A well-known doctrine of Aristotle’s is that of the unmoved mover, the first cause of the eternal motion of the universe. Aristotle’s key argument for the necessity of an unmoved mover is presented in Physics, Book VIII, chapters 4–5, where he argues, first, that everything that is in motion must be moved by something (ch. 4) and, second, that a thing cannot move itself (ch. 5). I have long been puzzled by this argument, and lately I have come to think it’s not just my ignorance—there really is something wrong with the argument. My aim here is to explain what it is.
To assess Aristotle’s argument for the unmoved mover, it is helpful to know how the unmoved mover fits into Aristotle’s cosmology. To review, Aristotle believes the universe is a finite sphere with the earth at the center. There is a fundamental division between the sublunary region and the superlunary region. Below the moon’s orbit, natural motion is linear, or, more exactly, radial in straight lines to and from the center. The four sublunary elements, earth, air, fire, and water, naturally move up and down along these lines. Earth moves naturally down until it reaches the center or is obstructed by something intervening (such as other chunks of earth). Fire moves naturally up to the extremity of the sublunary region unless something obstructs it. Earth and fire are thus “absolutely” heavy and light, respectively. Water and air, by contrast, are heavy and light “relatively”—to each other and to earth and fire. For example, water is heavier than air but lighter than earth. Left to themselves, the elements would form concentric spheres of earth, water, air, and fire. Their motions up and down are natural in that they result from a principle within themselves. In such motions, they realize their actuality (entelecheia) in a process like that of nutrition and healing, which operate naturally if nothing obstructs them (De Caelo, IV.3).
Beginning with the orbit of the moon, natural motion is circular, revolving around the center. Now, circular motion is one of the two simple motions. The other is rectilinear motion. But Aristotle thinks that circular motion is primary, since a circle is simple and complete, whereas the line of rectilinear motion is not. For, if the line is infinite, it is incomplete, having no endpoints, and if it is finite, it can always be extended. (Cf. De Caelo, I.2 and Physics, VIII.9.) But if rectilinear motion is simple and is the natural motion of simple bodies (earth, air, fire, water), and if circular motion is simple and prior to rectilinear motion, then circular motion must likewise be the natural motion of some corresponding simple body. But the simple, natural body of circular motion cannot be any of earth, air, fire, or water, because their natural motions are rectilinear, and a distinctive, internal principle of motion is the defining essence of any natural body. The simple body of the superlunary region therefore must naturally move in a circle. For various reasons, it also is ungenerated, indestructible, unalterable, and not subject to increase or diminution (De Caelo, I.3). People have given it the name aether, though Aristotle doesn’t seem to much care what it is called and rarely uses that word.
Thus, the superlunary region contains spherical bodies—the heavens—which are composed of aether, that rotate eternally. On these are mounted the observable heavenly bodies: the fixed stars, sun, moon, and planets. There are multiple spheres, whose rotation is semi-independent, in an arrangement that accounts for the semi-independent observable motions of sun, moon, etc. The outermost sphere, called the first heaven, carries the fixed stars, and it supplies the primary motion, all the other spheres being yoked to it. All of this is uncreated. It had no beginning and will have no end. Beyond the first heaven there is nothing. There are no other worlds. This is the universe.
Now, what moves the first heaven? This is the question that motivates the argument for the unmoved mover. The argument can be summarized as follows.
- Everything that is in motion is moved by something.
- A thing can’t be moved by itself.
- So, everything that is in motion is moved by some other thing. (from 1, 2)
- A thing that is a mover may itself be in motion or unmoved.
- A mover that is in motion is moved by some other thing. (from 3)
- On pain of endless regress, not every mover can be moved by some other thing.
- So, on pain of endless regress, not every mover is in motion. (from 5, 6)
- So, there is an unmoved mover. (from 7)
This argument seems perfectly intelligible—and, I am inclined to think, valid—given premises 1 and 2.
