Chrysippus’ Theory of Cosmic Pneuma: Some Remarks in Light of Medical and Biological Doctrines on Respiration, Digestion and Pulse

1 Reconstructing the Doctrine of Cosmic Pneuma

What emerges clearly from the extant texts is that the cosmos is permeated in all of its parts by a divine pneuma that makes it alive, coherent, sentient and rational. To this effect, Diogenes Laertius reports for Chrysippus the doctrine that the cosmos is governed according to intellect and providence (κατὰ νοῦν καὶ πρόνοιαν) and that intellect permeates each part of the cosmos, as does the soul in us.^[9] Alexander, in his de Mixtione, referring to the Stoics reports that they maintained that the cosmos was unified (ἡνῶσθαι) and sustained (συνέχεσθαι) by a pneuma that permeates the whole of it (πνεύματός τινος διὰ παντὸς διήκοντος αὐτοῦ).^[10] However, regarding what this pneuma amounts to and how exactly it works, considerable further exploration is warranted.

Scholars customarily define Stoic pneuma as a mixture of air and fire^[11] or as a warm breath. Indeed, this claim is rooted in the texts—even if this identification can be found in ancient sources less frequently than one might expect. Alexander of Aphrodisias in his de Mixtione repeatedly attributes this idea to the Stoics in general.^[12] Galen confirms this claim by saying that the Stoics call air and fire the pneumatic substance,^[13] and at PHP V 3.8, reporting specifically on Chrysippus’ psychology, he describes the pneuma in the hēgemonikon as possessing ‘two parts, elements or conditions, which are blended with one another through and through, the cold and hot, or if one wished to describe them with different names taken from their substances, air and fire; and it also acquires some moisture from the bodies in which it dwells’.^[14] Additionally, this definition of pneuma as a blend of air and fire was probably prompted by the division of elements into active and passive ones, attributed by some sources to the Stoics and largely accepted by modern scholarship.^[15]

However, a closer look at the sources suggests that the situation might be more complex and requires a broader view to be properly understood. First, the evidence that we have is considerably more nuanced about the composition of pneuma; in addition, as we shall see in the second part of this section, there is a complementary account of the soul as exhalation, which needs to be integrated into this picture. Alexander himself is hesitant to completely identify pneuma as a mixture of air and fire,^[16] and interestingly, he mentions a humid pneuma;^[17] other authors frequently name pneuma and fire or identify pneuma with air alone.^[18] Galen’s wording in PHP V 3.8 may also be symptomatic of a certain attitude from the sources, when he says, ‘or if one wishes to describe them by different names taken from their substance, air and fire’. He mentions a certain moisture acquired by pneuma, as well. One could object that in Stoic physics, fire and air definitely have the qualities of hot and cold, respectively, and that, since the Stoics accept the theory of the four qualities, referring to the quality or to the element was one and the same thing for them. However, I believe that an analysis of the texts shows that this notion needs to be partially rethought.

The sources are consistent in pinpointing the cohesive and sustaining power of pneuma as its main task. Indeed, it is often described as a cohesive force (sunektikē dunamis). Galen casts the relation between the four elements in terms of being sustaining-sustained: discussing the mass of a body, he cites the Stoic distinction between what sustains (συνέχον) and what is sustained (συνεχόμενον). This passage further specifies the role of cold and hot in the economy of the theory of pneuma, since, according to Galen, the Stoics distinguished between a sustaining power (συνεκτικὴ δύναμις), which is the pneumatic substance (ἡ πνευματικὴ οὐσία), and what is sustained, which is the material substance (ἡ ὑλικὴ οὐσία).^[19] Starting from this distinction, he says, they define air and fire as sustaining elements and water and earth as sustained elements.

More specifically, what allows pneuma to carry out its cohesive tasks is the peculiar pneumatic or tonical^[20] movement characterising it. Hensley has rightly pointed out that the traditional way of understanding the movement of pneuma is mistaken: usually, scholars understand tensile movement as a counterfactual movement but this would conflict with Chrysippus’ notion of movement as a change in place. Therefore, by following Sambursky to some extent, Hensley proposes that while a given volume of pneuma is stable, its parts or portions move in opposite directions.^[21] We find some hints of how this movement works in Alexander, who describes this movement as pneuma simultaneously moving from itself and towards itself (πνεῦμα κινούμενον ἅμα ἐξ αὐτοῦ τε καὶ εἰς αὐτό),^[22] and in Nemesius, who mentions a tonical movement that simultaneously moves inwards and outwards (τονικὴν κίνησιν … εἰς τὸ εἴσω ἅμα κινουμένην καὶ εἰς τὸ ἔξω).^[23] It is reasonable to understand the inwards movement, which tends towards the centre of the cosmos, as a contraction, and the movement towards the periphery as an expansion.

At least this is the way Simplicius understands it, as in his comment on the Categories, he mentions a power—or better, a movement—that is loosening (μανωτική) and thickening (πυκνωτική).^[24] If Simplicius understands it correctly, the agents of this movement must be heat and coldness. However, this should not surprise us: after all, Stoic physics is heavily based on the opposition between expansion and contraction, which are provoked by hot and cold, respectively.^[25] Thus, hot and cold, or the fiery and airy components of pneuma, are responsible for the movement of pneuma towards the periphery of the cosmos and the movement towards the centre, respectively.^[26] This is explicitly confirmed by a Galenic passage that attributes the inwards movement to cold and the outwards movement to fire.^[27] Thus, hot and cold play a central role in Chrysippean cosmology due to their ability to make things expand and contract. This opposition, as I shall discuss more at length later on, seems to be absent from Zeno’s and Cleanthes’ theories.

What is peculiar regarding Stoic cosmology is the fact that it is pneuma and not an incorporeal substance that has the creative power to impose form and order on the cosmos. Pneumatic movements are therefore central to Stoic cosmology; by its movements, pneuma endows beings with quality, unity and coherence, and these movements represent god’s providential action in the cosmos. The texts are at variance in defining the effects of hot and cold on bodies. Simplicius, for example, in the passage just mentioned, says that the movement towards the periphery is responsible for the being (αἰτία τοῦ εἶναι) of something, while the movement towards the centre is responsible for the thing being such (αἰτία τοῦ ποιὸν εἶναι). According to Plutarch’s understanding, the Stoics attribute both to air and fire the effect of providing the other two elements with tonos, substantiality (οὐσιῶδες) and permanence (μόνιμον).^[28] However, in another passage, Plutarch discusses at length Chrysippus’ theory that qualities are a form of air or aeriform tensions. After contrasting Chrysippus’ theory that air is properly and in itself cold and murky with the theory that Chrysippus expresses in his On Habitudes that hexeis are air, Plutarch quotes the following passage from that Chrysippean treatise: ‘For it is these that produce the cohesion in bodies; and each of the things that habitude makes cohesive owes its particular quality to the cohabiting air, which in iron is called hardness, in stone solidity, and in silver whiteness’.^[29] The only passage that clearly distinguishes the effects of cold and hot is from Nemesius’ de Natura Homini, where Nemesius reports that the hot, determining a movement towards the periphery, issues extension (μέγεθος) and quality (ποιότης), while the cold, through a movement towards the centre, imposes unity (ἕνωσις) and substantiality (οὐσία).^[30] Nemesius’ testimony, however, is at odds with Simplicius’ report and with the second passage mentioned for Plutarch, according to which the movement towards the centre is related to quality. It might also well be the case that the Stoics themselves neither had a univocal theory on this nor did they straightforwardly distinguish the effects of hot and cold. What can be said for now is that the sustaining task, which sometimes is attributed to the twofold movement (see Alex. Mixt. 224, 24), seems to be linked to the containing and contracting action of air more than to fire, which is rarefied and tends to expand.

