Stoic physics
The nature of the world is one of unceasing change, driven by the active part or reason (logos) of God which pervades all things.
The active substance of the world is characterized as a 'breath', or pneuma, which provides form and motion to matter, and is the origin of the elements, life, and human rationality.
The cosmos proceeds from an original state in utmost heat, and, in the cooling and separation that occurs, all things appear which are only different and stages in the change of primitive being.
But the Stoics adopted a compatibilist view which allowed humans freedom and responsibility within the causal network of fate.
[13] The Stoics did recognise the presence of incorporeal things such as void, place and time,[13] but although real they could not exist and were said to "subsist".
[3] In their earlier writings the Stoics characterised the rational principle as a creative fire,[8] but later accounts stress the idea of breath, or pneuma, as the active substance.
[2] In the Stoic system material substance has a continuous structure, held together by tension (tonos) as the essential attribute of body.
[16] 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.
[8] Like Aristotle, the Stoics conceived of the cosmos as being finite with the Earth at the centre and the moon, sun, planets, and fixed stars surrounding it.
[20] Similarly, they rejected the possibility of any void (i.e. vacuum) within the cosmos since that would destroy the coherence of the universe and the sympathy of its parts.
[16] In the original state, the pneuma-God and the cosmos are absolutely identical; but even then tension, the essential attribute of matter, is at work.
Motion backwards and forwards once set up cools the glowing mass of fiery vapour and weakens the tension.
[16] 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.
[32] Other prominent stoics such as Panaetius, Zeno of Tarsus, Boethus of Sidon, and others either rejected Ekpyrosis or had differing opinions regarding its degree.
[37] 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.
[45][47] The natural patterning of the world—life, death, sickness, health, etc.—is made up of morally indifferent events which in themselves are neither good nor bad.
[51] Humans appear to have free will because personal actions participate in the determined chain of events independently of external conditions.
[52][53] Divination was an essential element of Greek religion, and the Stoics attempted to reconcile it with their own rational doctrine of strict causation.
[35] To fully characterize the physical world, the Stoics developed a theory of mixing in which they recognised three types of mixture.
[55] 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.
[55] 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.
[56] Ancient critics often regarded this type of mixing as paradoxical since it apparently implied that each constituent substance be the receptacle of each other.
[60] 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.
[60] 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.
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.
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.
[60] 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.
[64] 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.
[64] 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.
[64] 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.