I also think that the case he makes for premise 2 is straightforward, whether or not it is true. Simplifying somewhat, the basic argument stems from the idea that motion is imparted by something that already possesses that motion. Thus, A heats B by being itself hot, and one teaches geometry by knowing it oneself. So, if a thing were to move itself, it would have to—at the same time and as a whole—both undergo and cause the same motion, which is impossible. Again, for Aristotle, motion is a progress from potentiality to actuality, and actuality is not reached until the potentiality is fulfilled. But the mover is in actuality already. What does the heating is already hot. So, continuing the heating example, what heats itself would have to be both hot and not hot at the same time and in the same respect. But this is impossible. The only way out of this would be to split the body in two, C and D, and say that C does the moving and D is moved. But now C must itself either be moved or unmoved. If C is moved, then the problem recurs for C. If C is unmoved, then D alone is moved and we can no longer say that a thing moves itself. So, nothing moves itself. (See Physics, VIII.5 257b1–258a26.)
My problem is with premise 1, which says that everything that is in motion is moved by something. And the problem is this: if the natural motion of aether is to rotate, why does it need a mover? The whole point of natural motion is that no external impulse drives it. It moves by itself because that is its activity. For example, fire goes up because that is its activity (De Caelo, IV.3 310b30–311a10). Fire is light, and lightness just is the principle of going up. Fire doesn’t need to be moved up. Likewise, earth is heavy, and heaviness just is the principle of going down. Earth doesn’t need to be moved down; it goes by itself. That is its activity. Now, if the natural motion of fire is to go up and the natural motion of earth is to go down and they don’t need to be moved up or down, and if the natural motion of aether is to go around in a circle, why can’t it just go around in a circle? Why does it need to be moved around in a circle?
Of course, in Physics, VIII.4, Aristotle provides a general argument for why, after all, everything does need a mover, including natural bodies such as earth, air, fire, and water. The reason, in the case of such natural bodies, is that their motion begins only when they are brought into being or, if they are already in being, then when whatever is obstructing their natural motion is removed. For example, when water evaporates into air, it rises of itself, but something else had to cause its evaporation, and that something is the mover.
So, too, with heavy and light: light is generated from heavy, e.g., air from water (for water is first such potentiality), and air is actually light, and will at once realize its proper activity unless something prevents it. The activity of lightness consists in the light thing being in a certain place, namely high up: when it is in the contrary place, it is being prevented. (255b8–12)
By “being prevented,” he means, for example, a wineskin full of air might be held under water by a stone. If something removes the stone, the wineskin at once rises. But although it rises of itself, it still has a mover—namely, whatever removed the stone (255b24–31). (See also De Caelo, IV.3 311a1–14.)
This is an admirably hardheaded doctrine. There are constraints on motion and hence on possible explanations of motion. And to be clear, by “motion” here Aristotle means practically any change; not only locomotion, but change of quality, change of quantity (increase and decrease), and generation and destruction. There is no spontaneous motion. All initiation of motion requires a mover. And transmission of motion is by contact.
Nevertheless, it is not quite right to conclude, as Aristotle does, that “all things that are in motion must be moved by something” (256b3). Rather, what he has shown is that all initiation of motion requires a mover. Once air comes into being or the wineskin is released, it simply moves with no further need of a mover.
Now, the thing about the circular motion of the aether is that it doesn’t need to be initiated. It has always been and will always be. So, why does it need a mover?
Policy of Truth 
I think Theophrastus was the first to ask this question.
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Damn, there goes my hope of a MacArthur genius grant.
What did Theophrastus conclude? Does he reject the unmoved mover on this basis or find a solution somehow?
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Theophrastus’s view appears to have been like yours: that the celestial sphere could be the cause of its own motion.
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Aristotle’s argument is perfectly valid GIVEN premises 1 and 2; but both premises are not quite right. In Aristotle’s time they may have seemed reasonable, but we know better now.
Everything is in motion. In your entire life you have never seen anything that was actually at rest. Therefore, there are vast numbers of things to cause other things to move. Looking for a Unmoved, First mover is pointless; nothing is unmoving, and the whole universe is full of moving things able to affect the motion of other things.
Objects in motion stay in motion until something changes their motion; this is because motion is NOT something that happens TO an object. Motion is a property OF an object; and remains constant until something intervenes. Again, the whole universe is full of moving things able to affect the motion of other things.
Aristotle did not realize these things, and can be forgiven. His physics is 23 centuries out of date. Not his fault, but there it is.
