In ancient philosophy the term “eclecticism” normally refers to philosophers who don’t adhere to a particular school of thought but borrow concepts and theories from multiple sources. In particular, the philosopher Antiochus of Ascalon, the teacher of Cicero, introduced an eclectic approach to the Platonic Academy, which attempted to assimilate elements of Stoic and Aristotelian thought into Platonism.
However, when people today talk about eclecticism they often just mean a more general notion of combining different philosophical elements. The Stoic school itself was originally “eclectic” in this sense. When Zeno, the founder of Stoicism, arrived in Athens, he found himself at a bookseller’s stall and allegedly read Xenophon’s Memorabilia of Socrates. (Possibly the section in which Socrates’ relates the parable called The Choice of Hercules, developed by his friend, the Sophist Prodicus.) According to other accounts, though, Zeno had already read the Dialogues of Plato in his youth.
Zeno was originally a wealthy Phoenician dye merchant who reputedly lost his fortune at sea, and was shipwrecked near Athens. The story goes that Zeno consulted the Oracle at Delphi and was told to “take on the colour of dead men”, which he interpreted as meaning he should study the philosophers of previous generations.
After reading about Socrates, Zeno turned to the bookseller and asked where he could meet such a man. The Cynic Crates of Thebes was walking past at that moment, and the bookseller pointed him out to Zeno, who became his follower for many years as a result. Crates had been the student of Diogenes of Sinope, the founder of Cynicism. Diogenes in turn was allegedly inspired by one of Socrates’ closest and most highly-regarded followers, Antisthenes. (Although modern scholars doubt Diogenes could have actually met Antisthenes, it’s quite possible he encountered his writings and perhaps even some of his followers.)
Hence, in the ancient world, authors such as Diogenes Laertius, claimed that the Stoic school founded by Zeno descended directly from Socrates via Antisthenes, and the Cynics Diogenes and Crates. This is sometimes called the “Cynic-Stoic succession” theory. Antisthenes was sometimes even referred to as the founder of Cynicism in the ancient world – he was reputedly nicknamed Haplokuon (“Absolute Dog”) and taught at the Cynosarges (“White Dog”) gymnasium.
We’re also told that Zeno studied in the Platonic Academy, under the famous scholarchs Xenocrates and Polemo. The Cynics did not teach Physics or Logic and it seems that Zeno felt this was a serious omission, which he could rectify by attending other more “academic” philosophical lectures. Nevertheless, we also know that Zeno attacked Plato’s philosophy, particularly in his Republic, a critique of Plato’s book of the same name.
We also know, however, that Zeno studied under philosophers of another Socratic sect: the Megarians and the associated Dialecticians. The Megarian school specialised in logic and was founded by Euclid of Megara, another one of Socrates’ circle. The head of the Megarian school in Zeno’s day, Stilpo, was considered one of the greatest intellectuals of his time. We find references to him and to Megarian Logic scattered throughout the surviving Stoic literature. Zeno also appears to have studied with another famous Megarian called Diodorus Cronus. Although the Megarians were particularly renowned for their Logic, they also had influential ethical teachings, which may have resembled those of the Cynics and other Socratic sects in holding that virtue is the only true good.
Zeno therefore studied under the main figures of all three major surviving Socratic sects of his day. Xenophon, whose Memorabilia Zeno had read, greatly admired Antisthenes. Like the Stoics, the Cynics believed that virtue was the only true good. Antisthenes also appears to have believed this, and perhaps to have attributed it to Socrates. Likewise, Xenophon appears to suggest this was Socrates’ ethical philosophy. There are some indications that this was also the position of the Megarian school, and possibly they too derived it from Socrates. The Stoics therefore possibly believed that the doctrine that became associated with them, that virtue is the only true good, was derived ultimately from Socrates himself, via most of his followers, with the notable exception of Plato, who portrays Socrates saying this in some of his earlier Dialogues, but equivocates elsewhere. The Platonic Academy later became associated with the doctrine that virtue is the highest but not the only good, and that the good life is composed of virtue in combination with bodily and external goods, which was also the position of the Aristotelian school.
On some accounts, Zeno is surprisingly silent about Aristotle. However, Plutarch claims that Zeno was heavily critical of the Peripatetic school. Aristotle stood somewhat outside of the Socratic tradition, and Zeno apparently did not choose to study in the Lyceum. The early Stoics generally appear relatively uninterested in Aristotle’s philosophy.
Those were the major Hellenistic influences on Stoicism. However, there are also many references to the pre-Socratic philosophy of Pythagoreanism in the surviving Stoic literature, particularly to the Golden Verses of Pythagoras. We know that Zeno wrote a book on Pythagoreanism. So he may have studied Pythagorean writings when developing his conception of the Stoic philosophy.
It’s also generally believed that the pre-Socratic philosopher Heraclitus was important to Stoicism. There’s no evidence Zeno studied Heraclitus. However, Cleanthes, Zeno’s successor, the second head of the Stoa, wrote a book on Heraclitus. For all we know, though, Zeno may have also referred to Heraclitus in his own writings on Physics and that may be why Cleanthes chose to write about him.
These philosophies all influenced the development of Zeno’s original Stoic school, and they continue to be influential among Stoics right down to the Roman Imperial age. That’s probably because references to these different schools of philosophy were interspersed in the canonical Stoic writings, which subsequent generations of Stoics continued to read. As far as we’re aware, among other topics, Zeno wrote books on Plato (The Republic), Pythagoras (Pythagorean Questions), and Memorabilia of Crates the Cynic. He also wrote books on Homer and other poets.
These different philosophical and literary traditions influenced Stoicism in many ways. However, the Stoic curriculum combined the scholarly study of Physics, Ethics and Logic, probably influenced by the scope of the Platonic Academy. It’s sometimes said, crudely, that the Stoics derived their Ethics from the Cynics, their Logic from the Megarians, and their Physics from Heraclitus. They also appear to have been particularly known for taking the Cynic ethical doctrine that virtue is the only true good (perhaps shared with Antisthenes, Xenophon, and the Megarians) and reconciling it with the Platonic (and Aristotelian) doctrine that the good life consists in a combination of virtue, bodily, and external goods. They did this by introducing the novel concept that virtue consists in making proper selections between things of secondary value (axia), known as “preferred indifferents”.
Although Zeno was the founder of Stoicism, it’s sometimes held that the third head of the Stoa, Chrysippus, was in fact the most influential Stoic scholarch. He was by far the most prolific of the school’s founders, writing over 700 texts. Chrysippus modified the earlier doctrines of Zeno and Cleanthes significantly. It was traditionally claimed that he did so in order to defend them against an onslaught of criticism from the rival Epicurean and Academic schools, particularly from the Academic Skepticism of Arcesilaus, which was just being developed, prior to Arcesilaus being appointed head of the Academy.
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