Stoic physics
Stoic physics is the natural philosophy adopted by the Stoic philosophers of ancient Greece and Rome used to explain the natural processes at work in the universe. To the Stoics, the universe is a single pantheistic god, but one which is also a material substance. The primitive substance of the universe is a divine essence which is the basis of everything which exists. This pneuma, which is the active part or reason of God, provides form and motion to matter, and is the origin of the elements, life, and human rationality. From their physics, the Stoics explained the development, and ultimately, the destruction of the universe in a never-ending cycle. The human soul is an emanation from the divine reason which permeates the universe, and knowledge is gained by the mind from sense impressions and subjecting them to reason.
Central tenets
In pursuing their physics the Stoics wanted to create a picture of the world which would be completely coherent. Stoic physics can be described in terms of monism, materialism, and dynamism.Monism
Stoicism was a pantheistic philosophy. The universe is active, life-giving, rational and creative. It is a self-supporting entity, containing within it all that it needs, and all parts depending on mutual exchange with each other. Different parts of this unified structure are able to interact and have an affinity with each other. The Stoics explained everything from natural events to human conduct as manifestations of an all-pervading reason. Thus they identified the universe with God, and the diversity of the world is explained through the transformations and products of God as the rational principle of the universe.Materialism
Philosophers since the time of Plato had asked whether abstract qualities of the soul, such as justice and wisdom, have an independent existence. In particular, could something that was not visible and tangible be said to exist. The Stoics' answer to this dilemma was to assert that everything, including wisdom, justice, etc., are corporeal. Plato had defined being as "that which has the power to act or be acted upon," and for the Stoics this meant that all action proceeds by bodily contact; every form of causation is reduced to the efficient cause, which implies the communication of motion from one body to another. Only Body exists. The Stoics did recognise the presence of incorporeal things such as void, place and time, but although real they could not exist and were said to "subsist". Stoicism was thus fully materialistic; the answers to metaphysics are to be sought in physics; particularly the problem of the causes of things for which the Platonic Theory of Forms and the Peripatetic "constitutive form" had been put forth as solutions.Dynamism
A dualistic feature of the Stoic system are the two principles, the active and the passive: everything which exists is capable of acting and being acted upon. The active principle is God acting as the rational principle, and which has a higher status than the passive matter. In their earlier writings the Stoics characterised the rational principle as a creative fire, although later accounts stress the idea of pneuma as the active substance. The universe is thus filled with an all-pervading pneuma which allows for the cohesion of matter and permits contact between all parts of the universe. The pneuma is everywhere coextensive with matter, pervading and permeating it, and, together with it, occupying and filling space.The Epicureans had placed the form and movement of matter in the chance movements of primitive atoms. In the Stoic system material substance has a continuous structure, held together by tension as the essential attribute of body. This tension is a property of the pneuma, and physical bodies are held together by the pneuma which is in a continual state of motion. The various pneuma currents combining in this way give objects their stable, physical properties. A thing is no longer, as Plato maintained, hot or hard or bright by partaking in abstract heat or hardness or brightness, but by containing within its own substance the material of these pneuma currents in various degrees of tension. This can be compared to Aristotle's essential form: in both systems the active principle, "the cause of all that matter becomes," is that which accounts for the existence of a given concrete thing, but in the Stoic system, the principle is itself material.
As to the relation between the active and the passive principles there was no clear difference. Although the Stoics talked about the active and passive as two separate types of body, it is likely they saw them as merely two aspects of the single material cosmos. Pneuma, from this perspective, is not a special substance intermingled with passive matter, but rather it could be said that the material world has pneumatic qualities. The whole universe is a single cohesive unit. The reason of things – that which accounts for them – is not some external end to which they are tending; it is something acting within them, "a spirit deeply interfused," germinating and developing from within.
Universe
Like Aristotle, the Stoics conceived of the universe as being finite with the Earth at the centre and the moon, sun, planets, and fixed stars surrounding it. Similarly, they rejected the possibility of any void within the cosmos since that would destroy the coherence of the universe and the sympathy of its parts. However, unlike Aristotle, the Stoics saw the universe as an island embedded in an infinite void. The universe has its own hexis which holds it together and protects it and the surrounding void cannot affect it. The universe can, however, vary in volume, allowing it to expand and contract in volume through its cycles.Formation
The pneuma of the Stoics is the primitive substance which existed before the universe. It is the everlasting presupposition of particular things; the totality of all existence; out of it the whole visible universe proceeds, eventually to be consumed by it. It is the creative force which develops and shapes the universal order. God is everything that exists.In the original state, the pneuma-God and the universe are absolutely identical; but even then tension, the essential attribute of matter, is at work. In the primitive pneuma there resides the utmost heat and tension, within which there is a pressure, an expansive and dispersive tendency. Motion backwards and forwards once set up cools the glowing mass of fiery vapour and weakens the tension. Thus follows the first differentiation of primitive substance – the separation of force from matter, the emanation of the world from God. The seminal Logos which, in virtue of its tension, slumbered in pneuma, now proceeds upon its creative task. The cycle of its transformations and successive condensations constitutes the life of the universe. The universe and all its parts are only different embodiments and stages in the change of primitive being which Heraclitus had called "a progress up and down".