Hensley has recently proposed an innovative reading of this testimony by Nemesius, which challenges the traditional interpretation of tensional movement, arguing that it cannot be the case that air determines the contracting and inwards movement of pneuma, while fire determines the outwards movement. More specifically, he proposes that it is fire that causes the blending of air and fire of which it is part to move;^[31] however, it is not exactly clear to me which role he assigns to air in his overall reconstruction of tensile movement.^[32] Contrary to Hensley, I am convinced that fire causes movement outwards, while air causes movement inwards. I try to show that by taking into account the notion of exhalation and the heavens’ effects on earthly water.

Regarding pneumatic movement, cosmic and individual pneuma behave in the same way. The texts that describe pneuma as made of a cold component that determines a movement towards the periphery and of a cold component that determines a movement towards the centre could be taken as descriptions of the behaviour of individual and cosmic pneuma, as well. Indeed, some fragments clearly refer to the pneuma in our body (such as Gal. PHP V 3.8, which refers to the moisture that pneuma acquires from the body in which it dwells), while other fragments report on cosmic pneuma only, and others could refer to both. Moreover, this movement seems to characterise pneuma in the state of hexis, physis and psychē without distinction.

This is, however, only a part—the most well-known one—of this story. Many sources report that the Stoics defined the substance of the soul as an exhalation. This account has been explored and considered regarding the individual soul,^[33] while its cosmic counterpart has been mostly ignored by scholarship. However, if we assume that the individual and the cosmic soul work in the same way and are analogous, it becomes important to understand what this would mean at the cosmic level and to analyse the evidence in this respect. Certainly, this could greatly enhance our understanding of how the cosmic soul works. Notably, this relation between pneuma and exhalation concerns psychic pneuma only, since exhalation is always said to be or to nourish the soul, and this process is to be found in human beings, celestial bodies and the cosmos as a whole. Thus, when speaking of exhalation, we are dealing with the soul properly.

I shall anticipate that the texts that we will take into account will oscillate between identifying the exhalation with the soul itself, on one side, and identifying it with the nourishment of the soul, whose substance is pneuma, on the other. Cherniss commented on this, saying that ‘what are probably more accurate accounts represent it as essentially “pneuma” which is nourished and sustained by the vaporous exhalation of the blood and the air inhaled in respiration … and it is critical interpretation which reduces this “psychic pneuma” either to ἀναθυμίασις αἵματος … or to a mere “blend” of fire and air’.^[34] I completely agree with the fact that directly identifying the exhalation with the soul is misleading; nonetheless, the reason why these two different accounts are conflated and confused might go beyond mere argumentative needs or inaccuracy of the sources. If, on the one hand, it is certainly true that exhalation does not constitute the substance of the soul per se, one should also keep in mind that there is no real discontinuity between exhalation and psychic pneuma, since the latter is a rarefied form of the former. This is the reason why an analysis of the exact relation that holds between pneuma and exhalation and of the extent to which they could be identified should be provided. I shall enquire further into this possibility, since a better understanding of the role of exhalation can help us to clarify some aspects of the pneumatic movements as well.

I will start by analysing the evidence that concerns the individual soul, since I believe that some aspects emerge more clearly. Several sources report that it was common for Zeno and Cleanthes to define the soul as an exhalation from the blood, and indeed, this definition could praise a Heraclitean pedigree. The most important testimonies come from Arius and Galen; the latter, in commenting on Diogenes of Babylonia’s doctrine, accuses him of having departed from his predecessors’ doctrine of the soul. While Diogenes’ argument has the shortcoming of resulting in identification of the substance of the soul with blood, Galen, Cleanthes and Zeno clearly claimed that the substance of the soul was pneuma and that blood was its nourishment.^[35]

This doctrine is described more extensively in a passage from Arius Dydimus, where Cleanthes is reporting on Zeno’s theory of the soul. Zeno, as Heraclitus did before him, claimed that the soul was a sentient exhalation (αἰσθητικὴν ἀναθυμίασιν) from the liquids (ἀπὸ τῶν ὑγρῶν). He also showed how the souls, by exhaling, are always generated and are intelligent, by saying that in the same rivers, some water flows away and some more steps in.^[36] Therefore, the role of exhalation in the constitution of the soul is already well established by Zeno and Cleanthes.^[37] Plutarch also refers to this doctrine when criticising Stoic epistemology; he starts from the presupposition that the Stoics consider the soul as an exhalation from liquids.^[38] This view is enriched by three Galenic passages.^[39] I shall discuss these passages more at length below; for now, it is important to notice that they seem to describe the soul as nourished both by exhalation from the blood and by inhaled air. In particular, Plutarch mentions its nourishment from liquids (τροφὴ … ἐξ ὑγρῶν) and the fact that this exhalation is renewed through breathed air.

While the relation between the individual soul and exhalation has been explored by scholars, especially, as we will see in the next section, in relation to Praxagoras, the situation concerning the cosmic soul is slightly different. To begin with, while the association between the individual soul and exhalation is made explicit in several texts, there is only one extant text that explicitly defines the cosmic soul as an exhalation: according to Arius Dydimus, the Stoics say that there is a soul of the whole, and they call it ether or air; it surrounds the earth and sea and from these it exhales. The other souls, which we find in animals and in the surroundings, grow from it.^[40] In this instance, Arius Didymus is reporting mainly on Zeno and Cleanthes. This, as mentioned before, is the only evidence where the identification between cosmic soul and exhalation is made explicit—and this might well be the reason why this connection has never been explored by scholarship. However, there are a number of testimonies that suggest that on the cosmic level, one can envisage the same process as the one just pictured for the individual soul. Indeed, given the relationship between the individual and cosmic soul, it would come as no surprise that they do work in the same manner.

Cleanthes explicitly stated that the sun was the hēgemonikon of the cosmos and that it beats the cosmos with its rays, thereby producing a harmonious exhalation by which it feeds itself.^[41] However, we do not have testimonies of this sort for Chrysippus; we only know that he identified the hēgemonikon of the cosmos with ether, i.e., the fire, in its most rarefied form, located in the heaven at the periphery of the cosmos. Several sources refer to a Stoic theory according to which the souls of the celestial bodies are made of exhalation and are nourished from the exhaling sea.^[42] Indeed, if Chrysippus located the hēgemonikon of the cosmos in the ether, the souls of celestial bodies are good candidates as the seat of the hēgemonikon. If this is the case, the cosmic psychē, which is the fieriest and finest part of the cosmic pneuma, should be considered an exhalation or nourished by exhalation, such as the human psychē.

