to the differing placement of the doxa, what stands out is the dissimilar content of Cicero's and Philodemus’ reports: in Cicero, Velleius denounces the unacceptable consequences of identifying God with air for the conception of the nature of divinity; in Philodemus, by contrast, Diogenes is the representative of a demythicized version of God, which leads him to praise Homer for having identified Zeus with air.

This is a necessary precondition for any attempt to resolve an important problem of Aëtius’ passage in question, viz. the doxa mentioned above, placed between those of Democritus and Pythagoras, which ascribes the idea that the deity should be identified with the world-soul to Diogenes, Cleanthes and Oenopides alike (Aët. 1.7.8 MR):

This doxa is transmitted only by Stobaeus (Ecl. 1.1.29b) and is placed between those concerning Democritus’ and Pythagoras’ theologies: the former identified the deity with the Mind residing in a spherically shaped fire (§7); the latter considered the Monad as the deity and the Good, and the Undetermined Dyad as a daemon and the Evil (§9). As for Oenopides (41 A 6 DK), Zeller supposed he was an otherwise unknown Stoic,Footnote ¹² but this unlikely suggestion has never gained credence. In the Placita, Oenopides appears only twice more, always along with Pythagoras.Footnote ¹³ Independently of the question of the Stoic interpretation of his thought in the Aëtian passage at hand, he should be identified with the homonymous astronomer of Chios (fifth century b.c.).Footnote ¹⁴ Scholars have long disagreed on the identity of the Diogenes cited by Aëtius in §8. Among the many scholarly arguments, Diels's stance is peculiar. From the index nominum of the Doxographi Graeci, it is clear that he previously ascribed the doxa to Diogenes of Babylon;Footnote ¹⁵ later, in Die Fragmente der Vorsokratiker, he changed his mind and included the doxa among the witnesses to Diogenes of Apollonia collected in 64 A 8 DK (however, except for Aëtius, in none of these witnesses is the soul, whether individual or cosmic, in question). By contrast, von Arnim recorded this doxa in SVF III 31 (= SVF I 532), among bits of evidence concerning Diogenes of Babylon's physics that transmit important fragments of his treatise, On the Regent Part of the Soul (SVF III 27–37).Footnote ¹⁶ In his edition of Diogenes of Apollonia, Laks treats this testimonium neutrally, placing it among the dubia (S 1).Footnote ¹⁷ In his collection of the Presocratics co-edited with Most (henceforth LM), however, Laks cautiously mentions the passage among the Stoicizing Versions of Diogenes (?) (28 R 17 LM), along with two other Aëtian witnesses pertaining to the chapters of Book 2 of the compendium that deal with the causes of the world's tilt (Τίς ἡ αἰτία τοῦ τὸν κόσμον ἐγκλιθῆναι)Footnote ¹⁸ and with the world itself, that is, its nature (Περὶ κόσμου), respectively.Footnote ¹⁹ Lastly, Mansfeld and Runia, in the commentary on their new monumental edition of Aëtius, reiterate that the issue remains unclear, as indicated in their apparatus testimoniorum on page 373, where both possibilities are given. However, they also offer arguments in favour of Diogenes of Apollonia and a ‘Stoicizing’ reading of his theory of the elements.Footnote ²⁰

