[MUSIC] The First Mover of the Cosmos. At the beginning of chapter six of book eight of the Physics, Aristotle claims, since motion must be everlasting and must never fail, there must be some everlasting first mover. In particular he goes on, there must be something that is itself unmoved and outside of all change, but initiates motion in something else. Now the view that motion must be everlasting and never fail, is the thesis he defended in chapter one. That there has always been change in the universe and that it will never stop. The thesis that there must be a first mover of this eternal change, is something he's been building up to in the intervening chapters. And that this first mover must be unmoved, Akhenaten, is one of the most famous and mysterious thesis' in Aristotle's philosophy. But before we can understand what that unmoved mover is, and why Aristotle thinks there has to be one, let's make sure we understand what he means by a first mover. Now you might wonder how he can think there was a first mover of the cosmos if there is no first movement. Recall, he thinks that as far back as you go in time, there has always been movement or change. But when Aristotle invokes a first mover, what he means by first isn't prior in time, but rather prior in the causal or explanatory order. Now we tend to think of causes as temporally prior to their effects. Like the lightening strike that causes the forest fire. But the causes Aristotle is thinking of as movers, typically exist simultaneously with the motions they bring about in the moves. Think about the acid turning the litmus paper pink. Or the teacher teaching the student. The acid and the teacher are movers, that is efficient causes of change, but they are still around while the change is taking place. Indeed their presence is crucial. The same can be true when there is a nested series of movers in an extended causal chain. In the example Aristotle gives in chapter five, we can see how the first mover in such an extended causal chain need not be temporally prior. Here's the example. A man moves a stone with a stick. What is moved, is the stone. But what's the mover? What moves the stone? Well in a way, the stick is what moves the stone. But in another way, the man is, since the stick is moved by the man. The man is the first mover of the series, in that we will credit or blame the man, rather than the stick for the stone's motion. If I position that stone very carefully in my rock garden and you come along and pry it up with a stick, you can't very well say, don't get angry at me, it was the stick that moved the stone. You moved the stick, and so you are the first cause, or the first mover in this causal sequence. Even though you're moving the stick is simultaneous more or less, with the sticks moving the stone. Okay, so that's the sense in which Aristotle thinks there must be a first mover of all the change in the cosmos. It needn't be temporally first, rather, it will be at the top of a nested causal chain, like the one in which the simultaneous movements of the man and the stick move the stone. Now, here's the nested causal sequence Aristotle has in mind for the cosmos. We can start with the output end down here on Earth, which Aristotle refers to as the sublunary region. That is, the area below the Moon. What's distinctive of this region, is that unlike the Moon, Sun, stars, and planets which are all eternal, the substances here in the sublunary region come to be and pass away. Plants grow in the spring and die in the winter, but set seeds that will sprout again the next spring. Animals are born, live to maturity, reproduce, and then die, and their offspring will repeat this cycle over and over. Even earth, air, fire, and water undergo a cycle of reciprocal transformation. Air turning into water when it cools off, water into earth when it dries out. Earth into fire when it heats up, and then fire back into air when it moistens. Aristotle describes all these cycles in a work called, On Generation and Corruption, where he raises the question what keeps these cycles going? His answer is that all these cycles depend on the yearly path of the Sun around the Earth, along the plane known as the ecliptic. Which is at an oblique angle to the equator. As Aristotle understands the Sun's path, the Sun draws nearer the Earth during part of the cycle, the summer. And retreats from us during another part of the cycle, the winter. The resulting cycle of seasons on earth then sustains the yearly cycles of birth, death and so on. But why does the Sun move along this particular path? Aristotle, like the astronomer Eudoxus, takes the motions of celestial bodies to be the products of the motions of concentric spheres. Each of them rotating with their own motion in different speeds and directions and around different axis. But with the outer spheres communicating their own motions to the inner spheres. So the path traveled by any particular celestial body, like the Sun, will be the product of the motion of it's own sphere, together with the motions of the outer spheres in which it is embedded. This is roughly the sort of picture of the cosmos we find in the Timaeus, although Aristotle thinks you need many more spheres than Plato invokes. So here is our nested series of movers of the cosmos. The cycle of coming to be and passing away in the sublunary realm, is due to the motion of the Sun in its sphere. While that sphere in turn is moved by another sphere in which it is embedded, and so on, until we get to the outermost sphere, whose revolutions are ultimately moving the sphere of the Sun. Thus, while we can say that both the Sun sphere and the outermost sphere are movers of the cycle of coming to be and passing away, the outermost sphere is the prior mover, since it influences the sphere of the Sun, rather than the other way around. So far, so good. We have seen that in invoking a first mover in the cosmos, Aristotle is not invoking a mover that got change started in the first place. Rather, he is seeking to explain what keeps those cycles of change ongoing. But, have we identified the first mover of that cycle of change? From what we've considered so far, you might think the first mover is the outer most sphere, since that is what moves the inner spheres and ultimately the Sun, whose movements cause the cycles of coming to be and passing away that Aristotle is seeking to explain. But Aristotle, in fact thinks there is something else that moves the outermost sphere, that's the unmoved mover of the cosmos. Now why Aristotle thinks there has to be such an unmoved mover, and what he thinks plays that role will be the subject of another lecture.