In particular, Plutarch explains how the soul is generated according to Stoics and reports their view that the sun becomes animate when the liquid changes into intelligent fire.^[43] The sense of the sentence can be better grasped if viewed against the background of the process of exhalation of the souls of living beings from liquid, which we have just discussed. This passage by Plutarch adds further elements to our picture; in other words, the transformation of water into intellectual fire is a change of state (metabolē is often used by the Stoics to indicate elemental change, cf. SVF II, 413). In this context, two changes are to be assumed, i.e., from water into exhalation—which, we suppose, should be understood as a sort of airy state—and from exhalation into fire. The intermediate state of exhalation should be conceived of as a heterogeneous phase that will be moister and thicker nearer the water (in this sense, it becomes clearer why pneuma can be humid, as well) but will become more rarefied and fierier as soon as it approaches the heavens such that the change into fire would be gradual. The progressive transformation of the exhalation from thicker to finer is provoked by the heat located in the heavens, similar to what happened in Cleanthes’ system. Additionally, we know that the sun and the stars obtain their own nourishment by heating and drying water. Water being heated expands and rarefies. Thus, the transformation of water into fire occurs through the state of exhalation such that the same substance progressively^[44] passes from a watery state to a fiery state due to the rarefying power of the heat located in the heavens that assimilates the substance to itself.^[45] It is important to keep in mind—and in this Plutarchean passage, this aspect is made particularly explicit—that it is fire, and more precisely intelligent fire, and not exhalation, that constitutes the final stage and the outcome of this process. Moreover, these passages clarify that this exhalation is able, thanks to the transformation it undergoes by fire, to endow certain apt bodies with life and reason. Thus, the souls of celestial bodies and of human beings, far from being incorporeal and unchangeable, are nourished by evaporating water. Yet they are divine, alive and rational because the whole process is governed by the intelligent and creative fire that takes care of his own nourishment.

This general frame is echoed in another text that seems to preserve a Stoic account of exhalation. A scholium to Theogony explains that when air becomes less moist, it expands into a greater mass and evaporates such that it becomes more mobile and is able to include the sun (ἐξ ἐλάσσονος ὑγροῦ εἰς πλείονα ὄγκον διαχεόμενον καὶ ἀτμιζόμενον. τετάρακται δὲ μέχρις ἂν λάβῃ τὸν ἥλιον· τότε γὰρ τῷ ἡλίῳ συμπαρεκτείνεται …).^[46] In general, the movements of the sun are explained by its need to nourish itself.^[47] Several tenets explicitly attribute to Chrysippus the theory that the sun is an intelligent mass (ἄναμμα or ἔξαμμα νοερόν) formed by exhalation from the sea^[48]—while the moon is formed by exhalation from streams.^[49] This formulation mirrors the definition of the soul of human beings as a sentient exhalation from liquid.

We know that god, being pneuma, is diffused everywhere in the cosmos, and he thereby fashions, gives shape to and organises matter.^[50] The great innovation introduced by the Stoics is to explain this creative power that providentially structures and organises reality in terms of exhalation and rarefaction—introducing the rather unusual idea that the logos could exhale from blood^[51] or from the sea. Let us attempt to sketch a provisional account of how cosmic pneuma works in Chrysippus’ system; the hēgemonikon of the cosmos, located in the heavens and made of ether, heats and dries the sea, thereby provoking exhalation, to receive its own nourishment. This exhalation consists of warm air or warm breath—which is one of the definitions of pneuma—and is produced by rarefaction and expansion of heated water, such as steam rising from boiling water. If, as they say, the exhalation nourishes the celestial bodies and constitutes their souls, this means that the exhalation, by being rarefied, moves upwards from the sea to the heavens to nourish the celestial ether. It is exactly this exhalation that constitutes, in my view, the best candidate to explain the pneumatic movement towards the periphery. It is more in line with the Stoic approach to elemental change to interpret this upwards movement as an expansion, as Simplicius does. Therefore, it is not fire itself that moves, but fire—or better, heat— causes the substance to move towards the periphery by heating it and making it expand. This movement is, of course, teleologically determined, in that its purpose is to nourish the psychic pneuma of the cosmos.

After this first and provisional attempt to provide an account of the Stoic cosmic soul and to state the terms of the issue, a number of questions arise: (1) what role does cold play in the economy of cosmic pneuma and in determining the pneumatic movement? I have mentioned the fact that the individual soul has two sources of nourishment, i.e., exhalation and breathed air; thus, it is important to understand what role air plays at the cosmic level and why it does so. (2) What is the relation between psychic pneuma and exhalation, and what type of transformation does exhalation need to undergo to develop psychic pneuma, i.e., soul? Finally, (3) how should we conceive of the whole picture? In particular, the relation between the hēgemonikon and the rest of the cosmos, as well as the causes and mechanisms of the pneumatic movements, remain unclear. It seems better to me to leave aside Chrysippus’ theory of cosmic pneuma and discuss two ancient models of respiration, digestion and pulse, which could, I think, clarify certain aspects of the processes we are discussing.

2 Two Models of Respiration and Pulse: Praxagoras and Aristotle

The questions raised may be answered if we compare Chrysippus’ doctrine to contemporary theories on respiration, pulse and digestion put forward within the frame of the medical and biological inquiries of the time. I have selected two ‘models’, i.e., two ways these processes could be accounted for, namely, (a) Praxagoras’ model on one side and (b) Diocles’ and Aristotle’s model on the other, since there is good reason to believe that Chrysippus was familiar to them, even if I do not exclude that other theories, e.g., Hippocratic, might turn out to be relevant to the present context.^[52]

1. Praxagoras ^[53]

The first model I analyse is the one put forward by Praxagoras of Cos, who was probably active during the late fourth and early third centuries and whose influence on Stoicism is confirmed by various sources and recognised by scholarship.^[54] In particular, what Chrysippus appreciated most about Praxagoras’ system was the fact that he assigned a central role in cognitive activity to the heart,^[55] which is also the reason why Chrysippus appealed to his authority against Herophilus and Erasistratus, who assigned this role to the brain.^[56]

According to Galen, Praxagoras conceived of pneuma as a dense airy substance (he described it as somewhat dense and vaporous),^[57] and he thought that it was nourished by breathing.^[58] Praxagoras seems to be perfectly aware of the fact that external air did not constitute pneuma as such and that it needed to undergo a qualitative alteration. Information on this alteration is rather scant; however, it seems that to become pneuma, breathed air needs to be heated^[59] and moistened and that this occurs somewhere between the lungs and the heart. Given the central role that Praxagoras assigned to the heart, it is likely that he thought that the most important stage of this alteration took place in this organ, where air was not only moistened and heated but also charged with certain information.^[60]

I will briefly mention Lewis’ discussion of why it is reasonable to suppose that the alteration of air into pneuma occurs due to the encounter of air with liquid and moist substances. This discussion is particularly relevant to assessing the role played by Praxagoras’ system in the development of Chrysippus’ theory. There are three possibilities as to how the alteration of air into pneuma occurs: (1) air encounters liquid and moist substances (such as humours and flesh), (2) it is enclosed in extremely narrow passages, or (3) it encounters and mixes with some airy substance inside the body.

