by Anitra Pavlico
Most of the modern revival of Stoicism has centered on the works of Seneca, Epictetus, and Marcus Aurelius–all from the “Late Stoa,” or the third phase in the history of Stoicism. No complete works survive from either the early or middle period.
Chrysippus (c. 279-206 B.C.E.) was one of the most influential early Stoics. He studied with Cleanthes, who studied directly with Zeno, who founded the school in 262 B.C.E. You won’t see his quotes on any inspirational desktop art, but Chrysippus was perhaps the one most responsible for keeping Stoicism alive in its early years. A later Stoic saying was, “If Chrysippus had not existed, neither would the Stoa.”
Chrysippus was prolific, having reportedly written over 705 books. No single book remains, and today we only have around 475 fragments. He was the second great logician, after Aristotle. Diogenes Laertius reported in his Lives of the Philosophers that most people believed that if the gods were to pursue dialectic, they would adopt Chrysippus’ system alone. The seemingly airtight logic of many Stoic approaches to life, as they have filtered down to us, stems directly from Chrysippus. The Stoic philosophy featured three branches: logic, physics, and ethics. The scope of logic also included the analysis of argument forms, rhetoric, grammar, propositions, perception, and thought.  Josiah Gould points out that Chrysippus felt that logic should be studied before the other two branches of the philosophy.  While Gould notes that it is lamentable that we do not have a single full logic treatise when they have such “tantalizing” titles as On Negative Propositions, An Introduction to the Study of Ambiguity, On Imperatives, and Reply to Those Who Think that a Proposition Can be Both False and True, he maintains that we can reconstruct some of his views based on the fragments we possess.
Chrysippus had groundbreaking views on epistemology, which was a key subset of Stoic logic. An important Stoic concept, discussed for example in Epictetus’ works hundreds of years later, is that of the “impression” and of our assent, or refusal to assent, after deliberation. Epictetus writes in his Enchiridion, “Make it, therefore, your study at the very outset to say to every harsh external impression, ‘You are an external impression and not at all what you appear to be.’”
An impression, or phantasia, comes from the word for light (phõs), Chrysippus believed, because “the presentation reveals both itself and that which caused it.” A presentation to the soul is sometimes the result of imagination, however: Chrysippus called this a phantastikon, and it can occur when we see an actor playing a part. He also classified other impressions, not based on real objects, as figments (phantasma). A schizophrenic who hears voices would be familiar with these figments. 
Chrysippus rejected the model of the soul proposed by his teacher Cleanthes, who said an object impresses itself on the soul just as a signet ring impresses itself on a wax seal. Chrysippus, meanwhile, argued that the soul, unlike wax, can receive and retain more than one impression at a time. Chrysippus recognized that some of the impressions we receive are not so obvious as a shape pressed into wax. Instead of a wax tablet, Chrysippus postulated that the soul was like air, in that it can be altered in many ways at the same time. Gould calls this “a brilliant endeavor to make Stoic epistemology more tenable without abandoning its materialistic premises.” Chrysippus rejected Plato’s tripartite soul, teaching rather that the soul was unified, but with various functions and conditions. Further, the soul was corporeal, interpenetrating the body, and composed of pneuma, a fiery, airy substance–in keeping with those aforementioned materialistic foundations. The ruling center of the soul, Chrysippus said, was located in the heart. 
While we have inherited relatively few fragments of Chrysippus’ writing, we can rely on other ancient writers’ exegeses of Chrysippus. The 4th-century philosopher Calcidius reported this on Chrysippus’ theory of how the soul obtains knowledge:
The whole soul extends its senses, which are functions of it, like branches, from that ruling part, as from a tree; and these senses are to be reporters of those things which they perceive, while the ruling part itself, like a king, passes judgment upon those things which the senses will have reported. Moreover these things which are sensed, namely, bodies, are composites, and thus each sense perceives some one ingredient in the composition; this one, color; another, sound […] And all this is concerned with what is present; however, no sense remembers what is past or apprehends what is future. Rather it is the peculiar function of inner deliberation and reflection to observe the affection of each sense and to infer what this object is from those data which the senses report, and to apprehend what is present, and moreover to remember what is no longer present, and to foresee what will happen.
When the soul is affected by something external to it, Chrysippus said, it undergoes a change. It seems that Zeno and Chrysippus thought of these external things as either “impressions,” as mentioned above, or “presentations.” Something outside of the soul impresses itself upon it as it presents itself. Chrysippus himself explained the process thus: “For example, when through the power of sight we see white, that which comes about in the soul through the act of seeing is a modification. And on the basis of this modification we are able to say that the white which is affecting us exists.”
Stoic logic was considered an indispensable tool in the development of knowledge (epistēmē). It was Zeno who formulated the notion of katalepsis, from the word “to grasp,” sometimes translated as “cognition.” (Cicero uses comprehensio.) An impression leads to a proposition, which the mind/soul then assents to or rejects. Katalepsis occurs when the mind perceives that an impression could not be wrong, and assents to it. Epistēmē is the body of knowledge that we gather from a lifetime of true impressions.
So how do we know that we are not falling prey to a false impression? Well, good question. This is one of those thorny problems in philosophy that has garnered capital letters in its title: the Problem of the Criterion. What is the criterion of knowledge? The Stoics had theories, but they were certainly not without their critics. As John Sellars writes in Hellenistic Philosophy, the Stoics–in opposition to those in the school of Academic skepticism–denied that false impressions “are ever convincing enough to be confused with genuine kataleptic impressions.” We are meant to judge an impression in relation to other impressions we have perceived, not in isolation. Stoics believed that the expert can always distinguish between a false and true impression, even if they appear identical. This might look like an optimistic view of humanity until we recall that they also believed that the Stoic expert, or sage, was so rare as to be practically unfindable.
Nevertheless, Chrysippus believed that humans’ “ruling center” was something we have all been given by nature. When the Stoics speak of living in accordance with nature, they mean, at least in part, relying on our rational capability to determine which impressions are correct and which ones are not. The ruling center helps us determine which impressions are worth assenting to. It was something to safeguard: Epictetus wrote that we should “take care to avoid harming” it.
As important as they felt it was to draw correct conclusions, the Stoics stopped short before agreeing with the Skeptics that no one should assent to any impressions. Cicero describes Stoics as believing that to live requires assenting to impressions; it was disingenuous to refuse to assent to anything. To live means to act. Action requires beliefs, and beliefs arise from impressions. There are times when we have to choose the conclusion that seems most reasonable.  Near-certainty, when reasonable, will have to suffice if we want to live fully. The Stoics did not necessarily want to ride an epistemological merry-go-round: they believed that one philosophized in order to live, and not the other way around.
 See “Stoicism,” Stanford Encyclopedia of Philosophy, at https://plato.stanford.edu/entries/stoicism
 Josiah Gould, The Philosophy of Chrysippus 45-60 (Leiden, E.J. Brill: 1971).
 See “Chrysippus,” Internet Encyclopedia of Philosophy, at https://www.iep.utm.edu/chrysipp/#SH6a
 Understandably, Galen criticized this element of Stoic teachings.
 John Sellars, Hellenistic Philosophy 42 (Oxford University Press: 2018), discussing Cicero’s Academica.