 Hi, I'm Peter Adamson, and you're listening to the History of Philosophy podcast, brought to you with the support of King's College London and the Leverhulme Trust, online at www.historyoffilosophy.net. Today's episode, Walking on Eggshells, the Stoics on Logic. When they invent time travel, I bet the first thing they will do is go back to ancient Athens to meet Socrates and tour the Acropolis. I also bet they will be disappointed. They'll probably forget to bring someone who can speak ancient Greek, so they won't get much out of Socrates. As for the sculptures and buildings, they will be surprised and a bit appalled to find that these things were all painted in bright, even gaudy colors. For us, classical sculpture is pristine and white, marble made flesh. But that's just because the paint has worn off. Downtown, they'll find more paintings in a space adjoining the Agora, or marketplace. It was a long-covered colonnade for walking to and fro called the stoa poikile, the painted porch, for the murals that decorated it. If the time travelers get their coordinates slightly wrong, and arrive a few decades after the death of Socrates, they'll still get to meet some philosophers if they venture to this public porch, because the stoa was not only the name of a porch, it was also the name of the greatest Hellenistic philosophical school, named after its favorite hangout. The early stoa, better known to us as the Stoics, sought to follow Socrates' example in many things, including his habit of doing philosophy in the middle of urban life. Their porch makes a striking contrast to the garden, academy, and lyceum. Whereas Epicurus, Plato, and Aristotle set up shop on the fringes of Athens, the Stoics planted themselves next to the marketplace. Like Socrates, they wanted to engage with their fellow citizens and make demands of them. Epicurus' philosophy is an invitation, a seduction, a promise. Pursue a moderate life of pleasure, he says, and learn to escape all disturbance. In the hands of the Stoics, philosophy is instead an accusation, a gauntlet thrown down. Those who fail to seek and obtain wisdom are not just missing out on a great good, they are irrational. They are, the Stoics went so far as to say, insane. Of course, Epicurus too disapproved of the way most other Greeks lived, but he was willing to meet them halfway. In pursuing pleasure, they had the right basic idea, it was just that their strategy for getting pleasure was all wrong. The Stoics were far more radical. Pleasure, along with honor and all the other things non-philosophers value, are actually valueless. Socrates had it right, seek wisdom or live a life of folly, a life befitting slaves. Now, I know what you're thinking. Isn't this an attitude towards philosophy and a way of imitating Socrates that we've already seen, with the Cynics? Why, yes it is, and that's no accident. The founder of the Stoic school, Zeno of Kitium, supposedly studied with the Cynic philosopher, Kratis. You might remember him as the successor of Diogenes the Cynic, the one who joined with Hipparchia in a Cynic marriage. To study with Kratis, Zeno first had to make the journey from Kitium, in Cyprus, to Athens. I mentioned in the last episode that 1st century BC philosophers like Cicero and Philodemus the Epicurean were still making the pilgrimage to Athens, and the city remained a center of philosophy for centuries after that. In the ancient world, Athens was to philosophy what Las Vegas is to gambling. The phenomenon was already well entrenched in the early Hellenistic period, with all the main Stoics turning up in Athens, even though they were born elsewhere. The ancient sources tell us that Zeno started his philosophical career as an admirer of Socrates and a follower of Kratis. We get the usual round-up of anecdotes about him, some of them describing how Kratis tried to school him in the Cynic lifestyle, for instance by making him carry a pot of lentils with him wherever he went. There is some independence evidence for this link to Cynicism. Zeno wrote a work called The Republic, known only through later reports. Despite its platonic title, Zeno's Republic apparently defended a broadly Cynic political program. It proposed abolishing many of the social conventions Diogenes the Cynic had rejected. Currency was to be not merely defaced, but eliminated. No temples were to be built to the gods. Traditional education was deemed worthless, and all other men should be taken as our fellows, joining in a community on the basis of virtue, rather than kinship. Zeno's utopian ideas were not retained as a core of the Stoic manifesto, but his uncompromising ethical stance certainly was. However, this is only one aspect of Stoicism, and doesn't come close to capturing the full breadth of their philosophical vision. The Stoics were pioneers in just about every area of philosophy and married their ethical absolutism to careful technical analysis. Because he was the school founder, Zeno's authority and status as a moral exemplar was unquestioned by all card-carrying Stoics. Many of their signature doctrines can be traced to him. But it was only with Zeno's successors that Stoicism matured into a system that would dominate ancient philosophy well into the age of the Roman Empire. After Zeno's death in 262 B.C., the