In recent years, scholars have been inclined to think that Praxagoras embraced the third option, in the belief that according to Praxagoras, pneuma was constituted by external air mixed with exhalation from the blood and that this exhalation constituted a sort of inborn source for pneuma responsible for the moistening and heating of external air. I have mentioned the fact that there are a couple of Galenic texts that attribute the idea that pneuma is nourished both by inhaled air and by exhalation to the Stoics; it had thus become commonplace among scholars to say that it was Praxagoras who passed this doctrine to the Stoics.^[61] However, Lewis shows in detail how the attribution of this doctrine of a double source for pneuma to Praxagoras finds no validation at all in the extant fragments and how attributing to him this theory is a misinterpretation. The attribution to Praxagoras of the belief in a preexisting inborn pneuma derived from exhalation (which would mix with breathed air, on the standard reconstruction) has to be dismissed due to a lack of evidence.^[62] After all, the claim that Praxagoras held this doctrine was entirely dependent upon the deeply rooted belief that the Stoics were strongly influenced by him rather than being based on textual evidence.

Praxagoras’ explanation of our inner system can, thus, be summarised roughly as follows: the external air we breathe reaches the lungs and the heart and somewhere along the way it becomes pneuma. This process entails a qualitative alteration based on moistening and heating. The central stage of this process is likely to take place in the heart, where the air is also charged with important information. From the heart, pneuma is then distributed throughout the body through a dedicated system of vessels. The arteries, by expanding, draw pneuma from the heart, and with their innate capacity to pulsate, send it to the limbs. In this framework, pneuma is only the transmitter of thought and movement (and perhaps sensation). Indeed, pneuma plays a central role in the execution of cognitive faculties, but it has more of an instrumental role, while the decisional and cognitive stages take place entirely in the heart.^[63] The heart, as mentioned, is the only, or at least the main, source of the pneuma in the arteries. Pneuma was, thus, responsible for the transmission of motion to the whole body, although it is not clear whether it acted directly or was just the carrier of the information. Regarding innate heat, we have no evidence that Praxagoras used this concept; we only know that he thought the body was hot. Additionally, the idea that bodily heat needs cooling is completely absent. Most likely, Praxagoras also had a theory on digestion, but the texts on this are almost completely lost.

1. Aristotle and Diocles

The second model that I shall outline takes into account both Diocles and Aristotle. I am pairing them together in the present context for no other reason than that they seem to share almost the same opinion on the processes that are relevant to the present inquiry. In particular, they both conceive of breathing as a means to cool innate heat. I am not, however, taking any particular position regarding the relationship between Aristotle and Diocles. While the fact that there was a certain exchange between the two is widely accepted, the direction and weight of it are greatly debated among scholars.^[64] When discussing this model in the final section, I mention mostly Aristotle, but this is only because, as we will see, truly little can be reconstructed of Diocles’ doctrine in this respect, while we possess a long text where Aristotle reports his own explanation of his views on respiration. I found it important to also mention Diocles in this instance, however, because he might well have been the channel through which the doctrine of breathing as cooling could have reached Chrysippus,^[65] especially if one is not inclined to accept Aristotle’s influence on the Stoics.^[66]

The evidence concerning Diocles is definitely scant, especially on the topics we are dealing with, and it is difficult to obtain a thorough picture of his system.^[67] From the fragments available, the extensive use Diocles made of the concept of innate heat clearly emerges, albeit perhaps not by calling it ‘innate’. Diocles located the commanding faculty in the heart, which he considered the source of reasoning and locomotion.^[68] What is important in this context is that he considered psychic pneuma to be a different type of breath than inhaled air. This latter serves rather to cool the innate heat^[69] and is completely expelled through expiration.^[70] The psychic pneuma originates in the heart and seems to be responsible for the distribution of locomotion to the whole body.^[71] This is, in outline, what we can draw from the fragments on his doctrine. Most probably, he must also have had doctrines on digestion and perhaps on pulsation, but nothing definitive has reached us.

The system developed by Aristotle is very similar to that of Diocles, especially regarding the aspects considered in this paper. However, importantly, unlike Diocles, we have Aristotle’s own explanation of his views on respiration. Similar to Diocles, he held a cardiocentric view and believed that respiration was for the sake of cooling the innate heat. Without breathing, the organism would have died because of the excess heat.^[72] Let us examine in more detail what this means. Aristotle locates the principle of innate heat in the heart since the soul is somewhat inflamed there. It is, thus, important that this heat never cools too much, since it always goes together with life, and its extinction amounts to death.^[73] According to Aristotle, fire is destroyed in two ways, i.e., either by exhaustion or by extinction; in both cases, the cause of its destruction is the lack of nourishment. In the case of extinction, this depends on the action of the opposite (cold), which blocks the concoction of nourishment. In the case of exhaustion, fire itself grows too large and exhausts the whole nourishment. In this latter case, to preserve the natural heat that is contained in the principle and is necessary for life, there should be a continuous process of refrigeration that prevents heat from growing excessively and consuming all its nourishment.^[74] As an example of this phenomenon, Aristotle mentions the charcoal fire that needs continuous draughts to be kept alive.^[75]

In this light, the process of respiration and its aim appear clearer: without refrigeration, heat would be dissolved, and the principle of life would be destroyed.^[76] A passage from On youth and old age is particularly relevant to the present context. There, Aristotle explains that when nourishment enters our body, it is transformed and reaches the heart. The natural heat located in the heart pneumatises nourishment and turns it first into vapour and, subsequently, into blood. The liquid nourishment, by being vaporised, expands thereby provoking the expansion of the heart. The heart, in turn, causes the surrounding parts (lungs and the whole thorax) to expand. Aristotle compares the functioning of the heart and lungs to that of bellows: by expanding, they let air flow in from outside. The heart’s expansion causes inhaled air to flow in. Inhaled air, being cold, quenches the excess heat and makes the heart and lungs contract (the contraction seems primarily to happen because of the extinction of heat). Air—this time being hot owing to its contact with the internal heat—is then expelled by the contraction of these organs.^[77] This explains why, according to Aristotle, the two processes of respiration and pulse are tightly intertwined. Pulse is defined as the expansion of the heart and lungs provoked by the vaporisation of the liquid.^[78] By expanding, the heart also drags the vessels, causing them to pulsate.^[79] The Aristotelian passage is as follows:

Breathing takes place when there is an increase of the hot stuff which contains the nutritive principle. For just as the other parts need nourishment, this does too, even more so than the others, since it is the cause of nourishment for the other parts. But when it increases, it must make the organ expand. This organ should be thought of as constructed like the bellows in a bronze-forge; for the shape of the heart and lung is not far from this. Such a shape is double, since the nutritive faculty must be in the middle of the natural † capacity. Now, as the stuff increases, it expands, and as it expands, the surrounding part must also expand. This is what breathers are seen to do: they expand their chest because the principle of this organ is present within the chest and produces the same result. For as it expands, air, which is cold, must enter in from the outside, just as in bellows; and (480b) because it has a cooling effect, it quenches the excess of the fire. But just as this part was expanding as the stuff was increasing, when the stuff decreases it must contract, and when it contracts the air that had entered must go out again. When the air enters, it is cold, but when it goes out, it is hot because of its contact with the heat present in this part, and this holds especially for animals that possess a blood-filled lung.^[80] (Transl. Miller 2018).

