41 - Richard Sorabji on Time and Eternity in Aristotle

Peter talks to Sir Richard Sorabji about Aristotle's physics, focusing on the definition of time and the eternity of the universe.

• R. Sorabji, Matter, Space, and Motion (London: 1988).

• R. Sorabji, Time, Creation, and the Continuum (London: 1983).

Prof Sorabji's website with a list of his publications

Note: this transcription was produced by automatic voice recognition software. It has been corrected by hand, but may still contain errors. We are very grateful to Tim Wittenborg for his production of the automated transcripts and for the efforts of a team of volunteer listeners who corrected the texts.

Peter Adamson: I thought I might start by asking you about time. Aristotle defines time as the measure of motion in respect of before and after. Can you explain what that means exactly?

Richard Sorabji: Well, I think that Aristotle prefers to define time as the number of motion in respect of before and after, but by number he means something special. He means what's countable. In full, I think he means that time is the countable, instantaneous stages of a motion. That would be the first part. Number of motion means countable, instantaneous stages of a motion. What about in respect of before and after? Well, he means in respect of what's spatially before and spatially after in the motion. Just two points. He means number, to go into the definition, rather than measure, because measure introduces an extra idea. When you are measuring, you need evenly spaced instantaneous stages. So that's a special case of being countable. But to have time, you have time whenever there are countable instantaneous stages of a motion, regardless of whether they're evenly spaced. So though he does also say that time is the measure of motion, I don't agree with those people who think that that's the actual definition. I think the actual definition is numbered in the sense of the countable instantaneous stages of a motion. And then you're thinking of these stages as some before and some after. Now you might think that that was circular, because doesn't before and after bring in the idea of time when we're trying to define time?

Peter Adamson: That would be bad.

Richard Sorabji: Yes, that would be bad. Now he thinks he gets out of it, because he says, look, he means spatially before and spatially after. He's talking about one point being before another point in the motion.

Peter Adamson: So if I get up and walk across the room, he means that the beginning I'm at the left side of the room, and then I'm in the middle of the room, and then I'm at the right side of the room.

Richard Sorabji: Exactly. But there is a snag. I'm not sure that he has avoided giving a circular definition of time, presupposing the idea of time within the definition, because you rightly said left and middle. And terms like left and middle clearly don't bring back in time. But why do we call the left hand side the before? It's because it's what the motion reaches in time before it reaches the middle. So I'm afraid that he probably has got himself into a circle, even though he's trying to avoid it. Not a stupid circle, but I think he hasn't quite succeeded in avoiding it.

Peter Adamson: Could it somehow depend on the idea that since this motion is a motion from left to right, the very nature or definition of the motion that we're considering brings with it the notion of priority. So if it's a motion from left to right, rather than a motion from right to left, then the motion has an inbuilt priority and posteriority, which time could then sort of map on to.

Richard Sorabji: But is left and right doing any of the work here to explain why we think in terms of priority and posteriority, or is it rather that it's because we've always got in our minds that the motion has earlier in time reached the left hand side or reached the right hand side, so that we've really got the time in our minds rather than the leftness or rightness in our minds when we talk about priority.

Peter Adamson: So that it would actually wind up being circular.

Richard Sorabji: So that it would wind up being circular.

Peter Adamson: I guess the other thing that's maybe worth saying here is that although he does tend to talk about it in terms of spatial motion, this account is surely supposed to apply to other kind of changes too, right? So if something becomes cold or changes color.

Richard Sorabji: Absolutely. Motion will be very useful in the end because with the stars moving around us, it provides a wonderful celestial clock - but you're quite right, of course, that other changes can be counted. The before and after stages of growing cold could be counted as well.

Peter Adamson: Right. Well, we'll get onto the stars in a moment. But first I wanted to ask you something else about this definition of time as a number. If time is a number, this is what makes it measurable, presumably. Now you would think that that would mean that Aristotle could hold the following: If there's a motion and the motion has some kind of number, then there will be time whether or not anyone measures it. And yet he sometimes seems to talk as if the only way there can be time is if there is some soul or mind to do the measuring. Do you think he's really committed to that? So do you think that he believes that there would be no time without someone to do the measuring of time?

