 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, Let's Get Physical, Aristotle's Natural Philosophy. Simplicity, they say, is a virtue. But is it really? I guess it depends what you're trying to accomplish. Producing a piece of minimalist art? Then simplicity should be your watchword. But designing a baroque church? Not so much. Scientists seem to side with the minimalist artists and not the baroque architects. As we can see from modern attempts to produce a unified theory of physics, scientists often seek to provide the simplest possible explanations of the world around us. If you can reduce the number of principles or concepts needed to account for what you see around you, that seems to count as a scientific advance, even if the simpler explanation doesn't, for instance, allow you to predict things with any greater accuracy. We seem instinctively to feel that explanations of nature should be simple, and that if we can restrict ourselves to a smaller number of explanatory factors, we must be getting closer to the underlying reality. Whatever you make of this feature of science, it has clearly been around for a long time, indeed for as long as there has been anything we can plausibly call science. If you cast your mind back to the first presocratics, you may recall that they sought to explain all natural phenomena in terms of a single principle, which might be water according to Thales, air according to Anaximenes, or the more obscure infinite of Anaximander. These Milesian thinkers are often called material monists because of their attempt to derive the whole natural world from a single original matter. The search for simplicity reached its climax with Parmenides, for whom everything is one and complexity is not just a theoretical blemish, but an illusion. When Aristotle sets out to do natural philosophy in his work, the Physics, he adopts his characteristic method of surveying previous views, and what he finds is that nearly all his predecessors sought to explain the natural world in terms of a small number of principles. The Milesian religions and Parmenides, as we just saw, get the number of principles all the way down to one. Empedocles doesn't get quite that far, since he invokes two opposed principles, love and strife, and it would be fairer to say that he has six, since there are also his four elements. Those who don't go for a small number of principles tend to go all the way in the other direction and say that there are an infinity of principles, which might be an infinity of atoms or the infinite seeds in the mixture of anaxagoras. In discussing this material, Aristotle does us the favour of preserving a lot of information about the pre-Socratics. Later commentaries on his Physics preserve further information. But Aristotle is not telling us all this out of mere historical interest. In fact, he risks distorting the views of his predecessors because he forces those views into a scheme which will allow his own ideas to emerge more clearly. In this case, the scheme pivots around this question of how many principles there are for nature. This is bound to be a misleading question to put to the pre-Socratics. Is Parmenides's one really a principle in the same sense as air, according to Anaximenes, or the infinite atoms of Democritus? Surely not. And the principles Aristotle will invoke are principles in yet another sense. Yet Aristotle has put his finger on something. His scheme reveals that the drive towards simplification was already a light motif of early Greek thought. Another characteristically Aristotelian thing to say would be, fine then, go ahead and strive for simplicity, but be careful not to oversimplify. This is, after all, the man who built a whole ethical theory around the golden mean. He will want to pursue simplicity, but within moderation. He argues that theories based on only a single principle are too simple. If you have only one thing, it cannot interact with anything else, so it will remain inert. This, of course, is exactly what Parmenides had in mind, that reality consists of an unchanging single being. Aristotle pauses to refute this idea, but remarks that the discussion doesn't really belong to natural philosophy as such. Parmenides's theory says that the natural world is an illusion, whereas natural philosophy should assume that the natural world does exist, and then attempt to understand it. Aristotle also faults two principle theories for being too simple. If we have only two contrary principles, they will just cancel each other out. No, what we need, says Aristotle, is three principles. A principle to be worked on, and then two other principles to work on the first one. Putting it this way makes his idea seem rather abstract, but we've already seen in a previous episode how he would spell it out. The key notion is going to be change. Parmenides and his followers claimed that change would mean that being emerges from non-being, as if by magic. Aristotle agrees that this sort of absolute coming to be from nothing is impossible, but this isn't the right way of understanding change. Rather, there is change when something that already exists gains a new feature, or loses a feature it already had. Aristotle illustrates with the example of a man becoming musical. Here the man undergoes the change, he is the subject of the change. He first lacks a form when he is not musical, and then gains a form, namely musical. So we have three principles, the subject of change, the absence of form, and form. Just the right number of principles, simple enough, but not too simple. This brings us to another key piece of Aristotelian terminology. You'll be hearing a lot of it from now on, so I hope you like it. Aristotle is going to say that if a man can acquire the form musical, then he is potentially musical. Once he acquires the form and is musical, the man is actually musical. The Greek word for potentiality is dunamis, which is where we get our word dynamic. It means power or capacity. Actuality meanwhile translates entelikia, which roughly means completeness, or alternatively energeia, which is of course where we get our word energy. Aristotle wields this pair of concepts, potentiality and actuality, as a basic weapon in his philosophical arsenal. He finds that perennial, and not-so-perennial, philosophical problems can be answered by distinguishing the potential from the actual. For instance, the distinction allows him to solve Parmenides's challenge against the possibility of change. He can say