But that means efforts to make sense of Aristotle’s reasoning now are somewhat moot. Even if his logic were otherwise flawless, since premises 1 and 2 are defective his conclusions are defective. As was Aristotle’s understanding of physics.
sean s.
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All of cosmology is contested, but it’s not clear that the Big Bang Singularity was in motion.
https://www.space.com/13347-big-bang-origins-universe-birth.html
Given that, contemporary physics does not conclusively demonstrate that everything is/always was/always will be in motion. By some accounts, it demonstrates the reverse. It’s more reasonable to say that the issue is up for grabs.
Given that, the interest of Aristotle’s argument is in getting clear on the logic of our concepts of rest and motion, something contemporary physics has not resolved. Aristotle’s physics is out of date, but his capacity for conceptual analysis isn’t.
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Irfan;
Regarding, “… it’s not clear that the Big Bang Singularity was in motion.”
It’s not clear there even was a “Big Bang Singularity”, much less whether it was in motion. And the properties of that “Big Bang Singularity” are not “unclear”; they are unknown. As your linked article plainly states:
“… the very beginning of the universe remains pretty murky. Scientists think they can pick the story up at about 10 to the minus 36 seconds — one trillionth of a trillionth of a trillionth of a second — after the Big Bang. At that point, they believe, the universe underwent an extremely brief and dramatic period of inflation, expanding faster than the speed of light.”
At the earliest point we know anything about, not only was everything in our universe in motion, but in EXTREME motion.
So even your linked article does not support your claim that, “Given that, contemporary physics does not conclusively demonstrate that everything is/always was/always will be in motion.”
There is nothing to demonstrate that there ever was a time that the bulk of the universe might not have been in motion; everything we see evidences that the contents of the universe have always been and are in rapid motion. What the far, far future may bring is not a concern for now.
Regarding, “By some accounts, it demonstrates the reverse.” Hmm. No. You’ll have to cite some of those accounts.
This issue is “up for grabs” only if you discount evidence.
The issues Aristotle argues about were resolved by contemporary physics some time ago. No “unmoved mover” is needed; motion is a property of objects. Aristotle had considerable analytic skills, but in this instance his premises were defective. No analytic skills can rescue conclusions based on defective premises.
sean s.
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Responding to Sean, Nov. 24, 11 am.
Some of what I’m going to say repeats what I said in a comment to Roderick. I think we need to get the overall logic of our disagreement clear.
David Potts’s version of Aristotle’s argument ends with the conclusion that there is an unmoved mover, where that unmoved mover is not itself moved. I’ll refer to that as claim (8). Your response to claim (8) is that, according to contemporary physics, everything in the universe that we know of is in motion. Given that, (8) is false, and the rest of the argument is unscientific and devoid of interest.
I focused on one problem with your reasoning, but I actually don’t think a single element of it is correct. For one thing, “everything in the universe that we know of is in motion” doesn’t really refute (8) on any construal of either claim. But set that aside. What I focused on was: does physics really prove, conclusively, that “everything in the universe that we know of is in motion”? The answer is no, it absolutely does not.
The article I linked to is a pretty standard recounting of the state of contemporary cosmology. What it says is that Big Bang theory is the predominant theory, granting that there are others in play. It then says that Big Bang theory postulates a singularity over which there is a lot of disagreement and unclarity. If Big Bang theory is itself the predominant theory, and it postulates a singularity, then I infer that a singularity is possible. The singularity cannot be ruled out until evidence comes in that rules it out. Nothing you’ve said rules it out. You’ve adduced no evidence that rules it out, and I know of none.
The fact that everything “we see” or “we know” is in motion is totally irrelevant to the issue. We’re talking about something we neither “see” nor fully know. (In truth, none of us in this conversation have literally “seen” the inflation of the universe one microsecond after the Big Bang, either.) Yet the singularity is not a whole cloth fabrication, either, as though it was on par with the claim that the universe was created by Kermit the Frog, or even with the claim that it was created by an omnipotent and omniscient personal God of the kind described in Judeo-Christian-Islamic Scripture. Big Bang theory and the Big Bang Singularity are scientific hypotheses in competition with other scientific hypotheses. Given that, they have to be granted the status of scientific possibilities. It doesn’t matter whether we know a lot about them, or a little, or close to nothing. It doesn’t matter whether we can “see” the singularity or not. Nor does it matter whether the singularity resembles the things we do know or see. Why would it? It’s not supposed to. The theory itself dictates that if it exists, it’s unique. Not being visible to us and being unique don’t (singly or jointly) rule out the possibility of the singularity.
