Stoic physics and metaphysics
Stoic physical theory is radically corporealist or materialist: only bodies exist in the full sense, because only bodies have the capacity to act or be acted upon. This has sweeping consequences — the soul is a body, and so are its attributes, meaning that wisdom itself is corporeal.
Bodies and incorporeals
Bodies do not exhaust Stoic ontology. Four "incorporeals" are said to subsist (huphistanai) — a distinct, lesser ontological status — rather than exist: time, place, void, and lekta (sayables). Void is intangible, shapeless, and passively receptive of body; it surrounds the cosmos and enables motion. Place is similarly a receptor of body without causal power. Time is more complex: some temporal entities (days, years) are bodies, while the past and future subsist and the present "obtains" (huparchein). Lekta — roughly, what can be said or expressed — form the contents of rational impressions and thoughts, and are discussed further under Stoic logic, language, and epistemology. Together, bodies and incorporeals share the status of being "something" (ti), the Stoic supreme genus broader than being itself.
The two principles, pneuma, and mixture
Fundamental to Stoic physics are two principles (archai): the active and the passive. The passive principle is primary matter — eternal, unqualified, formless, and inert. The active principle, identified with God or Zeus, acts on matter throughout the cosmos. Both principles are blended "through and through," and the Stoics developed a theory of mixture allowing two bodies to occupy the same place simultaneously.
The vehicle of the active principle is pneuma ("breath"), which pervades all bodies. Pneuma's inherent "tensility" — a simultaneous inward and outward movement — gives bodies their qualities and makes them unified objects. It comes in grades: as hexis (tenor), it sustains inanimate objects; as phusis (nature/physique), it regulates vital functions in plants; as psuchê (soul), it enables perception and self-locomotion in animals. The four elements — fire, air, water, earth — also play a role, with the active principle associated with fire and air and passive matter with water and earth.
Cosmic cycle and conflagration
The Stoics hold that the cosmos passes through endless cycles: periods of conflagration (in which all is fire) alternate with periods of ordered cosmic existence. The generation of the cosmos from fire and back again was the orthodox Stoic view, though Panaetius reportedly rejected it. Eternal recurrence raises deep questions: will there be numerically identical persons in the next world cycle, or merely indistinguishable ones? Different Stoic sources give different answers. The doctrine also bears on their conception of time as the "dimension of motion."
God and providence
God is corporeal — identical with the active principle — and is described as eternal reason (logos) or intelligent designing fire (pneuma) that structures matter according to a rational plan. The Stoic God is immanent throughout the cosmos, not transcendent; the cosmos is a living thing and God stands to it as an animal's soul to its body. Divine actions are orderly, rational, and providential, not arbitrary as in Greek mythology. The designing fire is likened to sperm containing the first principles of all that will develop — the logoi spermatikoi.
Causal determinism and compatibilism
The Stoics are determinists: every event is fully determined by a prior chain of causes, which they identify with the rational and inescapable will of Zeus (fate). This raises two practical worries: the "lazy argument" (if all is fated, why act?) and the threat to moral responsibility. Chrysippus responded to both.
Against the lazy argument, he argued that many events are co-fated: the doctor's arrival is fated only as part of the same causal chain that includes your calling her. Your action is not superfluous — it is itself the cause of the outcome.
Against the threat to moral responsibility, Chrysippus distinguished types of causes. Not all antecedent causes are sufficient for their effects; some are merely "auxiliary and proximate." His famous cylinder-and-cone analogy illustrates this: given an equal push, a cylinder rolls straight while a cone spins — because each moves according to its own nature, not merely the push. Likewise, a human agent's psychological profile is causally efficacious for action over and above any external stimulus. Assent — the rational soul's judgment about what is fitting — is "up to us" in a way that makes moral responsibility coherent even within a deterministic causal order. For detailed treatment of freedom and determinism in Stoicism, see Bobzien (1998).
Stoic psychology and the rational soul
The Stoic account of the soul belongs to physics. The soul is pneuma at a high degree of tensility, and its defining features — for animals — are impression (phantasia) and impulse (hormê). Plants have tenor- and nature-pneuma but not soul; ensouled life is marked by perception and self-locomotion.
Among ensouled creatures, mature humans possess reason (logos). The crucial faculty of rational souls is assent (sunkatathesis), located in the commanding faculty (hêgemonikon): the power to adopt a critical stance toward impressions and pass judgment on them. Rational creatures give assent to an impression that p when they deem p true; otherwise they suspend assent. Assent is "in our power" in a way that receiving an impression is not. What kind of person we are — virtuous or not — depends entirely on our acts of assent. Assent generates either a doxastic commitment (knowledge, cognition, or opinion) or a rational impulse. Since assent is necessary and sufficient for any rational impulse, the Stoics endorse strong motivational cognitivism: human action always reflects the rational mind's judgment about what is appropriate. This rules out genuine simultaneous akrasia, though rapid oscillation between opposing judgments was acknowledged by some Stoics. Human beings are not born with reason but acquire it around age seven; before that they employ a non-rational soul like other animals.