Out of it is separated elemental fire, the fire which we know, which burns and destroys; and this condenses into air; a further step in the downward path produces water and earth from the solidification of air. At every stage the degree of tension is slackened, and the resulting element approaches more and more to "inert" matter. But, just as one element does not wholly transform into another, so the pneuma itself does not wholly transform into the elements. From the elements the one substance is transformed into the multitude of individual things in the orderly universe, which is itself a living thing or being, and the pneuma pervading it, and conditioning life and growth everywhere, is its soul.
Ending and rebirth
The process of differentiation is not eternal; it continues only until the time of the restoration of all things. For the universe will in turn decay, and the tension which has been relaxed will again be tightened. Things will gradually resolve into elements, and the elements into the primary substance, to be consummated in a general conflagration when once more the world will be absorbed in God. This ekpyrôsis is not so much a catastrophic event, but rather the period of the cosmic cycle when the preponderance of the fiery element once again reaches its maximum. All matter is consumed becoming completely fiery and wholly soul-like. God, at this point, can be regarded as completely existing in itself.In due order a new cycle of the universe begins, reproducing the previous universe, and so on forever. Therefore, the same events play out again repeated endlessly. Since the universe always unfolds according to the best possible reason, any succeeding world is likely to be identical to the previous one.
God
The Stoics often identified the universe and God with Zeus, as the ruler and upholder, and at the same time the law, of the universe. The Stoic God is not a transcendent omniscient being standing outside nature, but rather it is immanent—the divine element is immersed in nature itself. God orders the universe for the good, and every element of the world contains a portion of the divine element that accounts for its behaviour.However, the universe is good only in as far as it contains the optimal rationality and virtue. The contents of the world may be appropriate and well-fitted, but they are morally indifferent. The goodness of the Stoic God lies in its rational and virtuous selection of the indifferent things of the world. Thus pain, suffering, and tragedy occur, even while the universe unfolds according to the rational good. World events are God events; they unfold according to the best possible reason, and they are knowable by reason. The good for humans lies in the understanding and acceptance of this combined with actions directed towards the good.
The Stoics attempted to incorporate traditional polytheism into their philosophy. Not only was the primitive substance God, the one supreme being, but divinity could be ascribed to the manifestations—to the heavenly bodies, to the forces of nature, even to deified persons; and thus the world was peopled with divine agencies. Prayers are of little avail in a rationally ordered cosmos, and surviving examples of Stoic prayers appear to be more like types of self-meditation than appeals for divine intervention.
Fate
To the Stoics nothing passes unexplained; there is a reason for everything in nature. Because of the Stoics' commitment to the unity and cohesion of the cosmos and its all-encompassing reason, they fully embraced determinism. However instead of a single chain of causal events, there is instead a many-dimensional network of events interacting within the framework of fate. Out of this swarm of causes, the course of events is fully realised. Humans appear to have free will because personal actions participate in the determined chain of events independently of external conditions. This "soft-determinism" allows humans to be responsible for their own actions, alleviating the apparent arbitrariness of fate.Divination
Divination was an essential element of Greek religion, and the Stoics attempted to reconcile it with their own rational doctrine of strict causation. Since the pneuma of the world-soul pervades the whole universe, this allows human souls to be influenced by divine souls. Omens and portents, Chrysippus explained, are the natural symptoms of certain occurrences. There must be countless indications of the course of providence, for the most part unobserved, the meaning of only a few having become known to humanity. To those who argued that divination was superfluous as all events are foreordained, he replied that both divination and our behaviour under the warnings which it affords are included in the chain of causation.Mixture
To fully characterize the physical world, the Stoics developed a theory of mixing in which they recognised three types of mixture. The first type was a purely mechanical mixture such as mixing barley and wheat grains together: the individual components maintain their own properties, and they can be separated again. The second type was a fusion, whereby a new substance is created leading to the loss of the properties of the individual components, this roughly corresponds to the modern concept of a chemical change. The third type was a commingling, or total blending: there is complete interpenetration of the components down to the infinitesimal, but each component maintains its own properties. In this third type of mixture a new substance is created, but since it still has the qualities of the two original substances, it is possible to extract them again. In the words of Chrysippus: "there is nothing to prevent one drop of wine from mixing with the whole ocean". Ancient critics often regarded this type of mixing as paradoxical since it apparently implied that each constituent substance be the receptacle of each other. However to the Stoics, the pneuma is like a force, a continuous field interpenetrating matter and spreading through all of space.Tension
Every character and property of a particular thing is determined solely by the tension in it of pneuma, and pneuma, though present in all things, varies indefinitely in quantity and intensity.- In the lowest degree of tension the pneuma dwelling in inorganic bodies holds bodies together providing cohesion. This is the type of pneuma present in stone or metal as a retaining principle.