headship of the school was taken over by a man named Cleanthes. Thirty years later he was succeeded by Chrysippus, who served as school head from 232 until his death in 206 B.C. It is to Chrysippus that we owe the subtle and rigorous systematization of Stoicism, without which it would never have become the greatest of the Hellenistic schools. Unfortunately, not a single one of Chrysippus' works, and he wrote well over a hundred of them, has survived to us today. Instead, the reports of his ideas are often found in hostile authors like Galen, early Christian authors, and members of rival schools. Nor is it always easy to pry apart the specifically Chrysippan material from general reports about Stoic doctrine. Still, the evidence at our disposal shows that Chrysippus was probably the most sophisticated and influential ancient philosopher to work between Aristotle and Plotinus. Sorry, Epicurus. He excelled in the cut and thrust of inter-school debate, and, for this sake, sharpened Stoic doctrines and techniques of argument. Until we reach the predominantly ethical approach of Stoics like Epictetus in the age of the Roman Empire, Stoicism centers around the teachings of Chrysippus. Nonetheless, the Stoics were admirably willing to make adjustments to their doctrines in response to internal and external debate. To understand the technical proficiency that Chrysippus brought to Stoicism, we need to consider a further historical influence from the age of Zeno. This, remember, was the generation or two after Aristotle, roughly the same time that Epicurus was putting forth his hedonistic ideas in competition with the more unrestrained pleasure principle of the Cyrenaics. At this time, cynicism too was a going concern, an influence on young Zeno, as we just saw. The Platonic Academy and Aristotle's followers were also active. So, this was a time of great intellectual ferment, with the early schools all engaging with one another in mutual criticism and polemic. A minor player in this period, but one important for our story, is Diodorus Cronus. Ancient sources make Diodorus the lead thinker of the dialectical school, specialists in logic and technical arguments. On several issues, it would seem that Diodorus and his followers goaded the Stoics, and especially Chrysippus, into careful reflection on argument forms, fallacies, and so on. The Stoics were thus provoked to develop some of their most subtle philosophical ideas, much as the Sophists had provoked Plato and Aristotle. It was in part thanks to fruitful competition with the dialectical school that the Stoics achieved so much in logic. The Stoics placed great value in logic, considering it to be one of the three main parts of philosophy along with physics and ethics. Taking their lead from Zeno, who was always good for a vivid metaphor, the Stoics compared the three parts of philosophy to an egg. Logic is the shell, physics the egg white, and ethics the egg yolk. Another version switches physics and ethics around, but logic is still the shell. There are other metaphors given too. Logic is a wall around a field, the trees in the field are physics, and the fruit is ethics. This threefold division might seem to leave some things out, like philosophy of mind and metaphysics, but the tripartite scheme is as much a declaration of what the Stoics do not do as a statement of what they do. The omission of any metaphysics distinct from physics is deliberate. The Stoics are materialists. Their theory of mind too is to be found in their physics. Part of the point of the metaphors though is that the three parts of philosophy are closely connected. Thus they also compare philosophy to a living body. Logic is the bones, ethics the flesh, and physics the soul. Later on, Aristotelians insisted that philosophical disciplines had to be taken in a strict order and that one should start with logic, meaning, Aristotle's logical works. The Stoics disagreed. For them, philosophy was an organic unity, and a question raised in ethics could lead you to physics or logic, and vice versa. Ultimately, wisdom consists in mastery of all three parts. Expertise in only one would not merely be inadequate, it would be impossible, because the three are so closely intertwined. The Aristotelians objected to the very claim that logic is a part of philosophy. As we already saw, for the Aristotelians, logic is something a bit less exalted than a part of philosophy. It is rather an instrument. This is why they called those logical works of Aristotle the organon, meaning tool or instrument. To refute the Stoic view, they pointed out that a logical argument form is empty until it is, so to speak, filled out with specific terms. The logician tells you that it is valid to argue, all A is B, all B is C, therefore all A is C. But you aren't doing philosophy until you substitute some words for those letters. Throw in giraffe for A, ruminant for B, and plant-eaters for C, and then you're in business. To be specific, the business of biology. You've just given a demonstration explaining why giraffes eat plants. More generally, we can say that for the Aristotelians, logic was not yet a part of philosophy because it could not on its own give us knowledge. It only gives us an understanding of the argument forms, which can be used to obtain