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ogether with the Epicureans and the Sceptics, the Stoics dominated philosophy during the Hellenistic period, i.e. during the three centuries afer Aristotle’s death in 322 B CE . Tey amously claimed that philosophy has three parts that are closely interrelated: physics, ethics and logic. Given their belie in the rationality o nature, they treated logic as inseparable rom the other parts o philosophy; whereas the end o physics is knowing the world, and that o ethics is living in accordance with the natural order, logic aims at distinguishing the true rom the alse and thus makes it possible to find out the truths o the other parts o philosophy. In other words, the purpose o logic, according to the Stoics, was the establishment o a true and stable understanding o the cosmos that was supposed to be essential to human beings, i they were to live a well-reasoned and happy lie. o show the special role o logic in the interrelation between the three parts o philosophy, the Stoics compared logic to the shell o an egg, to the surrounding wall o a ertile field, to the ortification o a city, or to the bones and sinews o a living being (e.g. Diogenes Laertius 7.40 = LS 26B; Sextus Empiricus,

Against the Mathematicians 7.19 = L S 26D).

Te Stoics did not use the term ‘logic’ (logikē ) as we do nowadays. Logic or them was the study ologos , that is, the study o reason as expressed in all orms o articulate speech; it was meant to help people ask and answer questions methodically, argue correctly, clariy ambiguous statements, solve paradoxes. Tus understood, logic was divided by the Stoics into rhetoricand dialectic: rhetoric was defined as the art o speaking well in the orm o whole, continuous speeches; dialectic, on the other hand, was defined as the art o conducting discussions by means o short questions and answers, though in a much broader sense it was also defined as the science o what is true and what is alse. More

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specifically, Stoic dialectic was subdivided into the topics o significations and utterances, or it separately studies what is signified by our utterances and the utterances themselves. Te study o utterances includes purely linguistic and grammatical phenomena; or instance, it includes a physical account o sound appropriately ormed by the speech organs, a discussion o the phonemes or letters o the alphabet, an analysis o the parts o speech, an examination o the

criteria or good style. Te study o what is signified covers what gets said by using all sorts o utterances, but mainly by using propositions, the relations between them, the arguments composed o such propositions, and especially their validity (e.g. Diogenes Laertius 7.41–4 = LS 31A; Cicero,On the Orator

2.157–8 = LS 31G). Hence, the modern sense o logic was treated by the Stoics under dialectic, i.e. under the subpart o the logical part o Stoic philosophy, although they also treated under dialectic what we would nowadays call grammar, linguistics, epistemology and philosophy o language (Gourinat 2000). In what ollows, I ocus on the part o Stoic dialectic that corresponds to our modern sense o the term, reerring to it as Stoic logic.

Chrysippus, the third head o the Stoic school, developed Stoic logic to its highest level o sophistication. It is reported that he was so renowned in logical matters that people believed that i the gods had logic, it would be no different rom that o Chrysippus (Diogenes Laertius 7.180 = LS 31Q). Te ounder o the Stoic school, Zeno o Citium, and its second head, Cleanthes, were not logicians in the sense that they constructed a ormal logical system, but they both used valid arguments o a considerable degree o complexity; given the rather standardized patterns o their arguments, they must have been aware o the logical orms in virtue o which these arguments were considered as valid (c. Schofield 1983; Ierodiakonou 2002). On the other hand, Zeno’s pupil, Aristo, ervently advocated the view that only ethics should be studied, because physics is beyond us and logic is none o our concern. He claimed that logic is not useul, since people who are skilled in it are no more likely to act well, and he compared logical arguments to spiders’ webs (Diogenes Laertius 7.160–1 = LS 31 N). In general, though, the Stoics studied logic systematically and used it assiduously. Tey were, in act, ofen criticized or being overconcerned with logical orm, or elaborating empty theories, and or ignoring the useul parts o logic such as scientific proos. Diogenes Laertius’ catalogue o Chrysippus’ books lists 130 titles in its section on logic, while its section on ethics contains several titles that suggest a logical content; moreover, other ancient sources offer a ew supplementary titles o Chrysippus’ logical treatises. But although Chrysippus was undoubtedly the authoritative figure in Stoic logic, there seem to have been

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divergences within the Stoa over logic, so that it is reasonable to suppose that, at least in minor ways, Stoic logic changed in the course o its long career (e.g. Sextus Empiricus, Against the Mathematicians 8.443 = L S 36C).

o better understand the emergence o Stoic logic, it is useul to look into the logical background out o which it historically developed (c. Barnes 1999b). Tere are two philosophical schools that may have influenced Stoic philosophers

in their logical endeavours: first, Aristotle and his ollowers, namely Teophrastus and Eudemus; and second, the Megaric or Dialectical school, namely Diodorus Cronus and Philo the Logician. Tere is no need to search or the influence o Epicurus, since he explicitly rejected logic, though later Epicureans did show some interest in such things as the truth conditions o conditional propositions. And there is no evidence that the philosophers in Plato’s Academy, who at the time adhered to the sceptical stance, attempted to advance logic.

Te established view in the nineteenth century was that Stoic logic should be considered as a mere supplement to Aristotle’s logical theory; or Stoic logic, so it was alleged, does nothing more than either copy Aristotelian syllogistic or develop it in a vacuous and ormal way. It is only since about the middle o the twentieth century, afer the important advances in ormal logic, that it has become obvious how Stoic logic essentially differs rom Aristotle’s (c. Łukasiewicz 1935; Kneale and Kneale 1962). It has even been suggested that the Stoics could not have been influenced by Aristotle, since his logical treatises were not available to them and only recovered in the first century BCE (c. Sandbach 1985). Tis view, however, has been extremely controversial. Afer all, even i the Stoics were amiliar with Aristotle’s works, there can still be no doubt concerning the srcinality o their logical system. For although it is true that Teophrastus and Eudemus published treatises on what they called ‘syllogisms based on a hypothesis’, which Aristotle had promised to write about in hisPrior Analytics

but never did, and these syllogisms have a great deal in common with the types o arguments discussed by the Stoics, there is no evidence that the Peripatetic logicians anticipated the outstanding eature o Stoic logic; that is, the construction o a logical system to prove the validity o a whole class o arguments o a different kind than those Aristotle ocused on in his syllogistic (c. Barnes 1999b).

As ar as the Megaric or Dialectical school is concerned, the historical connections between the philosophers o this school and the Stoics are well documented. Zeno knew both Diodorus’ and Philo’s works well, and Chrysippus wrote treatises in which he criticized their logical theses. Although Diodorus and Philo are usually presented as mainly occupied with the study o logical

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puzzles or paradoxes, it is clear that they also put orward srcinal views about, or instance, logical modalities and the truth conditions o conditional propositions (see below). But they never came close to ormulating a logical calculus as elaborate and sophisticated as that o the Stoics (c. Bobzien 1999).