Diogenes tells us that the Stoic physics had three general divisions: the investigation of the cosmos, of the elements that make up the cosmos, and causal explanations of what occurs within the cosmos (7.132). We shall begin with a description of the cosmos as such.
The universe is said to fall under two principles: an active principle, which is rational, and a passive principle which is matter without any qualities. In fact, these two principles are abstractions, in that the rational only exists in a material medium and matter is always qualified. The principles are said to be permanent: they do not come to be or pass away. Indeed, the cosmos which they comprise is itself eternal, though it undergoes periodic cataclysmic changes. The origin of this active/passive dichotomy seems to be in Plato's Sophist (247d-e). There, the Eleatic Stranger suggests that the mark of real beings is power: that they act or be acted upon. The rational, active, principle is further escribed by the Stoics as being god, who is eternal and penetrates all matter, the "craftsman" of all things (Diogenes Laertius, Lives of Eminent Philosophers, 7.134).
In another book, the Timaeus, Plato had described god as a craftsman, but one who existed outside the material world and was determined to make it better, and so gave it soul and body modeled on the eternal, immaterial forms. The Stoic active principle, by contrast, is immanent, as are the forms which matter takes on. It interpenetrates all matter, although in different ways. Most importantly, it mixes with some matter in the form of mind, which is the "leading part" of the soul of human beings. Here we begin to see the use of analogical reasoning. First, the world is "a substance which is alive and capable of sense-perception"— an animal (Diogenes Laertius, Lives of Eminent Philosophers, 7.143). The argument is that "an animal is better than a non-animal, and nothing is better than the cosmos; therefore the cosmos is an animal. And [it is] alive, as is clear from the fact that the soul of [each of] us is a fragment derived from it." Here, it must be assumed that the premises that assert what kind of thing is better than another must come from "basic grasps," those conceptions which are naturally and universally accepted. It is not difficult to make the further case that reason is the leading part of the human soul, insofar as its deliberations are the basis for voluntary action; so the cosmos has a rational leading part. (For further discussion, see below.)
The cosmos is spherical and surrounded by an unlimited void. Given that its size is variable, there is need for a place into which the cosmos might expand. The reason the size of the cosmos varies is that the cosmos itself is subject to periodic generation and destruction. The argument for this claim shows again the heavy reliance by the Stoics on analogy: "in the case of things conceived by sense-perception, that whose parts are destructible is also destructible as a whole; but the parts of the cosmos are destructible, since they change into one another; therefore, the cosmos is destructible" (Diogenes Laertius Lives of Eminent Philosophers, 7.140). The destruction of the cosmos is not an annihilation or return to nothingness, but rather a massive transformation, on the model of the way objects of sense-perception change into one another.
The Elements and the Pneuma
The abstract description of the universe as having an active and a passive principle needs to be supplemented by a description of the kinds of bodies which make up the world. It is the elements which have form and which are subject to generation and destruction. The four elements acknowledge by the Stoics are fire (which is pre-eminent), air, water and earth. Fire is the pre-eminent element in the sense that all the other elements are generated from it. It has already been mentioned that the cosmos is subject to periodic destruction. The Stoic model of destruction is conflagration, and they held that the universe periodically burns itself up, changing everything to fire. This notion comes from the pre-Socratic philosopher Heraclitus, who is quoted as having stated that "This universe, which is the same for all, has not been made by any god or man, but it always has been, is, and will be--an ever-living fire, kindling itself by regular measures and going out by regular measures" (Wheelwright, The Presocratics, p. 71). Heraciltus also compared the change from fire and everything else to an exchange of goods for gold in commerce. After the conflagration, the Stoics maintained, "the cosmos comes into being, when substance turns from fire through air to moisture, and then the thick part of it is formed into earth and the thin part is rarified and this when made even more thin produces fire" (Diogenes Laertius, Lives of Eminent Philosophers, 7.142).
All the elements for the Stoics are qualitative: "fire is the hot, water the wet, air the cold, and earth the dry" (Diogenes Laertius, Lives of Eminent Philosophers, 7.137. They can be arranged in pairs based on the opposition of the qualities which they embody: fire/air, water/earth. These pairings allow for a tight connection between the elements and the two principles: "The Stoics say that some of the elements are active and others passive: air and fire are active, earth and water passive" (Nemesius 164, 15-18). Fire as an active element makes appropriate the Stoic description of their god (the active priniciple) as a "craftsmanlike" (or "designing") fire. As usual, this position is based on an analogy with the perceptible world. Cicero credits the following claim to Cleanthes: "Everything which lives, whether an animal or a vegetable, lives because of the heat contained within it. From this one should understand that the nature of heat has within itself the power of life which penetrates the entire cosmos" (On the Nature of the Gods 2.24).