knowledge. Now, the Stoics were certainly no slouches when it came to considering argument schemes, but their understanding of logic included much more than this. For them, logic embraced analysis of language and rhetoric. They also realized that logical points are not always philosophically innocent, as the Aristotelians pretend. For instance, their understanding of the logical notion of possibility is intimately connected with their physics. In fact, the Aristotelians had a similarly wide-ranging conception of logic. Remember that they too wound up counting rhetoric as part of the logical curriculum, and that Aristotle's posterior analytics—a work on philosophy of science or epistemology—was seen by them as the capstone of the organon. In light of this, I tend to think the Stoics had the better of this particular debate. The disagreement between Stoic and Aristotelian logic is deeper than this, though. When the Stoics actually start doing logic proper, they take a very different approach to what we find in Aristotle's prior analytics. As we saw, Aristotle's logic is a theory of predication. It examines the relationships we can draw between claims like A is B and some B is not C. The Stoics, by contrast, tend to give examples involving claims like, It is day! or This man walks. They call these simple assertions and say that they are complete because they can be either true or false. An incomplete assertion would be, for instance, This man. You have to add something further, like, Walks or hosts a podcast or is devastatingly handsome, to get something that can be true, or, you know, false. The next step is to think about how simple assertions can combine into more complex assertions. There are basically three ways to connect them. If, or, and, and. You wouldn't think that these three little words would cause controversy, but they do. The troublemaker is that word if. Suppose I link two irrelevant statements with if and then. For instance, if giraffes are animals, then Socrates died of hemlock. Is that true? Or, I might include a false statement like this, If giraffes fly, then Socrates died of hemlock. Not so obvious what to say, Today's logicians and some ancient ones tend to think that if-then statements are true, just so long as they do not infer something false from something true. Suppose I say, If Socrates died of hemlock, then giraffes fly. That has to be wrong. After all, Socrates did die of hemlock, but giraffes don't fly, any more than pigs do. But the other way around looks fine, at least harmless. Since giraffes never fly, there's no harm in saying that if they did, Socrates would die of hemlock, or for that matter, not die of hemlock. But this was not satisfactory to some ancient logicians, for instance our new friend, Diodorus Cronus. And one is tempted to agree with him. Surely the point of asserting if X then Y is to claim that X and Y have some kind of connection. Diodorus gets closer to this by adding another requirement, it can never be the case that X is true and Y is false. This helps him rule out the truth of assertions like, If chickens are birds, then Socrates died of hemlock, because there was a time when chickens were birds, but Socrates had not yet died of hemlock. Chrysippus basically agrees with Diodorus, but goes further. He and other Stoics speak of a coherence between the two parts of an if-then statement. I am only allowed to infer Y from X if the truth of X somehow rules out the falsehood of Y. For instance, suppose it is always true that penguins are birds, and that giraffes are ruminants. Still, it is not true to say, If penguins are birds, then giraffes are ruminants, because the fact that penguins are birds has nothing to do with giraffes. Here we catch a glimpse of how Chrysippus sees logic as relating to physics. The relation Chrysippus is describing sounds like a causal relation. Given the cause, the effect must follow. Once we sort out these issues about complex assertions, we're ready to build some arguments. These will involve at least two assertions, and of those, at least one will need to be complex. So, a standard Stoic argument might be, If it is day, then it is light. It is day, so it is light. Just as Aristotle itemized syllogistic forms and considered whether or not they count as valid, so Chrysippus identified five argument forms which he called indemonstrable. In other words, they are obviously valid arguments. The example I just gave, the one about daylight, would be the first indemonstrable, which has the form, If the first, then the second, but the first, so the second. This looks a bit like what we find in Aristotle. Logic is the study of arguments, so you begin by considering parts of assertions like, This man, or, Walks. Then you see how they combine into complete assertions, like, This man walks. Finally, you work on combining the assertions into argument forms, like, If this man walks, then he moves. This man walks, therefore, he moves. But, the differences with Aristotle are more striking. The Stoics do not lay particular stress on predication, and in fact, they are downright uncomfortable with the kind of predication that Aristotle considered most important. If you remember, these were universal predications, like, All giraffes are ruminants. The Stoics would be happier to put this