that change does not require that nothing becomes something or that something becomes nothing. Rather, one thing that is potentially something else becomes actually that other thing. For instance, a cold rock that is potentially hot is heated and becomes actually hot. Simple, right? Now, I know what you're thinking. What does any of this have to do with natural philosophy? That's supposed to be the subject of the physics, right? Indeed, and this is the last etymology I'll trouble you with today, the very word physics comes from the Greek word for nature, namely phusis. Why does Aristotle start out a general investigation of nature by talking about change? The answer is revealed when we get to Book 2 of the physics, where Aristotle attempts to give a definition of nature. According to his definition, nature is a principle of change. So, we won't be able to understand nature or the natural world without understanding change. To see why, consider the difference between a man-made object and a natural object. In honour of those heroes of simplicity, the minimalist artists of the 1960s, let's take as our example of a man-made thing a steel cube. And let's contrast this to, oh, a giraffe. Well, they're clearly very different. About the only thing they have in common is that they are sometimes put on public display and greet their audiences with cool indifference. Steel cubes don't, for instance, eat leaves off trees. In fact, to be honest, they don't do much of anything. Even their shape is imposed upon them from the outside by a minimalist artist, or more likely, the artist's team of assistants. If the cube changes, the change is forced upon it. For instance, it will move when people pick it up and bring it to a museum, perhaps wondering why minimalist artwork has to be maximally heavy. How different is the giraffe, which begins as a fetus in its mother and naturally transforms into a baby giraffe, then moves itself around to get at plants which nourish it, enabling it to grow into an adult? Aristotle, pondering this sort of contrast, decides that the key difference between the man-made and the natural is that the natural things can move and change themselves. Even when they are at rest, without changing or moving, for instance when giraffes go to sleep, it is their nature which explains this and not some force from outside. Aristotle gives us a vivid example to push the point home. If you take a wooden bed and bury it, and it sprouts up as a plant, what you'll see growing is a sapling, not another bed. The nature of the wood or the tree remains even though it is forced into the shape of a bed. Similarly, the minimalist steel cube may have some natural tendencies, as we'll see in a moment Aristotle would think that it does, but it is not natural for it to be cube-shaped or to be in a museum, since these features are imposed from outside. Hence Aristotle understands phusis, or nature, as an internal principle of motion and rest. Aristotle's conception of nature means that, for him, physics is in one way strikingly like modern physics, and in another way strikingly different. It is strikingly similar because he thinks that physics, or natural philosophy, should investigate motion. Motion is of course one of the things we all studied as teenagers in physics class, rolling balls down ramps and learning about acceleration, and so on. Of course, spatial motion is only one kind of change, but it is an important kind, and Aristotle has a tendency to see spatial motion as just one of the kinds of change that natural philosophy can investigate. It also investigates processes like heating and cooling, changing color, and so on. Really, any change that things do naturally will come under the heading of Aristotelian natural philosophy. All this seems reasonably relevant to physics as we conceive it nowadays. Yet Aristotle's physics is also unlike modern physics, because it puts so much emphasis on innate tendencies towards motion and rest. Now that we can build machines that seem able to move themselves, Aristotle's distinction between the artificial and the natural might seem in danger of breaking down. And anyway, do all natural things really have an innate tendency to change? This may seem plausible when it comes to giraffes and the like, since they do have an internal principle of motion. They can stand up and gallop across the savanna in search of succulent acacia leaves. But what about things like rocks and clouds? These are not alive, but surely they are natural. And they don't have an internal principle of motion and rest. Here, Aristotle definitely parts ways from our modern physicist. For him, a rock does indeed have an internal principle of motion. Just drop one to see it in action. First, get your foot out of the way, because that rock is going to go straight down. Whereas we would think of this as the rocks falling due to gravity, Aristotle, who lived about two millennia too early to discuss the fine points of falling bodies with Isaac Newton, thought that the rock falls due to its natural tendency to move down. To explain this, we need to glance at how Aristotle believes the natural world as a whole is structured. I'll be looking more at his cosmology in a future episode, so for now I'll just lay out the basics. For Aristotle, as for Plato before him, the earth is at the center of a spherical universe, and is tiny in comparison to the heavenly bodies that surround it. The realm below the heavens, where we live, is made of the four elements familiar from Empedocles—fire, air, water, and earth. Unlike Plato, Aristotle thinks that the heavens are made not from particularly pure versions of the four elements, but rather from a fifth element, sometimes called aether. The fifth element is not, as a science fiction movie from the 1990s would have you believe, a young lady with bright red hair, but rather a transparent, indestructible material. This material makes up the spheres carrying the visible heavenly bodies in their circular orbits around the earth. As for the four lower elements, they move not in circles, but rather in straight lines—down in the case of earth and water, towards the midpoint of the universe, and up, away from the midpoint in the case of air and fire. This is why rocks and raindrops fall, but bubbles of air move upwards in water, and flames flicker upwards in air. These motions are natural to the four elements. So Aristotle can say that the falling rock does indeed have an internal principle of motion. Due