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 (pneuma) which is the basis of everything which exists. The separation of force from matter produces a divine fire (aether) which, as the basis of all matter, is differentiated into elements and shaped by the tension caused by the pneuma working according to the divine reason (logos) of the universe. These processes are responsible for the formation, the development, and ultimately, the destruction of the universe in a never-ending cycle (palingenesis). The human soul is an emanation from the fiery aether which permeates the universe, and sensation is transmission of pneuma-currents from objects, which interact with the substance of the mind, which is the soul's ruling part. The Stoics also recognised the existence of other gods and divine agents as manifestations of the one primitive God-substance.
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. 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.
Materialism was also a doctrine of the Epicureans. The characteristic difference with the Stoic system was the idea of tension as the essential attribute of body. The Epicureans placed the form and movement of matter in the chance movements of primitive atoms. To the Stoics, however, nothing passes unexplained; there is a reason (Logos) for everything in nature. In everything that exists there are two principles, the active and the passive: everything which exists is capable of acting and being acted upon. By the passive principle a thing is susceptible to motion and modification; matter determines substance (ousia). The active principle characterizes matter, and gives it its quality. For all that happens there is a cause, and as only body can act on body, this cause is as corporeal as the matter upon which it acts. The active principle or "force" is everywhere coextensive with "matter," pervading and permeating it, and, together with it, occupying and filling space. A thing is no longer, as Plato once thought, 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 qualities, conceived as air-currents (pneuma) in various degrees of tension. The virtues are corporeal, as indeed are actions. The Stoic quality corresponds 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. Here, too, the reason of things – that which accounts for them – is no longer some external end to which they are tending; it is something acting within them, "a spirit deeply interfused," germinating and developing from within. By its prompting a thing grows, develops and decays, while this "seminal reason," the element of quality in the thing, remains constant through all its changes.
As to the relation between the active and the passive principles there was no real difference. The active cause was always a corporeal current, and therefore matter, although the finest and subtlest matter. Aristotle's technical term Form (ethos) the Stoics never used, but always Reason or God. The Stoics laid down with rigid accuracy the two chief properties of matter – extension in three dimensions, and resistance, both being traced back to force. There were, it is true, certain conceptions, creations of thought to which nothing real and external corresponded, such as time, space, and void, but though each of these might be said to be something, they could not be said to exist.
A Stoic might maintain that World-Soul, Providence, Destiny and Seminal Reason are not mere synonyms, for they express different aspects of God or different relations of God to things, but there were no different substances underlying the different forces of nature. The pneuma neither increases nor diminishes; but its modes of working, its different currents, can be conveniently distinguished and enumerated as evidence of so many distinct attributes.
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 (God) which develops and shapes the universal order (cosmos). 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. The pneuma cannot long withstand this intense pressure. 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 primitive substance is not Heraclitus's fire, but rather it is a fiery breath or aether, a spiritualized element. 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, first, elemental fire, the fire which we know, which burns and destroys; and this, again, condenses into air or aerial vapour; 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 (e.g. only a part of air is transmuted into water or earth), so the pneuma itself does not wholly transform into the elements. The residue that remains in original purity with its tension is the ether in the highest sphere of the visible heavens, encircling the world of which it is lord and head. 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. But this process of differentiation is not eternal; it continues only until the times of the restoration of all things. For the world which has grown up will in turn decay. 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 (ekpyrôsis) when once more the world will be absorbed in God. Then in due order a new cycle of the universe begins, reproducing the previous, and so on forever.
The influence of Heraclitus upon Stoicism is a matter of dispute, but the earliest Stoics, such as Cleanthes, Aristo and Sphaerus all wrote commentaries on the writings of Heraclitus, which point to a common study of these writings under Zeno. In Heraclitus the constant flux is a metaphysical notion replaced by the interchange of material elements which Chrysippus stated as a simple proposition of physics. Heraclitus offers no analogy to the doctrine of four elements as different grades of tension; to the conception of "fire" and "air" as the "form" of particulars; nor to the function of organizing fire which works by methodic plan to produce and preserve the world. Nor, again, is there any analogy to the peculiar Stoic doctrine of universal intermingling.
In Stoicism every character and property of a particular thing is determined solely by the tension in it of a current of pneuma, and pneuma, though present in all things, varies indefinitely in quantity and intensity. So condensed and coarsened is the indwelling pneuma of inorganic bodies that no trace of elasticity or life remains; it cannot even afford them the power of motion; all it can do is to hold them together, pneuma is present in stone or metal as a retaining principle. In plants it is manifested as something far purer and possessing greater tension, called a "nature," or principle of growth. A distinction was drawn between irrational animals, and the rational, i.e. gods and humans, leaving room for a divergence, or rather development, of Stoic opinion. The older authorities conceded a vital principle, but denied a soul, to the animals. Later on it was a Stoic tenet to concede a soul, though not a rational soul, throughout the animal kingdom. The universal presence of pneuma was confirmed by observation. A certain warmth, akin to the vital heat of organic being, seems to be found in inorganic nature: vapours from the earth, hot springs, sparks from the flint, were claimed as the last remnant of pneuma not yet utterly slackened and cold. They appealed also to the speed and expansion of gaseous bodies, to whirlwinds and inflated balloons.