differently and say, If something is a giraffe, it is a ruminant. In this case, with their rephrasing, the Stoics are registering their unwillingness to countenance universal entities. For them, everything that exists is material, so there is no such thing as a universal form or species, giraffe. There are only individual giraffes. Again, we see that logical points can have serious philosophical consequences. The Stoic view puts clear water between them and Plato, with his theory of forms, and also Aristotle, who tends to think that universal features of the world have some degree of reality. Let's end with a few logical puzzles. These were a favorite topic for Chrysippus, who wrote entire books about single paradoxes and puzzles, including the famous Liar and Psorites paradoxes. I've mentioned the Liar paradox before. It consists simply in a statement like what I'm now saying is false. If it's true, it's false. If it's false, it's true. Great paradox. As per the Psorites, the title comes from the Greek psoros, which means heap. The puzzle is this. You are asked whether one grain of sand constitutes a heap. Well, obviously not. How about two grains? Still not a heap. Your opponent keeps going, three, four, five, until you admit that we now have a heap. As soon as you do that, they say, you're telling me adding one grain of sand turns it into a heap? It's absurd to suppose that adding or subtracting only one grain would make a decisive difference. Yet, at some point, a heap must come into existence. So where do you draw the line? I'm going to leave you in suspense and wait until next time to tell you Chrysippus's solution. For now, I'll consider another logical puzzle he confronted, which introduces a major topic in Stoicism—modality. In other words, the concepts of necessity, possibility, and impossibility. Again, Diodorus Cronus provided the provocation with something called the ruling or master argument. The idea seems to have been this. Let's suppose I never get to be a ruler, for instance, the president of a country. It seems like I could be a president, even if I never am a president, but Diodorus argues otherwise. He exploits the fact that everything about the past seems to be necessary. After all, it's too late to do anything about the past. We cannot, for instance, change the fact that Socrates died of hemlock, so Socrates having died of hemlock is necessary. Now if, as we supposed, I will never rule, then it must have been true already in the past that I will never rule, but then this must be a necessary truth. After all, everything about the past is necessary. So, if it was already true yesterday, or a hundred years ago, that I will never be president, then a ship has already sailed. It is impossible for me to be president. This argument, and other arguments from the dialectical school, yield an unnerving result, namely that if something will never happen, then it cannot happen. Other deterministic arguments tried to show something even stronger, namely that each thing must happen at the time that it happens. Aristotle already confronted this prospect in his famous sea battle argument, and also in a passage from his metaphysics. There, he discusses a group he calls the Megarians, who claimed that everything that actually happens must happen. Chrysippus, like Aristotle, found these consequences troubling, so he proposed a novel conception of possibility and necessity. For Chrysippus, something is possible so long as it is not in itself impossible, and furthermore is not prevented from happening. Thus, many things that could happen do not happen. I don't have a sister, but I could have had one, because my parents were able to produce female children, and nothing intervened from the outside to stop them. But other things that never happen are impossible. Firstly, things that are in themselves impossible, like me lifting a skyscraper. Secondly, things that could occur in themselves, but are prevented from occurring. The classic example is a piece of wood at the bottom of the ocean, which in itself could burn, but is hindered from burning by being permanently under water. Chrysippus' interest in puzzles like the liar, sorieties, and master arguments was not mere idle speculation. We already saw that the dispute over conditional assertions connects to Stoic physics, because causation is a conditional relation. If I watch a Buster Keaton movie, I will laugh. The same goes for the dispute over modality. Chrysippus and other Stoics were causal determinists. They thought that true causes inevitably give rise to their effects, and that all things arise in this way. But they were not logical determinists or fatalists. They still wanted to distinguish between possible and necessary truths. We'll see why in a couple of episodes. As for the liar and the sorieties, Chrysippus needed to solve these two. This results from his epistemology, which envisions the possibility of an infallible sage who always knows the right answer, or knows that there is no right answer. This would be a kind of super-philosopher who never makes mistakes. It's among the most ambitious and sophisticated epistemological theories of the ancient world. Clearly, then, it would be a mistake for you to miss the next episode of The History of Philosophy without any gaps.