to its earthy nature, it has a tendency to move towards the center of the universe if nothing gets in its way. That minimalist steel cube is much the same. It's made of metal, and metal is obviously an earthy kind of material. As you'll discover if you drop the steel cube while carrying it, it has a very powerful tendency to move downwards. Like I said, mind your foot. Even though the steel cube is man-made, it retains natural tendencies, as do all man-made items, like the bed that sends up shoots when it is buried. Thus motion and change are central notions in Aristotle's physics, indeed definitive of what it is for something to have a nature. This means that Aristotle has good reason to provide a careful analysis of motion and change, which is exactly what he goes on to do in the physics. In fact, part of the physics was called in the later tradition, the books on motion. He relies heavily on his distinction between potentiality and actuality. For Aristotle, any change is going to be a transition from potentiality to actuality. This will be true of both spatial motion and change more generally. If I go from London to Paris, then on the way, I gradually actualize my potentiality for going to Paris. If it is the middle of summer, so that Paris heats up and, as it is wont to do, sizzles, then its potentiality for heat is actualized. A striking feature about change, which becomes obvious once we start thinking in terms of potentiality and actuality, is that changing inherently involves a degree of incompleteness. When I am on the way from London to Paris, I have partially, but not completely, actualized my potential for moving towards Paris. At the very moment where I reach Paris, my potentiality is fully actualized, because I am actually in Paris. As soon as that happens, the change is over, it is complete. Likewise, something that is potentially hot is partially actualized as it is heated and fully actual once it is hot. This is one reason why Aristotle uses that word entelacheia, or completeness, for the form or actuality that is reached in change. Another resonance of entelacheia would be with the related word telos, which we met last time when I explained the term teleology. As we saw, telos means purpose or end, and we can think of the end state of a change as the completion, fulfillment, and purpose of the process of change. Every change is identified by the end state towards which it is directed. Hence, if I am on my way from London to Rome by way of Paris, my journey is incomplete when I reach Paris. I am still in a state of potentiality, whereas my goal is reached if I intended only to get as far as Paris. A side benefit of this analysis is that it allows Aristotle to understand something which has remained central in the study of physics—time. Here, he engages in the simplifying tactic so popular among scientists. He defines time in terms of change, so that he reduces the question of time's reality to the less contentious reality of change. Plato too connects time to motion. He says in the Timaeus that time is produced by the movement of the heavens. It's no surprise that both philosophers see motion and time as having an intimate connection. If you try to envision time passing, you will likely imagine some change or motion, such as a clock's hand sweeping around its face. Aristotle endorses this way of understanding time, and in fact, proposes that time is simply the measure of motion and rest. So, for him, our conception of time is entirely dependent on our awareness of change and potential change. Most obviously, as Plato emphasizes, we measure days via the motion of the sun around the earth. But Aristotle doesn't link time exclusively to heavenly motion. Any motion can be measured by time, for instance when someone moves 100 meters in 10 seconds. As you might recall, this was the basis for Aristotle's response to Zeno's dichotomy paradox, according to which we can never move because we'd first need to move half of the distance. Aristotle answers that it does not take an infinite time to move a finite distance. Zeno is right that the distance is infinitely divisible, but time is infinitely divisible in the same way. Rather than moving through an infinite number of divisions, we move over the whole distance which can be divided up however we like. The time is divided along with the motion, which only stands to reason, since it measures the motion. But no matter how we divide the distance and the time, we will still have a finite total amount of time and of distance. In episode 38, we saw that Aristotle considers particular beings, like Socrates and Hiawatha the giraffe, to be primary substances, the basic building blocks of reality. So it isn't surprising that he makes time ultimately dependent on these substances. Time exists because it is a measure of the changes that primary substances undergo, whether these substances are the massive spheres of the heavens as they rotate, or tiny fleas as they leap. Aristotle has something similar to say about place. Rather than setting forth a concept of absolute space, as Newton will do, Aristotle contends himself with the notion of place as the limit surrounding a particular body. My place is simply the border where the air around me is touching me. Like Aristotelian time, Aristotelian place is entirely dependent on the particular objects that he calls substances. This feature of Aristotle's physics is comforting in a way. He does not postulate metaphysically mysterious things like absolute time and space, but contends himself with the familiar substances that surround us. Time and place emerge only as phenomena dependent on these substances. But this does not mean that his physical theories are just commonsensical, or that they are immune to objections. For instance, could there be an empty place, a void? How would that work, if place is just where a body is surrounded by another body? As for time, we might worry that, if it is a measure of motion, then it will only exist when someone does the measuring. Couldn't time pass without measuring, indeed without there being any motion that could be measured? To look at these and related puzzles, we're going to need more space and time than this episode provided. So, I will be revisiting these issues in an interview with a very special guest, one of the world's leading scholars of Aristotle, Richard Sarabji. Don't miss my interview with him about Aristotle's physics next time on The History of Philosophy, without any gaps.