- In the next degree of tension the pneuma provides nature or growth to living things. This is the highest level in which it is found in plants.
- In a higher degree of tension the pneuma produces soul to all animals, providing them with sensation and impulse.
- In humans can be found the pneuma in its highest form as the rational soul.
Soul
In the rational creatures pneuma is manifested in the highest degree of purity and intensity as an emanation from the world-soul. Humans have souls because the universe has a soul, and human rationality is the same as God's rationality. The pneuma that is soul pervades the entire human body.The soul is corporeal, else it would have no real existence, would be incapable of extension in three dimensions, incapable of holding the body together, herein presenting a sharp contrast to the Epicurean tenet that it is the body which confines and shelters the atoms of soul. This corporeal soul is reason, mind, and ruling principle; in virtue of its divine origin Cleanthes can say to Zeus, "We too are thy offspring," and Seneca can calmly insist that, if man and God are not on perfect equality, the superiority rests rather on our side. What God is for the world, the soul is for humans. The cosmos is a single whole, its variety being referred to varying stages of condensation in pneuma. So, too, the human soul must possess absolute simplicity, its varying functions being conditioned by the degrees of its tension. There are no separate "parts" of the soul, as previous thinkers imagined.
With this psychology is intimately connected the Stoic theory of knowledge. From the unity of soul it follows that all mental processes – sensation, assent, impulse – proceed from reason, the ruling part; the one rational soul alone has sensations, assents to judgments, is impelled towards objects of desire just as much as it thinks or reasons. Not that all these powers at once reach full maturity. The soul at first is empty of content; in the embryo it has not developed beyond the nutritive principle of a plant; at birth the "ruling part" is a blank tablet, although ready prepared to receive writing. The source of knowledge is experience and discursive thought, which manipulates the materials of sense. Our ideas are copied from stored-up sensations.
Just as a relaxation in tension brings about the dissolution of the universe; so in the body, a relaxation of tension, accounts for sleep, decay, and death for the human body. After death the disembodied soul can only maintain its separate existence, even for a limited time, by mounting to that region of the universe which is akin to its nature. It was a moot point whether all souls so survive, as Cleanthes thought, or the souls of the wise and good alone, which was the opinion of Chrysippus; in any case, sooner or later individual souls are merged in the soul of the universe, from which they originated.
Sensation
The Stoics explained perception as a transmission of the perceived quality of an object, by means of the sense organ, into the percipient's mind. The quality transmitted appears as a disturbance or impression upon the corporeal surface of that "thinking thing," the soul. In the example of sight, a conical pencil of rays diverges from the pupil of the eye, so that its base covers the object seen. A presentation is conveyed, by an air-current, from the sense organ, here the eye, to the mind, i.e. the soul's "ruling part." The presentation, besides attesting its own existence, gives further information of its object – such as colour or size. Zeno and Cleanthes compared this presentation to the impression which a seal bears upon wax, while Chrysippus determined it more vaguely as a hidden modification or mode of mind. But the mind is no mere passive recipient of impressions: the mind assents or dissents. The contents of experience are not all true or valid: hallucination is possible; here the Stoics agreed with the Epicureans. It is necessary, therefore, that assent should not be given indiscriminately; we must determine a criterion of truth, a special formal test whereby reason may recognize the merely plausible and hold fast the true.The earlier Stoics made right reason the standard of truth. Zeno compared sensation to the outstretched hand, flat and open; bending the fingers was assent; the clenched fist was "simple apprehension," the mental grasp of an object; knowledge was the clenched fist tightly held in the other hand. But this criterion was open to the persistent attacks of Epicureans and Academics, who made clear that reason is dependent upon, if not derived from, sense, and that the utterances of reason lack consistency. Chrysippus, therefore, did much to develop Stoic logic, and more clearly defined and safeguarded his predecessors' position.