The active nature of air also supports the doctrine of the pneuma (literally, "breath") which was a basic explanatory principle for the Stoics. Pneuma is a blend of air and fire that is the soul of animals as well as nature as a whole.
For they [the Stoics] claim that the soul is a kind of pneuma, as is nature too; the pneuma of nature is more fluid and cool, while that of the soul is drier and hotter. Consequently, [they also think this]: that pneuma is a kind of matter proper to the soul, and in form the matter is either a symmetrical blend of airy and fiery substance; for it is not possible to say that it is either air alone or fire alone, since the body of an animal does not appear to be either extremely cold or extremely hot, but rather it is not even dominated by a great excess of either of these . . . (Galen, On the Habits of the Soul, 4.783)The cosmic pneuma, the soul of the world, is also the divine soul, which accounts for the connection of things in the universe: "These things [meteorological phenomena], and the mutual harmony of the parts of the cosmos, certainly could not happen as they do unless they were bound together by one divine and continuously connected pneuma" (Cicero, On the Nature of Gods, 2.19).
This brings out an aspect of the active nature of the elements air and fire: they are "sustaining powers," while earth and water are "sustained." The Stoics explained this by holding that the two sustaining elements work against each other to create a dynamic tension: "there exists in bodies, a kind of tensile movement which moves simultaneously inwards and outwards, the outward movement producing quantities and qualities and the inward one unity and substance" (Nemesius, 71). Since what is hot expands and what is cold contracts, it is reasonable to suppose (with Long and Sedley) that fire produces the quantities and qualities of things, while air produces unity and substance.
Pneuma is responsible for the "condition" (or "tenor," "hexis") of all corporeal things. More specialized objects, living things, also have a specific "nature" (or "physique," "phusis") which is also due to the pneuma that is mixed with its other elements. It is the condition of the plant in motion, a reference, it seems, to the plant's capacity to grow. Irrational animals have a nature, soul, which has sense-perception and "impulse," ("horme") which is responsible for the animal's self-movement. Finally, the human being has a rational soul ("psuche"). Ultimately, this soul is a condition which is the mode of existence of the pneuma which the soul really is.
Intelligence . . . has many powers, the tenor kind, the physical, the psychic, the rational, the calculative. . . . Tenor is also shared by lifeless things, stones and logs, and our bones, which resemble stones, also particpate in it. Physique also extends to plants, and in us there are things like plants -- nails and hair. Physique is tenor in actual motion. Soul is physique which has also acquired impression and impulse. This is also shared by irrational animals. (Philo, Allegories of the Laws, 2.22-3)It should be noted that the entire cosmos is an amimal which has a rational soul. Lifeless things, then, are lifeless only insofar as they are considered as limited things, like the bones of a human being cited in the quotation above. Souls, which are ultimately pneuma, are corporeal, and for that reason are subject to generation and destruction. These differentiations of pneuma have consequences for change in the cosmos.
Of things that move, some have the cause of motion in themselves, while others are moved only from the outside. Thus things which are moved by being carried, such as sticks and stones and every form of matter held together by hexis [condition] alone, are moved from the outside. . . Plants and animals, and in a word everything held together by nature and soul have within themselves the cause of motion. . . . Of things which contain the cause of motion in themselves, they say that some move from themselves and others by themselves. 'From themselves' applies to soulless objects, 'by themselves' to things with souls. For ensouled things move by themselves when a presentation occurs which stimulates the impulse. . . . But the rational animal has reason too in addition to the power of persentation. Reason judges the presentations and rejects some and admits others. (Origen, On Principles, 3.1.2-3 [II-25])Thus the basic mechanism for animal motion is a combination of the two characteristic animal powers: sense-perception, which presents an object, and impulse, which moves the animal toward or away from the presented object.
The Rational Soul
We are told that the part of the human soul (and the world-soul) which is rational is its "leading" or "commanding" part, and its principal role is that of judgment and consequent assent or rejection of presentations. (Sometimes the Stoics use "soul" to refer just to the leading part). The subordinate parts of the soul are "the five senses, the spermatic principles in us, [and] the vocal part," each of which is explained physically. For example, sight is explained by there being a pneuma leading from the rational part of the soul to the visual organ, which meets the "tensed" air outside the body, which in turn meets the object. "So the observed object is 'announced' [to us] by the tensed air, just as [the ground is revealed to a blind man] by his walking stick" (Diogenes Laertius, Lives of Eminent Philosophers 7.157 [II-20]). The rational part of the soul is commanding because its assent to and rejection of presentations is the basis of action. Diogenes states that that impulses are "in" the commanding part of the soul (7.159), which would make impulse as a source of motion dependent on the its commands.