If the singularity is possible, so is the possibility that it (the singularity) was not in motion. That’s what makes the singularity a singularity: at the point of singularity, there was neither time nor motion; there was no “before” it, and there were no distances for any motion to traverse. That’s all I meant by saying that on some interpretations, the Big Bang Singularity proves the opposite of “everything is always in motion”: on any interpretation of the Big Bang that involves a singularity, there were no distances; if there were no distances, there was no traversing them; if there was no traversing them, there was no motion.
Again, it’s totally irrelevant whether we have conclusive evidence that this is the form the singularity took. As the Space.com article makes clear, there is no conclusive evidence to be had on this topic at all. But some possibilities can be granted consistently with the evidence we have, and some can be rejected. That there was a singularity, and that there was neither motion nor time at it, is a possibility consistent with the evidence. So I flatly reject your claim that I’m ignoring the evidence. The Big Bang theory itself is the evidence for what I’m saying. And the claim I’m making is not a very strong one. It simply takes the form of saying: here is a possibility consistent with, though not conclusively demonstrated by, the evidence. Since there is no contrary possibility that demonstrates the opposite, my claim stands.
One thing that can conclusively be rejected is your assertion that physics proves that everything is and was always in motion. No, it doesn’t prove that at all. And it can’t prove that, until it conclusively rejects the Big Bang Singularity, which it hasn’t done. Every quotation you cite from the Space.com article underscores the fact that modern cosmology has not conclusively rejected the Big Bang Singularity. I’ve never said that it has conclusively confirmed it, either, but I don’t need to. I’m not one making a categorical assertion. You are. I’m saying that something is possible. You’re saying that something is impossible. The evidence better supports my claim than yours.
This disagreement brings out the larger interest of Aristotle’s “physics.” You’ve used your rejection of (8) to claim that Aristotle’s argument is irrelevant to contemporary thought. I don’t think you’ve successfully rejected (8), but even if you had, it wouldn’t make Aristotle’s argument irrelevant in the way you suggest. To repeat something I’ve said before: the interest of Aristotelian physics today is not that it gets physics right (it doesn’t), but that it displays greater attention to conceptual precision and logic than most contemporary people (including professional cosmologists and physicists) do when they invoke “science” to prove this or that.
We’re seeing that in this very conversation. You’re making an inference from what “we see” to what is possible about the universe as such. Aristotle shows why that’s a non sequitur. You’ve referred to the origin of the universe as a “creation.” But physics doesn’t say anything about “creation,” and Aristotelian philosophy clarifies why we need not refer to origins as creations. The Space.com article quotes physicists who believe that “the universe came from nothing.” Can anything really come from nothing? Should we accept “something can come from nothing” simply because physicists say so? Again, “something can come from nothing” seems to violate basic logic. Nothing can come from nothing.
The interest of Aristotelian philosophy (and philosophy generally) is that it shows that the assertions of contemporary physics cannot be taken to dictate everything we believe or say about the physical world–as though physics was the only epistemic authority about the physical world, and no other body of thought had any. Philosophy has a role to play in what we believe/say about the physical world, because logic has an authoritative role to play (and logic is part of philosophy). When physics contradicts logic, we have a stalemate, not a victory for physics. As we do when it comes to the claim “everything is always in motion.”
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Irfan;
I have to say your responses rather surprised me.
You wrote that “David Potts’s version of Aristotle’s argument ends with the conclusion that there is an unmoved mover, where that unmoved mover is not itself moved. I’ll refer to that as claim (8).”
Then you wrote that, “Sean’s response to the whole argument and whole discussion is that it’s all irrelevant: (8) is false because everything is in motion;”
Umm … no. That is not what I claimed. I never said it’s false.
I wrote that, “looking for an Unmoved, First mover is pointless”; that “no unmoved mover is needed”; that “there is no motion observed anywhere that requires Aristotle’s unmoved mover”; that “Aristotle’s defective understanding required an Unmoved Mover, but now we know that [THE REQUIREMENT] is wrong.”