There are four strong arguments in favour of identifying the Diogenes in Aët. 1.7.8 MR with the Stoic rather than with the Presocratic philosopher. The first three rest on formal aspects of the passage, while the fourth focusses on the philosophical content of the testimonium. 1) It has been observed that the only argument that Diels could have used to rule out the possibility that Diogenes of Babylon is meant in the doxa at issue is that the Stoic would then appear only here in the Placita.Footnote ²¹ But this conclusion is clearly untrue. In Aët. 2.32.9 MR, Diogenes is explicitly indicated as ὁ Στωικός and is said to maintain that Heraclitus’ Great Year is to be multiplied by 365 times (SVF III 28). But even if Diogenes of Babylon appeared only here in the compendium, this would not be reason enough to rule out a priori that Aëtius was referring to him in the passage under consideration: his name would not be the only hapax legomenon in the Placita.Footnote ²² Among several examples, I find the case of the Stoic philosopher, pupil and successor of Diogenes of Babylon, Antipater of Tarsus, significant. His name appears only in the chapter On Fate (Περὶ εἱμαρμένης) of Book 1 of the compendium, where, immediately after the doxa on Zeno of Citium, he is said to support the theory that God and Fate coincide.Footnote ²³ 2) Moreover, although it is true that in the doxai concerning him Diogenes of Apollonia is rarely cited by Aëtius with his ethnicon, viz. 1.3.10 and 4.3.8 MR, Diogenes of Apollonia is also cited alongside a Stoic only in this passage of the Placita, to the best of my knowledge. 3) The sequence of philosophers in Cicero and Philodemus does not automatically indicate that Aëtius’ Diogenes must be the Presocratic philosopher. I have already highlighted above the differences in the content of Cicero's and Philodemus’ reports on Diogenes of Apollonia. However, in neither of these reports is Diogenes’ theology connected with that of other philosophers, let alone with that of the Stoics. In addition, in the long list from Aëtius, each Presocratic theological doxa is devoted to a single thinker. The only exception is Melissus, who is linked with Zeno of Elea (within the same doxa) in saying that the deity is the One/All, the only entity that is everlasting and unlimited (§18). 4) The deciding reason, however, for ascribing the Aëtian doxa to the Stoic Diogenes is the philosophical argument. I now develop this point in greater detail.

None of the witnesses in 64 A 8 DK relates Diogenes of Apollonia's view of air qua (physical) principle to the idea of a world-soul. As Laks points out, none of the sources allows us to argue that Diogenes of Apollonia ever upheld the world-soul doctrine or, more generally, the existence of a cosmic intelligence.Footnote ²⁴ Neither is this reported in Augustine's account (De ciu. D. 8.2 = 64 A 8 [4] DK), where Diogenes of Apollonia's aer is only said to be provided with a diuina ratio, nor in the other two bits of evidence that are recorded, not in DK, but only in Laks's edition: Clement of Alexandria (Protr. 5.64.1–3 = T 7b Laks) and Minucius Felix (Oct. 19.5 = T 7d Laks). In the doxographical sources specifically devoted to Diogenes of Apollonia's psychology in the Vorsokratiker (64 A 20 DK) as well, no link between the theory of the world-soul and Diogenes of Apollonia appears. In On the Soul (1.2.405a21–5), Aristotle only says that for Diogenes soul is air and, since it is the thinnest of all things, it is also its principle (above all the principle of its motion).

In Book 4 of Aëtius’ compendium (4.7.1 MR), the doxa on the indestructibility of the soul is ascribed to Diogenes (of Apollonia) along with Pythagoras, Anaxagoras, Plato, Empedocles and Xenocrates. Note that no Stoic appears here. The Stoics as a general group are discussed instead at 4.7.3 MR. At the end of 64 A 20 DK, Diels adds in parentheses Aët. 4.5.8 MR, where the opinion that the regent part of the soul is placed in the heart's arterial cavity, which is pneumatic (Διογένης ἐν τῇ ἀρτηριακῇ κοιλίᾳ τῆς καρδίας, ἥτις ἐστὶ πνευματική), is ascribed to a Diogenes who is not otherwise specified. Scholars strongly suspect that Aëtius is speaking here not about Diogenes of Apollonia but about Diogenes of Babylon.Footnote ²⁵ Moreover, an apparently heterodox opinion in comparison to the official Stoic position—which maintained that the regent part of the soul is found in the entire heart or in the heart-embracing pneuma (Aët. 4.5.7 MR), not in a part of the heart—is ascribed to Diogenes of Babylon. If, as I am inclined to believe, Aët. 4.5.8 MR concerns Diogenes of Babylon's psychology, then its close connection to Galen's witness in SVF III 30—in which the regent part of the soul is said simply to be in the heart—is evident.Footnote ²⁶ Finally, among the evidence for Diogenes of Apollonia's psychology in the Vorsokratiker, Diels did not include Aët. 4.3.8 MR (T 5b Laks = 28 D 12 LM), where the essence of the soul is said to be air (ἐξ ἀέρος τὴν ψυχήν). Now, it is clear that Aëtius also speaks in these passages about the soul tout court, not about a world-soul, and that he introduces the doxa under the name of Diogenes, who is unequivocally provided with his ethnicon (Διογένης ὁ Ἀπολλωνιάτης).Footnote ²⁷