In the treatise On youth and old age, Aristotle explicitly states several times that it is not for the sake of nourishing the innate heat that we breathe, meaning that inhaled air is not conceived by Aristotle as fuel for the fire. Rather, Aristotle argues against this position, claiming, as previously mentioned, that the aim of inhaled air is to preserve heat by cooling it.^[81] What I would like the reader to note in the quoted passage is that the effect of cold air is exclusively linked to cooling and contraction and that the relationship between heat and cold is primarily characterised in terms of expansion and contraction. Inhaled air seems to be completely expelled by expiration, and its main—and apparently only—task is to preserve vital heat, both by preventing it from dissolving and scattering and by stopping it from growing so large that it consumes all the fuel. Additionally, air returns the heart to its previous and normal condition (since expansion is defined as an altered state). What is particularly important in Aristotle’s treatment of respiration for the present instance is that respiration is strictly related to nutrition. Inborn heat, which is necessary for nutrition (more specifically, for its concoction) must be preserved,^[82] preventing it from growing too large and becoming exhausted. It is interesting to note that Aristotle provides a teleological explanation of these processes: the complex mechanism of respiration serves the sake of preserving our vital heat and balancing the heat produced by digestion.

Even if Aristotle’s treatment of respiration and of cold inhaled air is rather clear, his treatment of pneuma and inborn pneuma is, regrettably, rather ambiguous in the texts in our possession. Thus, it is difficult to define a univocal meaning for this term in his system.^[83] In our passage, however, there is an interesting term that is a unicum in the Aristotelian corpus, namely, ‘pneumatisation’ (πνευμάτωσις),^[84] which is used to describe the evaporation from the liquid nourishment caused by heat. Additionally, in the passage where Aristotle compares pulse and boiling, the verb that describes the process of heat turning water into vapour is πνευματόω. I think that this might be at the root of an idea that sometimes can be found in Aristotelian scholarship, i.e., that pneuma is produced by the vaporisation of the blood.^[85] This might be extremely interesting for the present context, especially if one is ready to grant the Aristotelian pneuma psychic functions. In this case, exhalation would possess a sort of psychic role in the Aristotelian system—pneuma being the only corporeal substance able to accomplish certain psychic tasks. Nevertheless, the information on pneuma in Aristotle’s surviving texts is too problematic to build something on it.

3 A ‘Biological’ Account of the Cosmic Soul

We might now take a fresh look at Chrysippus’ theory of cosmic pneuma and see whether, by reading the tentative reconstruction made in §1 in light of Aristotle’s mechanism of respiration and digestion, we gain a better understanding of how Chrysippus’ cosmic psychology could work. Previous studies highlighted the influence that Praxagoras’ theories on pneuma and on the seat of mental activity had on Chrysippus’ psychology, especially in his quarrel with Herophilus and Erasistratus.^[86] The fact that Chrysippus argued for the corporeality of the soul led him to engage with the main findings in the medical field—even if one should keep in mind that physicians were mostly not interested in discussing the soul or its substance; at most, they identified its functional equivalent with certain substances, such as pneuma or heat.

It has been argued that at Chrysippus’ time, pneuma was being progressively established as the main substance in charge of perceptive and motive functions at the expense of heat and that this influenced Chrysippus’ choice for the substance of the soul.^[87] In general, it is safe to assume that Chrysippus had quite good knowledge of the main theories of the time on themes such as pneuma, pulse, and respiration. What he might have found most interesting in such theories as Praxagoras’ and Aristotle’s (or the Peripatetics’) is the attempt to provide a teleological explanation for such phenomena. Additionally, Chrysippus seems to be interested in the opposition between hot and cold to account for processes and in the notions of contraction and expansion. Needless to say, Chrysippus’ interest in the functioning of living beings is always philosophical—and often ethical—as well as scientific.

In this section, I will suggest that Chrysippus applied to his cosmology some features of the complex mechanism that we find in Aristotle’s doctrine on breathing—the mechanism Aristotle developed to provide an integrated explanation of digestion, pulse and respiration. As we have seen in the case of Chrysippus’ psychology, the process of nutrition determines an exhalation, which in turn provokes an expansion. The role of air in this process remains to be determined. I shall argue that, besides Praxagoras’ influence on Chrysippus concerning the role of air in nourishing the substance of pneuma (discussed below), one crucial idea of Chrysippus’ theory about the cosmic soul is that air has a primarily contracting role. Being cold, air has the task of quenching and contracting heat to keep it alive and preventing it from being scattered and consuming all its fuel.

There are various reasons for which it is economical to assume that Aristotle’s theory (or a version of it) influenced Chrysippus. Aristotle provides a complex mechanism where breathing is given a teleological role, and digestion, pulse and respiration are integrated into one single teleological explanation. Similarly, Chrysippus’ theory of pneuma seems to require an integrated account of nutrition (exhalation), pulse (expansion and contraction, pneumatic movements) and breathing. Moreover, Aristotle’s theory is able to address a problem that consistently troubled the Stoics, i.e., that cosmic heat is dispersed and scattered. In addition, Chrysippus did not seem to be particularly keen on developing new physiological theories;^[88] rather, he was a great systematiser. Therefore, it is more likely that he built on a physiological theory he had already found at his disposal and used it to create a strong cosmological system—and, as said, Praxagoras’ system is not enough to explain the process we saw at work in the case of the Stoics. I shall discuss these points more at length in what follows.

Certainly, Chrysippus’ reception of these concepts was not merely passive; he seems to have consciously appropriated both the models we have analysed, taking what, from a philosophical point of view, was more convenient to his system. In this section, I briefly recall what we can say of Chrysippus’ theory of the cosmic soul thus far, highlighting his concerns. I then discuss the role of air, analysing the evidence we have for its cooling action (and how it carries out this role) on the one hand, and its nourishing function on the other. Finally, I highlight the philosophical advantages of the innovative solution proposed by Chrysippus with respect both to the two medical models of §2 and to his predecessors, Zeno and (possibly) Cleanthes.

First, let us return to Chrysippus’ cosmology and attempt to define more precisely certain aspects that were left open in the first section.^[89] Presumably, the higher the exhalation gets, once evaporated from the sea, the more rarefied and expanded it becomes because of the heat in the hēgemonikon. It is not clear whether exhalation can already be considered pneuma or whether it needs to undergo a qualitative transformation on its way to the hēgemonikon—as was the case for air in Praxagoras’ system. That there should be a transformation is suggested by a passage on the formation of rain, which states that rain occurs when the exhalation has not yet gone through the sun’s katergasia.^[90] This transformation in the case of exhalation might well be its vaporisation or pneumatisation, such that it reaches the state of pure heat or ether and becomes ensouled fire.^[91] Indeed, we know that this exhalation is conceived as nourishment for the hēgemonikon; admittedly, the proper nourishment is the sea, but to be assimilated, it should be vaporised.^[92] It remains unclear whether exhalation nourishes the heaven as fuel or by becoming an actual part of the sun and the stars. I would suggest that by using it as fuel and nourishment, the sun and stars transform this exhalation into matter that they can incorporate, effecting an actual digestive process. This would account for the frequent use of terms such as ‘nourishment’ or ‘consume’ and ‘exhaust’^[93] and for Chrysippus’ definition of the sun as an intelligent mass from the exhalation of the sea.^[94] Once the heavens have been nourished by this exhalation, it is sent back to the centre of the cosmos in a continuous exchange.

