Chrysippus was the most prolific of all the Stoic philosophers. Sadly, only a few fragments of his over 700 works survive today. His philosophical style was to summarize the arguments of his opponents, usually quite fairly, and then provide his own position in a reply.

He was third to head the Stoa, after the founder, Zeno of Citium, and Cleanthes, both of whom were his teachers. He may also have studied with Arcesilaus, head of the (Platonic) Middle Academy. Chrysippus not only synthesized earlier Stoic thought into a philosophical system, but also integrated his great understanding of Hellenistic physics and formal logic, including propositional logic.

Many of the classical unsolved problems in philosophy are the direct result of thinking that they could be solved by logic, reason, and a deterministic physics. Although Chrysippus established deterministic physics, he challenged the idea that logical truths necessitated physical events (logical determinism). An event is only necessitated if the physical causes for that event exist in the present.

Chrysippus opposed the atomists and Epicurus' idea of irreducible chance in the universe. Stoic physics had no room for discrete entities like atoms. It was a continuum theory perhaps inspired by Parmenides, a plenum of material infinitesimals in contact everywhere, although Chrysippus admitted an external void (κενόν) surrounding the cosmos (ὅλον κοσμος).

In early Stoic philosophy, every event has a cause, and causes necessitate their effects.

Stoics, including Chrysippus, thought they could prove the existence of causes from a logical analysis of statements about the future. This problem of future contingents was dealt with by Aristotle (de Interpretatione, IX), who denied the present truth or falsity of statements about the future. As Cicero describes it:

Chrysippus argues thus: If uncaused motion exists, it will not be the case that every proposition (termed by the logicians an axioma) is either true or false, for a thing not possessing efficient causes will be neither true nor false; but every proposition is either true or false; therefore uncaused motion does not exist.
(Cicero, De Fato X, 20)

Chrysippus claimed that a single uncaused cause could destroy the universe (cosmos), a concern shared by some modern philosophers, for whom reason itself would fail.

Everything that happens is followed by something else which depends on it by causal necessity. Likewise, everything that happens is preceded by something with which it is causally connected. For nothing exists or has come into being in the cosmos without a cause. The universe will be disrupted and disintegrate into pieces and cease to be a unity functioning as a single system, if any uncaused movement is introduced into it.

Despite his fear of chance, Chrysippus later loosened the strictness of determinism by separating logical necessity from causal determinism and the idea of fate. He argued that all things are fated, including human decisions. But although the past is fixed and unchangeable, and all the antecedent events fated, future events are not necessitated logically unless the causes for the future event exist at the present time.

Chrysippus thus eliminated the non-causal and arbitrary fatalism which maintains that a future event will happen no matter what we do in the meantime. This gave him room for his subtle compatibility between free will and determinism. Our fated actions are a necessary part of the causal chain that brings about the future which Chrysippus needed to establish moral responsibility.

Since future events are not necessary (though they are fated), human decisions are not constrained or forced by antecedent events or anything external to the mind. This lack of coercion, including one's heredity and environment, was critical for Chrysippus' idea that we have a freedom to assent (or not to assent) that made our decisions "depend on us." He called this πάρ’ ἡμᾶς or ἐξ ἡμῶν, depending on us, similar to Aristotle's ἐφ ἡμῖν.

This is the core idea of modern compatibilism. Chrysippus was thus the first compatibilist.

Although on the surface, being able to act (assent) or not act in a given circumstance seems inconsistent with causal determinism and the Stoic belief in an "eternal return" or "great cycle" in which the world would repeat everything exactly as they occurred in the past, modern philosophers (e.g., G.E.Moore) take this to mean "could have acted differently if one had chosen to do so."

Notice that in Chrysippus' compatibilist freedom our decisions are determined by our character and values, which were partially determined by factors beyond our control like heredity and environment. But they also include factors that we acquired freely in learning and training by our parents and educators.

Thus our character in not necessitated, though it is fated by the Stoic dogma of universal reason and lawful causal nature. Since the Stoics saw God as Nature, Chryssipus' idea of a fate compatible with freedom seems parallel to the religious idea of divine foreknowledge of our decisions that is compatible with our free will.

In this respect, Stoic determinism is less a physical determinism than a teleological or theological determinism. (See our dogmas of determinism.)

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