At no point did I say there was no unmoved mover. Maybe there is one.
What is certain is that there is no reason to think there actually is one.
I don’t think that is too subtle a distinction.
On other matters:
I haven’t tried to rule out the big bang singularity. I merely noted that it is a thing about which we know nothing. It’s just a vague idea at this time. Your linked article referred to alternatives; which have been considered for decades now.
For the sake of argument, I will not contest your description of a singularity, but I can contest your conclusions. Zero distance does not mean zero motion; instantaneous motion would also be consistent. At any finite speed, traversing zero distance takes the same time: zero seconds. Absence of motion is a possible inference for zero distance; but infinite motion can also be inferred; as well as anything in between. All velocities are the same in this scenario. This is the problem with singularities; all things are possible, and nothing is certain.
It is important to understand that “singularities” arise in physics when an explanation or model reaches a point beyond which it can sensibly say what’s going on. This applies both to the “big bang singularity” and to black hole singularities. These do not imply actual real-life infinities. We know something is going on, but we’re not sure what.
Whether motion inside the singularity is possible is a different question from whether the singularity as a whole was in motion. Was the “big bang singularity” itself in motion? In motion relative to what? We don’t know. We can only say we don’t know and that it does not appear to matter.
Suppose this singularity were not in motion; what would that change in our understanding? Nothing. The singularity ceased to exist at the moment of the “big bang”; and everything in the universe that was created is in motion. It may be that the overall motion of our universe is zero; but everything in it is in motion, sometimes at extreme velocities. An unmoved mover remains unnecessary. Whether the singularity itself was in motion or not, all the evidence indicates that everything created by it is in motion and has been since the first moment we know anything about.
You wrote that, “One thing that can conclusively be rejected is your assertion that physics proves that everything is and was always in motion. … You’re saying that something is impossible.”
Since I never even used the words “proves” or “impossible’; I can regard this as misunderstanding needing no further response. BTW, “proof” is for math, printing, and booze. Science deals in evidence, not proof.
Finally; physics does not attempt to dictate “everything we believe or say about the physical world”.
Say what you want. But physics does have expertise regarding many aspects of our knowledge of the physical world; if a philosopher asserts things about the physical world, then physics is not obligated to remain silent to protect the philosopher’s ego. There are many philosophers who pay close attention to physics; and many physicists who pay close attention to philosophy. Neither submits to the other.
Philosophy has a role to play in what we believe or say about the physical world, because logic has an authoritative role to play, and logic is part of philosophy. But logic is also a part of physics, mathematics, and many other disciplines. Logic does not belong to any single discipline. Physics has a role to play in what we believe or say about the physical world because studying the physical world is what this discipline is about.
No discipline scores victories by contradicting logic. But whether a claim is logical or not is not left to one discipline to decide; physics has the same standing to evaluate logic as mathematics or philosophy.
Everything we see evidences that the contents of the universe have always been and are in rapid motion. If that means we can cease consideration of an unmoved mover; so be it. Nothing about that contradicts logic or facts. Other than supposing the bare possibility that the conjectured-but-uncertain singularity might-maybe-possibly have been motionless, you have mentioned no evidence to the contrary. And since the singularity may have been in rapid motion, your bare possibility is even more tenuous than it first appears.
What is certain is that your bare possibility cannot not override everything we can see.
An unmoved mover could exist. As could faeries, unicorns and flying pigs. But why would we think so?
sean s.
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The Big Bang is not a first mover in Aristotle’s sense (since Aristotle thought the past was infinite). His first mover isn’t something way back in the past that kicks off a series of motions, it’s something in the present that keeps all motions going.
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(Responding to Roderick)
I don’t know if your 11/24 7:09 pm comment is addressed to Sean or to me, but I wasn’t likening Aristotle’s unmoved mover to the Big Bang. I was invoking the Big Bang Singularity as a counter-example to Sean’s rejection of the conclusion of David Potts’s argument.
DP’s conclusion reads:
So, there is an unmoved mover. (from 7)
Sean’s response to the whole argument and whole discussion is that it’s all irrelevant: (8) is false because everything is in motion; given the patent falsity of the conclusion, the argument is of no interest. (The number 8 refuses to show up above, but the conclusion was line 8 of DP’s argument.)