This process, however, entails the following two risks: on the one hand, the progressive rarefaction and expansion of the substance can lead the fire to be dispersed in the void that surrounds the cosmos; on the other hand, the fire can become so large that it completely exhausts its nourishment. That these were two main concerns for the Stoics is evident; Chrysippus continues to engage in his predecessors’ attempt to account for the cohesion of the cosmos^[95] by introducing new theories, probably to answer some criticism on why the cosmos does not dissolve in the void. The worry that the heavens might burn the whole cosmos as fuel is present, as well; during the whole diakosmēsis the cosmos must be preserved, and it is only at a precise moment, when the conflagration should occur, that the hēgemonikon consumes all the fuel^[96]—god is said to consume in himself the whole substance (ἀναλίσκων εἰς ἑαυτὸν τὴν ἅπασαν οὐσίαν)^[97]—and reaches its maximum degree of rarefaction and expansion.^[98] Interestingly, these are exactly the concerns (even if applied to the individual organism) that Aristotle has when assigning that specific role to cold air in breathing, i.e., quenching fire to keep it alive, preventing it from consuming (καταναλίσκειν) all its fuel, from being scattered and from dissolving.^[99]

What exactly, then, is the role of cold with regard to pneuma and, in particular, to psychic pneuma? Scholarship had long agreed in attributing to the Stoics the theory that inhaled air is, together with exhalation, the source of nourishment for the (individual) soul. Indeed, as mentioned in §1, some authoritative sources, such as Plutarch and Galen, seem to confirm this claim. However, pneumatic movements, which seem to happen both at the cosmic and at the individual level, suggest that air has a contracting and thickening property that serves to contrast the action of fire and preserve heat. Thus, does cold air nourish or does it cool pneuma? In what follows, I shall argue that the evidence we have suggests that the main task of air in this context is to cool and contract—and this constituted an innovation introduced by Chrysippus that can be better understood against the background of the Aristotelian theory of respiration. However, a nourishing function of air is to be assumed, as well, such that this element appears to have a twofold role. If this is the case, Chrysippus’ theory appropriates and combines Praxagoras’ and Aristotle’s models, retaining the advantages of both.

I shall start analysing the texts that provide evidence for the cooling and contracting role of air. Several texts that provide such validation are those mentioned in §1 regarding pneuma and pneumatic movements. A passage from Galen’s De tremore is particularly illuminating in this respect. This passage has been included by von Arnim among the testimonies on Chrysippus’ theory of pneuma (II, 446), although Galen does not explicitly mention the Stoics—the only authority mentioned in these lines, besides Hippocrates, is Heraclitus.^[100] However, this passage seems to be attributed to a Stoic background without hesitation by scholars,^[101] and indeed, if we accept it as evidence of the Stoic doctrine, our understanding of how pneuma works is considerably enhanced.^[102] Although this passage is somewhat long, it is worth quoting in full, as follows:

Since the innate heat is in constant motion, it moves not merely either inward or outward; rather, one motion continually succeeds the other. For the inward motion alone would quickly end in immobility, the outward one would disperse and thus destroy the heat. Being now quenched and now kindled as Heraclitus said, it thus remains constantly in motion. Hence it is kindled when it converges downwards, reaching for sustenance; it is quenched when it is raised up and dispersed in all directions. But it has acquired motion upward and outward and, as one might say, an unfolding from its own starting place because it is hot by nature; it has also acquired motion inward and downward, that is, towards its own origin, because it contains a share of cold, having come to be a mixture of hot and cold. By reason of its heat, it is self-moved, and very much needs this property in order to act. Yet cold too is very useful for it. For heat is accustomed to rise up and bring along its nourishment; if cold did not prevent this, it would [618] proceed to the greatest heights. But cold does prevent heat from moving in this way and thereby keeps it from dispersing and perishing. For there is danger that heat will leave the bodies because of its natural lightness and its tendency to move upwards. But the cold checks, hinders, and reduces the violence of this excessive motion.^[103] (transl. Sider and McVaugh 1979)

In this section of the treatise, Galen is concerned with the definition of rigor as an affection of natural heat and as distinct from tremor. In the previous passage, he explained how the outwards-inwards movement could be considered equivalent to expansion and contraction and, in this respect, it differs from the upwards-downwards movement, which is a local movement. Galen explains that for natural heat to be preserved, a double movement is needed, namely, an outwards and an inwards movement. This is because outwards movement alone, which is characteristic of heat, would condemn natural heat to be scattered and dispersed such that it would be quenched or become sufficiently light to leave the bodies in which it dwells. Natural heat will thus be preserved due to a balance between hot and cold and the movements that they determine; neither of them should prevail, or else heat would be quenched due either to an excess of movement or to immobility. The movements here are related not only to the natural tendencies of hot and cold but also to the supply of nourishment for the heat. Its nourishment is likely to be identified with something moist: nourishment and the cold have a different role in the economy of the explanation since when the heat takes the nourishment upwards, the cold obstructs it. Cold air does not have the function of fuel here, its task being rather to kindle heat. Heat moves downwards to obtain nourishment and then upwards, bringing nourishment with it. This upwards movement is hindered by the cold, which prevents the heat from being quenched. The concerns we find in the passage, as well as the teleological solution adopted by nature to overcome these risks, which have been considered Stoic, are explained in much the same terms as in Aristotle’s passages on breathing and on charcoal fire.

Both the Galenic and the Aristotelian quoted passages include the idea that cold keeps fire alive and keeps it from being quenched by partially quenching it, i.e., by tempering its excess. This is a central mechanism in another Stoic doctrine, namely, their theory on embryology. I shall quote some Stoic text on this topic, which allow us to get a better understanding of the contracting force of cold air. Several sources agree that according to Stoics, as long as the embryo is contained in the womb, it is to be considered a natural pneuma, i.e., as physis. It is only at the moment of birth that the physis is cooled and hardened (the verb usually used is στομόω) by external air. This process enables the transformation of natural pneuma into psychic pneuma, that is, of physis into psychē.^[104] This theory surely bewilders our sources, who understandably ask themselves how it is possible that psychē, the warmest nature according to the Stoics, can ever originate from a process of cooling. It is a question that has troubled scholarship as well.

Plutarch discusses this problem at length, and his texts are the main and the best source we have to aid in understanding this problem. He criticises Chrysippus’ affirmation that the soul originates from the cooling and hardening effect of air (ψυχόμενον ὑπὸ τοῦ ἀέρος καὶ στομούμενον τὸ πνεῦμα μεταβάλλειν καὶ γίνεσθαι ζῷον) and that it is called soul precisely from cooling (ὅθεν οὐκ ἀπὸ τρόπου τὴν ψυχὴν ὠνομάσθαι παρὰ τὴν ψῦξιν). In contrast, however, he also states that according to Chrysippus psychē is a more rarefied and subtler type of pneuma than physis (αὐτὸς δὲ πάλιν τὴν ψυχὴν ἀραιότερον πνεῦμα τῆς φύσεως καὶ λεπτομερέστερον ἡγεῖται μαχόμενος αὑτῷ). Plutarch then goes on to ask himself how it can ever be possible that something rarefied and subtle comes from a dense substance through a process of cooling and condensation (πῶς γὰρ οἷόν τε λεπτομερὲς ἐκ παχυμεροῦς καὶ ἀραιὸν γενέσθαι κατὰ περίψυξιν καὶ πύκνωσιν;). The main absurdity is certainly that Chrysippus asserts that animation is the result of both chilling and exhalation (that is, a process of heating).^[105] Notably, throughout the entire passage, Plutarch clearly opposes heating and rarefaction to cooling and contraction, treating them as reverse processes,^[106] such that it clearly emerges that Chrysippus uses the terms of the two couples as synonyms—and this confirms what we previously observed.