God is a rational animal on the cosmic scale. The hallmark of rationality is command, which is based on assent to what is thought to be best. Thus the Stoic god is the ruler of the universe, setting all things into motion based on what it knows to be the best for the universe. We are told that the principal concerns of divine providence are "first, that the cosmos be as well suited as possible for remaining in existence, second, that it be in need of nothing, but most of all that it should possess surpassing beauty and every adornment" (Cicero, On the Nature of the Gods, 2.58).
We know that god exists because the cosmos exists, and nothing is better than the cosmos. Being rational is the best condition of any being, so the cosmos is rational. Moreover, the rationality of the cosmos is revealed in the impressive organization of the universe, from vegetative life to motions of the heavenly bodies, which is described at great length by Cicero.
The fact that everything that occurs on earth is due to divine providence is what led the Stoics to identify god as the active principle with "fate": "the common nature and the common reason of nature are fate and providence and Zeus" (Plutarch, Stoic Self-Contradictions 34, 1050a-b [II-81]). Fate is defined as "a string of causes, i.e., an ordering and connection which is inescapable" (Aetius, 1.28.4 [II-79]), which has "an element of logical consequence" (Alexander De Anima Mantissa CIAG Supp. 2.2 p. 185.1-5 [II-80]). Chrysippus is quoted as stating that fate "is a sempiternal and unchangeable series and chain of things, rolling and unravelling itself through eternal sequences of cause and effect, of which it is composed and compounded" (Aulus Gellius 7.2 [I-89]). Thus fate for the Stoics is what we would now call a thoroughly deterministic causal order.
The Stoic view shows an integrity lacking in the Epicurean physics, with its doctrine of the swerve. But it comes at a philosophical price, as summarized in this objection:
Chrysippus thinks that everything is moved and governed by fate and the sequences and revolutions of fate cannot be turned aside or evaded, then men's sins and misdeeds should not rouse our anger, nor should they be attributed to men and their will but to a kind of necessity and inevitability which comes from fate, mistress and arbiter of all things, by whose agency all that will be is necessary. And therefore the penalties aplied by the law to the guilty are unfair, if men do not turn to misdeeds voluntarily but are dragged by fate." (Aulus Gellius 7.2 [I-89])The Stoic response to this objection is to distinguish between ways in which necessity manifests itself in human action. When an action is necessitated by a person's character, the deed is to be "laid at the door" of that person, and not to outside agency. The exercise of will according to one's character is a "principal and perfect" cause, as opposed to an "auxiliary and proximate" cause. Only the latter is strictly speaking in the clutches of fate. To illustrate this distinction, Chrysippus used an analogy. The proximate cause of a cylinder's rolling down an incline is the hand that pushed it. But it continues to move due to its "rollability," which it has in itself, independently of its being pushed. So in the case of assent (which is how human action begins), a presentation is the proximate cause but assent is due to our own will. Another metaphor is that of a dog leashed to the back of a cart. If the cart moves, the dog might not voluntarily follow, in which case it will be dragged behind. But it could follow voluntarily, in which case it would still be dragged, except that it would be "exercising his autonomy in conjunction with necessity" (Hippolytus Philosophoumena 21 [II-92]).
There are a couple of interesting side-issues related to this topic. One has to do with the so-called "lazy argument," according to which it is not worthwhile to pursue one's ends if all is fated. For example, if it is fated that I shall die of my illness, it doesn't matter whether I call the physician or not. On the other hand, if it is fated that I survive it, it still makes no difference whether the physician attends me. Chrysippus opposed this argument, claiming that the act of calling the physician would figure into the causal chain resulting in one's health (if he called the physician and survived). The summoning of aid is "co-fated" with recovery.
A second issue concerns "future contingents," statements about the future which presumably may or may not be accurate, such as that there will be a sea-battle tomorrow. If it is true now that there will be such a battle, it would seem that the battle is inevitable. Although these statements posed a problem for Aristotle and the Epicureans, who both seem to have altered their view of truth to avoid the consequence of the inevitability of the future event, the Stoics could accept the outcome with equanimity. Cicero, who was horrified by those "who would strip the mind of free will and bind it by the necessity of fate," puts it this way.
Chrysippus reasons thus. "If there is a motion without a cause, not every proposition, which the dialecticians call an axioma, will be either true or false. For what will not have effective causes will be neither true nor false. But every proposition is either true or false. Therefore, there is no motion without a cause. And if this is so, everything which happens happens in virtue of prior causes; and if this is so, all things happen by fate. So it is shown that whatever happens happens by fate." (On Fate, 20-21 [I-15])It seems clear from Cicero's original examples that the emphasis on the universal truth and falsehood of propositions is meant to include the future contingents, which would be neither true nor false (the Epicurean doctrine) if there were not now causes which would bring them about later.