There are many things wrong with Sean’s reasoning, but my response was limited to one point: if there was a Big Bang Singularity, we have to grant the possibility that it (the singularity) was not in motion. So Sean’s response fails. I don’t have to prove that Big Bang theory is true, or that a singularity is required, or that the singularity was not in motion. It’s enough to say that Big Bang theory has some warrant, as does a singularity, as does the claim that the singularity was not in motion. Those possibilities are not whole cloth confabulations.
In other words, whatever the ultimate status of DP’s (version of Aristotle’s) argument, you can’t reject it by asserting that physics has now conclusively proven that (8) is false. It hasn’t. Big Bang theory is still a player in cosmology, and if so, the Big Bang Singularity is still a possibility we (non-experts) have to grant. So there is no way to assert a categorical rejection of (8), as Sean does, by flatly asserting that everything is in motion, or that everything “we see” is in motion, or that everything “we know” is in motion, etc. None of those claims rules out, or can rule out, that: it’s possible that at least one thing wasn’t in motion once. So my point is, Sean’s strategy fails, and can’t be rescued by invoking physics in the way he does.
I’ll respond more directly to Sean in a bit.
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Doesn’t the fact that there is no sphere of the fixed stars, and hence no example of circular motion that continues forever, actually improve Aristotle’s argument to the Unmoved Mover? That is, all motions – in Aristotle’s definition – that occur in the real universe are of the kind that Aristotle thought were confined to the sublunary region. Every change that really happens has to be initiated somehow. So every real change requires the Unmoved Mover to explain how it can happen. The perpetual motion of the heavenly spheres is a problem for Aristotle’s metaphysics only if the heavenly spheres exist, and Isaac Newton proved that they don’t.
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In our contemporary scientific cosmology, it is unknown what value of total net angular momentum is had by the universe. But it is known that that value is conserved, i.e., a constant. So unless it turns out that that constant has the value zero, it remains that there is one circular motion that cannot have a mover.
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I should say further, however, that just because we could take the angular momentum vector of one galaxy and add it to that of another galaxy and get single resultant value pointing in a certain direction in space, does not mean that in the global topology of the universe full-scale, it makes sense when adding up all the angular momenta of the galaxies one end with a single resultant direction of the angular momentum that is rightly pictured as a single straight-line axis about which the universe is spinning like the earth spinning about an axis of rotation. Still, if there is a total nonzero net angular momentum to the universe and its value is a constant (according with the law of conservation of angular momentum), then there is a motion that does not require something else having brought it about or keeping it going.
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Actually, there are many examples of “superlunary circular motion”: every observable galaxy is rotating continuously. Their rotations are so well observed and quantified that a discrepancy observed in it has lead to the “dark matter” mystery.
And of course, our solar system comprises thousands of observable objects in “circular motion” around the Sun, which is itself in “circular motion” around the center of the Milky Way galaxy.
I put those in quotes because actual circular motion is very, very rare; almost all objects we see in the sky move in elliptical orbits.
There is no motion observed anywhere that requires Aristotle’s “unmoved mover” to explain. The initial creation of our universe imparted so much energy (motion) to the contents of our universe that things are still banging around now 13-plus billion years later. Will they do so forever? We don’t know.
sean s.
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1) The orbits of planets around stars, and stars around galactic centers, are not natural motions (in the sense Aristotle uses for “natural” here.) They are the result of continuous alterations to the inertial movement of the bodies by the gravitational field. And those alterations have to be explained by something outside the body – so the argument to the Unmoved Mover applies to them.
2) The difficulty with the heavenly spheres was that, in principle, they could have existed forever in the past, following their natural motion of rotation, and thus needing no initial impetus. Now that we know the universe’s age is finite, even if we suppose that the motions of everything in it are completely explained by the Big Bang, the Big Bang itself needs to be explained. That is, if the Big Bang initiated everything else, what initiated it?
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1) The orbits of planets around stars, and stars around galactic centers, are perfectly natural motions. Aristotle’s use of “natural” was defective.The continuous alteration of inertial movement of bodies by gravitational fields is quite natural and carries its explanation: gravity ( which is “outside the body”) acting on the body in question. Aristotle’s detective understanding required an Unmoved Mover, but now we know that is wrong.