We can infer from Plutarch’s account of the problem that, according to Chrysippus, for the soul to be engendered—and, now we can say, also nourished—both a process of heating/rarefaction and one of cooling/contraction are needed. The way to solve the contradiction, I believe, is to think that these processes do not have the same aim and that, therefore, there is no contradiction at all in Chrysippus’ claim. As Plutarch rightly says, the soul is the finest and most rarefied—and therefore also the warmest—substance. It would thus be quite unreasonable of Chrysippus to claim that it is engendered by cooling a substance that is already cooler than fire (as physis is). It is not necessary to tackle this complicated question here or to suggest a possible interpretation. However, I think it is clear that the main task of air in this process is not primarily to cool but rather to contract and give cohesion to a substance that, otherwise, owing to its excess heat, would be scattered. In other words, it is not the temperature that is at stake but coherence; the substance will maintain the same degree of rarefaction, although averting the risk of being too dispersed. A testimony from Tertullian helps gain an understanding of this process. According to the Stoics, when nature comes out of the womb, as from a furnace, it is smoking and scattered by heat (fumantem et calore solutam); it is then stricken by cold and, consequently, assumes the power of the soul, as happens in the case of a white-hot iron that is plunged into cold water.^[107] Therefore, it is reasonable to believe that cold air stops the dispersion and heating. It does not begin to actively cool the substance and make it colder than it was; it just stops it from heating further.

If the reconstruction of this process is correct, this sheds light on some notions of Stoic embryology. A slightly later Stoic text on the first inhalation of the newborn reports the same mechanism as the one that we find in Aristotle’s passage. This passage is a fragment from the ps. Plutarchean Placita that the scholarship has always attributed to Diogenes of Apollonia. Tieleman, however, in an article from 1991, convincingly demonstrated that the author must be identified with the Stoic Diogenes of Babylonia.^[108] The importance of this text for the present context is that it provides a testimony of breathing and cooling being linked together and offers a possible explanation of how exactly an embryo becomes ensouled. It is particularly interesting since it clarifies that the cooling agent is not external air per se but external air once inhaled by the first act of breathing.^[109] The text is as follows: ‘Diogenes [believes] that the embryos are generated without soul but in heat; that is why the innate heat is drawn into the lungs as soon as the child is poured forth’.^[110] Diels rightly inserted ‘the cold’ as the direct object of ἐφέλκεσθαι on the basis of Ps.-Galen,^[111] so that the meaning becomes: ‘Diogenes [believes] that the embryos are generated without soul but in heat; that is why the innate heat draws the cold into the lungs as soon as the child is poured forth’.^[112] I believe that here the function of breathing (drawing the cold into the lungs) and of the cold is not primarily to nourish the soul^[113] but rather to cool, harden and contract it, as was clearly explained in the other Stoic fragments on embryology. Therefore, there is no need to think that the surrounding air cools and the inhaled air nourishes the soul as soon as it is inhaled air that accomplishes both tasks. Furthermore, this is made explicit in the Arabic translation of this passage, whose ending is as follows: ’Daher wird, wenn sich die angeborene Wärme ausbreitet, bis sie zur Lunge gelangt, sofort die Luft herbeigezogen’.^[114] This might explain the Greek ἐφέλκεσθαι as something analogous to the bellows-mechanism described by Aristotle.

Thus, it emerges clearly, and the fragments on embryology confirm that in relation to the soul, the main task of air (at birth and throughout life) is to cool and that this task is teleologically determined—having the purpose of preserving heat. In light of the texts analysed, we can say more precisely that the cooling effect of the first breath of the newborn does not aim to make him less warm, but it rather constitutes the start of those pneumatic movements that will characterise his soul throughout his whole life.

However, there is a well-established tradition in Stoic scholarship, according to which inhaled air nourishes pneuma. This belief is corroborated by the alleged influence exerted by Praxagoras on the Stoics. This theory is mainly attested in three Galenic passages (two of which are in the same work), which are usually quoted when discussing this issue and have been understood as referring to the Stoics and as attributing to them both of these sources for pneuma’s nourishment.^[115] However, only in the passage from the de Usu Respirationis (nr. 3 in the note), it is clearly stated that air does nourish the soul, but it is not clear if this part of the sentence can be attributed to the Stoics, as well. In the other two passages, inhaled air is said to be necessary for the maintenance of the soul (διασῴζεσθαι in the first text and πρὸς διαμονήν in the second), and in the second passage, it is even opposed to nourishment (here, intended as the result of exhalation). In these two passages, the maintaining task of air could also well be connected to its refrigerating property too. Another passage that explicitly attests that inhaled air nourishes the soul comes from Plutarch; when questioning the Stoic theory that conceptions come from mental images that are nothing but impressions on the soul, he expounds their theory that the soul’s nourishment and generation are from the liquids and that this vaporous exhalation is forever renewed by inhaled air.^[116]

Additionally, further evidence that inhaled air nourishes the soul could be drawn from Zeno’s and Chrysippus’ argument that demonstrates the corporeality of the soul. The argument, as reported by Calcidius, runs as follows: Zeno said that living beings die when the natural pneuma or breath (spiritus naturalis) leaves their body; thus, it is surely the soul. Chrysippus used the same argument to demonstrate that the soul is pneuma by saying that due to the same cause, we both live and breathe, and this is natural pneuma or breath.^[117] Therefore, the texts attest that air also nourishes the substance of the soul. All the texts reporting on this theory refer, as we have observed, to the soul of living beings. We should now understand what it could mean at the cosmic level that air nourishes the soul.

The testimonies in our possession allow us to conclude that in Chrysippus’ psychology, air has both the role of nourishing and cooling pneuma.^[118] The strong evidence we have for the cooling effect of air, however, enables us to grant this task a heavier weight in the economy of the theory of the soul.^[119] Indeed, this effect is the one stressed at the birth of the soul and can provide a reasonable account for the pneumatic movement. If we can accept the testimonies on both the cooling and the nourishing task of air as trustworthy, this means that Chrysippus has appropriated and deeply innovated the two medical models that we have analysed,^[120] and he has also applied them to the cosmic soul. Not only, as is well known, does he accept the idea present in Praxagoras’ system that breath nourishes the substance of pneuma, but he also integrates the complex mechanism of expansion and contraction of the Aristotelian model. In this way, he can benefit from the philosophical advantages of both systems; retaining the idea that breath nourishes and constitutes the substance of the soul offers an expedient strategy to those who, as the Stoics, want to maintain the corporeality of the soul—and it is no coincidence that our main evidence comes exactly from the proofs of the corporeality of the soul. At the same time, the mechanism of expansion and contraction expounded by Aristotle in his theory of respiration represented a great solution for Chrysippus. On the one hand, it fitted perfectly with Stoic physics, whose elemental cycle was based on expansion and contraction. On the other hand, developing this mechanism provided Chrysippus with the tools to overcome some structural difficulties faced by Zeno.