The Logos is quick and powerful, and sharper than any two-edged sword. Tension itself Cleanthes defined as a fiery stroke; in his Hymn to Zeus lightning is the symbol of divine activity. As to the fundamental properties of body, extension and resistance, extension results from distance; but distances, or dimensions, are straight lines, i.e. lines of greatest tension. Tension produces expansion, or increase in distance. Resistance, again, is explained by cohesion, which implies binding force. Again, the primary substance has rectilinear motion in two directions, backwards and forwards, at once a condensation, which produces cohesion and substance, and an expansion, the cause of extension and qualities.
In the rational creatures – humans and gods – pneuma is manifested in a high degree of purity and intensity as an emanation from the world-soul, itself an emanation from the primary substance of purest aether – a spark of the celestial fire, or, more accurately, fiery breath, characterized by vital warmth more than by dryness.
The soul is corporeal, else it would have no real existence, would be incapable of extension in three dimensions (and therefore of equable diffusion all over the body), 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. It follows that of "parts" of the soul, as previous thinkers imagined, there can be no question; all that can consistently be maintained is that from the centre of the body – the heart – distinct air-currents are discharged to various organs, which are so many modes of the one soul's activity.
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. This excludes all possibility of innate ideas or any faculty akin to intuitive reason. The source of all our 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.
The relation of the soul of the universe to God is quite clear: it is an inherent property, a mode of its activity, an emanation from the fiery aether which permeates the universe.
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 from without. Sensation reacts, by a variation in tension, against the current from the sense-organ; and this is the mind's assent or dissent, which is inseparable from the sense presentation. 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. The law which regulates our action is thus the ultimate criterion of what we know – practical knowledge being understood to be of paramount importance. But this criterion was open to the persistent attacks of Epicureans and Academics, who made clear (1) that reason is dependent upon, if not derived from, sense, and (2) that the utterances of reason lack consistency. Chrysippus, therefore, substituted for the Logos the new standards of sensation and general conception, and more clearly defined and safeguarded his predecessors' position. For reason is consistent in the general conceptions in which all people agree. Nor was the term sensation sufficiently definite. Chrysippus fixed upon a certain characteristic of true presentations; provided the sense organ and the mind be healthy, provided an external object be really seen or heard, the presentation, in virtue of its clearness and distinctness, has the power to extort the assent which it always lies in our power to give or to withhold.
The work of reason was assimilated to the force which binds together the parts of an inorganic body and resists their separation. There is nothing in the order of the universe other than extended mobile bodies and forces in tension in these bodies. So, too, in the order of knowledge there is nothing but sense and the force of reason maintaining its tension and connecting sensations and ideas in their proper sequence. 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. The illustration is valuable for the light it throws on the essential unity of diverse intellectual operations as well as for enforcing once more the Stoic doctrine that different grades of knowledge are different grades of tension. Good and evil, virtues and vices, remarked Plutarch, are all capable of being perceived; sense, this common basis of all mental activity, is a sort of touch by which the ethereal pneuma which is the soul's substance recognizes and measures tension.
For the Stoics, God is everywhere as the ruler and upholder, and at the same time the law, of the universe. Zeno declared cult images, shrines, temples, sacrifices, prayers and worship to be of no avail. A really acceptable prayer, he taught, can only have reference to a virtuous and devout mind. The Stoics however attempted to defend and uphold the truth in polytheism. Not only was the primitive substance God, the one supreme being, but divinity could be ascribed to the manifestations – to the heavenly bodies, which were conceived, like Plato's created gods, as the highest of rational beings, to the forces of nature, even to deified men; and thus the world was peopled with divine agencies.
The practice of divination and the consultation of oracles afforded a means of communication between God and man – a concession to popular belief, which may be explained when we reflect that divination was an essential element of Greek religion. Chrysippus did his best to reconcile the superstition with his own rational doctrine of strict causation. Omens and portents, he 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.
- Plato, Sophist, 246C ff.
- Plato, Sophist, 247D
- Heraclitus, DK B60
- Diogenes Laërtius, vii. 174, ix. 5, 15
- Seneca, Epistles, liii. 11–12
- Diogenes Laërtius, vii. 54
- Cicero, Academica, ii. 4