2) Now we know the universe’s age is finite; even if the motions of everything in it are completely explained by the Big Bang, the Big Bang itself needs to be explained AT SOME POINT. Something must have caused it. But about that something we currently know absolutely nothing. Inventing magical entities as ” explanations” does not get us even one step closer to understanding
sean s.
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1) If you are looking for a contradiction in Aristotle’s work, you must begin by using words as Aristotle did, not as you would prefer. In this context Aristotle used “natural motions” to mean what we now call inertial motion: how a body moves if no force acts upon it. Of course there is a sense in which the gravitational field is a natural entity, but that sense isn’t relevant to the argument in De Caelo.
Now, David’s question was, if the rotation of the heavenly spheres is natural in this sense – that is, inertial – and has gone on forever as far as we can know, why do we need a mover to explain it? That was a problem for Aristotle, but it isn’t one for us today, because the orbits of the planets and stars are not inertial motions, and have not gone on forever. It’s like asking how one explains the existence of unicorns – it’s enough to say that they don’t exist, and never did.
2) I don’t agree that we know nothing about the cause of the Big Bang. We know what can be deduced by analyzing the nature of changes in general (since the Big Bang was certainly a change!) Physics, as an empirical science, fails us, but metaphysics remains.
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Michael;
If all we were doing is looking for contradictions or consistency in Aristotle’s work, then DP should not have gone down the path of questioning Aristotle’s premises 1 and 2. But he did.
So the rest of us can too. It’s been written that, if we take premises 1 and 2 as givens, then Aristotle’s argument works. But since we are permitted to question those premises, we are not obligated to take them at face value. At that point, those premises fail.
sean s.
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Shouldn’t “creation” also be put in scare quotes? Surely physics doesn’t prove that the universe was “created” in any familiar sense of “created”? There’s a great deal more evidence for a Big Bang Singularity than there is for the “creation” of the universe. A creation presupposes a creator. A singularity doesn’t. A non-created singularity is more obviously consistent with the findings of contemporary physics than a created universe.
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No, a creator and an uncreated singularity are both equally consistent with the findings of modern physics. What physics tells us is that there is an initial singularity; no measurement or experiment can tell us, even in principle, what existed beyond that singularity. It could be a creator, another universe, or nothing at all – any of those are consistent with the empirical evidence.
The problem with “nothing at all” (if that’s what you mean by “uncreated”) is logical – it would mean that something, in fact everything, came from nothing. But surely “nothing can come from nothing” is a self-evident truth?
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There is no coming of the Initial Singularity into existence. The “initial” of it is only our finding that there was no time before it. The Initial Singularity possessed the same mass-energy as the universe has today. All of that has by now good observational support. It is indicated by sensory evidence under conception not qualitatively different from our natural, sensory indications that, for example, the Bible exists.
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How is the argument for premise (2) compatible with the fact that even apart from the Unmoved Mover, some things in Aristotle’s world are self-movers? If nothing moves itself, how can moral agents be a source of action, taking action to be a kind of motion?
Same with “all initiation of motion requires a mover,” where the mover is not the thing initiating the motion. Wouldn’t that entail that Aristotle rejects metaphysical freedom? Maybe he does, but he often sounds like he’s endorsing it.
This is what happens when you open the Pandora’s Box of “puzzles in Aristotle.”
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Aristotle distinguishes different senses of self-motion, so that in one sense all living things are capable of self-motion and in another sense they’re not. When I was writing my dissertation I could have told you precisely what the senses are and how they apply, but it’s been a while.
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I figured as much. But my point is that while DP’s worries about the soundness of Aristotle’s argument focus on premise (1), you could just as well worry about premise (2). As stated in DP’s argument above, premise (2) is either false, and/or inconsistent with Aristotle’s other claims, and/or misleading. Given what you say, (2) has to be tweaked (if that’s the right word) so as to capture all the different senses of self-motion. (It may need more than tweaking.) Once we do, premise (2) changes, and so does the argument as a whole. I don’t know whether the changes are relevant to the topic at hand, but they affect how we conceptualize self-motion, which seems relevant to conceptualizing an unmoved mover.
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