All the evidence just cited refers only to the physiological aspects of Diogenes of Apollonia's psychology, without ascribing to him the idea of a cosmic soul. Conversely, to prove on a philosophical level that Aët. 1.7.8 MR refers to the Stoic, we need a source that, even indirectly, connects Diogenes’ God to psychology, and in particular to a cosmic concept of the soul. This source does exist: Philodemus, in the last section of his treatise On Piety, provides an account of Diogenes of Babylon's theology which is much more detailed than that of the parallel passage in Cicero's On the Nature of Gods.Footnote ²⁸ Below I give a synoptic overview of the two texts with a translation.

Philodemus goes into detail here concerning Diogenes’ allegorical method for approaching the gods, viz. an ‘inductive’ (or ‘rationalistic’) theological method in which the Stoic identifies the gods’ names with physical entities and renders some of the most important members of the Olympic pantheon—Poseidon, Hera, Athena (and indirectly Demeter as well)—mere emanations or parts of Zeus. But who is Zeus for Diogenes? Philodemus maintains that the Stoic, in his work On Athena, said that the cosmos is identified with Zeus, or that the cosmos contains Zeus as a man does his soul.Footnote ³⁵ Such an allegory is in itself a cosmo-theological tenet which posits that the cosmos and man, on the one hand, and deity and soul, on the other, neatly correspond to each other. Hence, in analogical terms, Diogenes’ Zeus is the soul (that is, the life) of God/cosmos that some sources ascribe to the early Stoics.

In fact, immediately after the report on Diogenes of Babylon's theology Philodemus maintains that all the Stoics, generally labelled as ‘Zeno's followers’, held that God is one, and is the same entity as the universe endowed with a soul.Footnote ³⁶ The Epicurean philosopher is here plainly employing the macrocosm/microcosm model that Democritus (68 B 34 DK) introduced and that is ubiquitous in Plato's Timaeus. In addition, according to Arius Didymus, Chrysippus claimed that the cosmos is also called Zeus because it coincides with God and is for us the cause of life.Footnote ³⁷ But even before Chrysippus, Cleanthes in his Hymn to Zeus outlined a ‘cosmic theology’ of this sort when he said that the entire universe obeys Zeus.Footnote ³⁸ In Cicero's On the Nature of Gods, Velleius describes this aspect of Cleanthes’ theology in even more clear-cut terms: Cleanthes—he says—maintained, among other things, that God is the cosmos in that he is both the mind and the soul of all nature.Footnote ³⁹

When Aëtius cites Diogenes and Cleanthes together in Book 1 of his compendium (1.7.8), he must be alluding to a divinity provided with all of these features. These characteristics go beyond a God identified with a natural element according to the model of Presocratic physics: they clearly subscribe to an ‘immanentistic’ concept of deity that aligns with the Stoic ‘pantheistic’ and ‘monotheistic’ tenets.Footnote ⁴⁰ In light of the textual correspondences discussed above, the Diogenes to whom Aëtius ascribes the concept of a God qua world-soul cannot represent a ‘Stoicized’ Diogenes of Apollonia. He must be rather identified as the Stoic Diogenes of Babylon.Footnote ⁴¹