From the testimonies available to us, we can deduce that Zeno struggled to address two problems that do not seem completely solved in his system, namely, (1) the coherence and tension of the cosmos and (2) the immanent and providential action of god in the cosmos. If we take it that the theory of cosmic pneuma was first developed by Chrysippus (or that Cleanthes started to develop it and then with Chrysippus it reached full development), the benefits of the doctrine of cosmic pneuma in providing a philosophically satisfying solution to the problems faced by Zeno become apparent. Zeno’s system (and perhaps partly also Cleanthes’ one) was based on the predominant role of heat, the only power capable of sustaining and giving life to the cosmos and all the living beings in it. The primary opposition in his system was that between heat and moisture and was conceived of as an opposition between an active and a passive power.^[121] As opposed to heat, what distinguished moisture as a sort of principle was its ability to be receptive, receiving heat and providing the conditions for it to develop and generate. An important instance of this interaction between heat and moisture is the cosmogonic accounts developed both by Zeno and Cleanthes,^[122] and valid antecedents can be found in both the medical and philosophical traditions.

In contrast, the main opposition in Chrysippus’ system is the one between heat and cold or fire and air.^[123] This primary opposition, however, is between two active forces, i.e., hot and cold, that have opposite directions, effects and aims but cooperate to maintain the cosmos. As we have observed, heat rarefies and makes the substance expand, determining its size, while cold makes the substance contract and condense and is responsible for its coherence, having a specific cohesive tension.^[124] This opposition was absent from Zeno’s and Cleanthes’ systems,^[125] and cold air seems primarily seen as a destructive force. In contrast, in the economy of Chrysippus’ cosmology, heat remains the moving principle, but cold—if in the right balance—acquires a teleological role too. It moderates and orients this moving force to maintain life: while heat and movement are the source of life, an excess of them would jeopardize life.

(1) This opposition between hot and cold was particularly helpful in facing one of the two problems Zeno had struggled with, i.e., the cohesiveness and coherence of the cosmos. Chrysippus’ solution presents the advantages of explaining with a valid physical theory the reason for cosmic coherence and unity. It is likely that Cleanthes attempted to solve this problem by introducing the concept of tonos; however, he considered tonos to be an effect of the heat from the sun.^[126] Heat, being rarefied and having the tendency to expand, could hardly provide something with coherence and cohesiveness. Chrysippus’ solution was neater and more economical in many ways. It attributed the contracting power to air, which was treated by the Stoics as a cold and thickening element that was able to contract and harden. In this way, heat could retain its expansive and rarefying power, which was the ultimate source of movement and life.

(2) Moreover, Zeno’s heat or fire could hardly provide an effective account of the immanence of the active principle during diakosmēsis. The immanence of the all-permeating reason in the cosmos was explained by Zeno only through the action of fire at the moment of the cosmogony. During the diakosmēsis, however, this principle of life was relegated in the heavens, at the periphery of the cosmos, where it constituted the substance of the stars, and it was present in living beings only as a result of cosmogony. It was not, in other words, constantly and actively permeating the cosmos. In this way, the immanence of the active principle in the cosmos was somehow jeopardised.^[127] Cleanthes’ conception of the hēgemonikon making the cosmos exhale harmoniously was undoubtedly a step forwards in this direction. Chrysippus’ solution, taking into account Cleanthes’ improvement as well, greatly enhances Stoic cosmology in this regard: exactly as the pneuma that moves about in our organism is able to make the whole body moveable, sentient and alive and is present at any time in any part, in a continuous exchange of information with the hēgemonikon, the cosmic pneuma ensures that the providential and divine action of god is present in any part of the cosmos at any moment. In this context, too, the role of air is quite central, since it is the contracting action of air that kickstarts the circulation of pneuma, which otherwise would have been only expanding. Additionally, in this way, the elemental cycle is framed in the wider context of the providential movements of the cosmic soul.

To conclude, the analysis I have been developing thus far leads to the following narrative. The heat present in the hēgemonikon, i.e., in the heaven and in the celestial bodies, heats up the sea (and probably also the earth), causing it to exhale, as when a heat source heats water and makes it boil. This exhalation is moist and thick at the beginning, but when rising towards the heavens, it becomes hotter and is gradually rarefied, dried and expanded. This process is a vaporisation or pneumatisation of the moist substance, and this substance constitutes the nourishment of the hēgemonikon. When it reaches the hēgemonikon, this exhalation undergoes a transformation, probably by being partly assimilated by the celestial bodies and partly resent to the earth in a continuous cycle of exchange. Nothing, or almost nothing, of the substance is dispersed since the heavens send back to the earth the remaining nourishment (probably, e.g., in the form of rain).^[128] The expanding exhalation, however, has also caused the cosmos to expand such that it needs something that partially quenches and contracts its substance. This is where air comes in: according to the Stoics, air can be moister or warmer, depending on its contact with the other elements; however, taken per se, it is cold. In pneumatic movement, its role is to block and balance this expansion and return the cosmos to its normal size. During the whole diakosmēsis, air and fire balance each other, but when it is due time for conflagration to occur, air cannot balance the action of heat anymore and heat dries and consumes everything.^[129]

Moreover, we have observed that on the individual level, air also nourishes pneuma, meaning that it is likely that at the cosmic level, one should envisage the same process, or this was at least our presupposition.^[130] However, it might be the case that this has a slightly different meaning at the cosmic level. The theory that air constitutes the substance of the soul could also mean that the agent of the inwards pneumatic movement should also be considered as part of the pneumatic substance, together with exhalation. Indeed, as said, some testimonies describe pneuma as being a mixture that includes air. Thus, in the individuum, inhaled air is not entirely breathed out, it actually mixes with the exhalation, and this mixture is the product that is called pneuma and moves about in the vessels. In the cosmos, the region between sea and heaven is always occupied by both exhalation and air that mix together and moves about, transmitting their proper movement to the product of this mixture (exhalation outwards and air inwards).

Thus, even if the usual definition of pneuma as a mixture of air and fire might be useful in a certain context, it is nonetheless important to specify what exactly we mean by adopting this definition. Indeed, the scenario is far more complex: fire and air are primarily agents of pneumatic movement. The solution I am proposing is to consider pneuma in Chrysippus’ system as a catch-all term to describe the motion of matter through the structure of cosmos, which can describe fire, air, or exhalation, as long as they are involved in the pneumatic process.^[131] This may well be the reason why pneuma is sometimes identified with air alone—exhalation being an airy substance—and sometimes defined as warm breath with reference to exhalation. Thus, by appropriating two already available models and by combining them, Chrysippus not only highlights their limitations and the need to rethink them against another background, i.e., Stoic physics, but above all, he shows how it is only within the frame of Stoic cosmology, where pneuma is ‘artlike’^[132] and acts according to a providential and divine plan, that medical and biological theories can work—and the other way around.