Richard Sorabji: Unfortunately he does explicitly say that. And I think that it is a mistake, but not a stupid mistake at all because it's due to a very difficult question which in various contexts he tackles four times. What he's got in mind is this: that if there were no conscious beings at all in the universe, then there would be no possibility of counting. And from that he infers that there wouldn't be anything countable and so there wouldn't be any time. It's not a stupid idea because the idea would work for certain other concepts. But I think what he's overlooked, and he overlooks it more than once, is the difference between an ability and an opportunity. What I think he should have realized is that if there were no conscious beings in the universe, there would be no opportunity for counting. But there still might be something which had the ability to be counted.

Peter Adamson: Right, just the way that a visible thing could be visible even if there's no one around to see it.

Richard Sorabji: That's true. But take another example where his way of looking at it would I think be comparatively plausible. Supposing we all became immune to mosquito bites. All the animals in the world became immune and none of them got malaria or any other disease from mosquito bites. Now would mosquito bites still be lethal? You see lethal, lethal is a word. It implies an ability on the part of mosquitoes to kill. But in that case, their ability seems to depend very much on something about their victims, doesn't it? Change the victims, make the victims immune, and they've lost their lethalness. So Aristotle is not making a stupid mistake. It's very difficult to work this out.

Peter Adamson: The difference between lethality and countability.

Richard Sorabji: We don't want to say in this case, oh no, but the mosquitoes are still lethal. It's just that there's no opportunity for them to kill. Now that's the wrong answer. Aristotle's view is absolutely right for a concept like lethal. So it's really quite surprising that it doesn't work so easily for the concept of visible, or the one that concerns us, countable. With countable I still insist we want to say not that there wouldn't be anything in a world without consciousness that was countable, merely that there wouldn't be any opportunity of counting.

Peter Adamson: Something else I wanted to ask you about time is that Aristotle has this view about time, which is that time is infinite. And not only does he think that time is infinite, but he thinks that the cosmos has always existed, the universe in which we live, which is a sphere with these astral bodies surrounding the earth, which is in the middle. And he also thinks that the way that this cosmos is constructed is permanent. So he not only thinks that time is infinite, but he thinks that the cosmos is eternal and has always been pretty much the way it is now. For example, all the species of animals are eternal as well. For example, there have always been mosquitoes, according to him, if there are mosquitoes now. Why does he think this? I mean, it seems like a very bold thing to think. And it's not the sort of thing that he could have just looked around and observed. So what kinds of reasons does he give for believing this?

Richard Sorabji: Well, first, just the smaller point for the present purpose. Why does he think that there's always been a universe? Well, he thinks that motion and therefore matter and also time couldn't possibly have had a beginning because if it suddenly began, you would need to have some triggering motion before the beginning to cause that supposed beginning. Some force would have to come closer or change in some way in order to bring about this supposed beginning of motion. So there couldn't be a beginning of motion.

Peter Adamson: Because whatever first motion was supposed to happen would have to be kicked off by a previous motion.

Richard Sorabji: It would have to be kicked off by a prior motion.

Peter Adamson: So it wouldn't be first after all.

Richard Sorabji: So it wouldn't be first after all. Right. But now your main question was about the animals always having been the same. Now there were, and you've discussed this, I think, at least two people in the early phase of Greek philosophy who did talk about the change that has occurred to species of animals. First, in the 200 years before Athens became the center of philosophy, when we had just individuals doing philosophy all over the Greek-speaking Mediterranean world, first we had Anaximander very early on in the sixth century before our era, saying that humans had originally been inside fish and that other animals had come out of mud. And then more importantly, we had in the fifth century BC, Empedocles saying that there was natural selection, that there had been a certain kind of evolution. And he very clearly expresses the idea of natural selection. Why at least did Aristotle not notice that? Natural selection had been invented already by Empedocles.

Peter Adamson: Well, I guess it can't then be that he assumes that animals are, that animal species are eternal just because nothing else has occurred to him. The alternative is actually on the table.

Richard Sorabji: Exactly. So, Empedocles' great discovery of the idea of natural selection left out a very important element which Darwin introduced, which meant that Aristotle could easily refute Empedocles' version, and he didn't anticipate Darwin's counter argument. Empedocles, unfortunately, suggested that there were chance mutations, but he made the mistake of not thinking of these as originally isolated and then becoming widespread because they were favorable. He didn't think of that Darwinian idea. He thought that these chance mutations occurred everywhere at the same time, in lots and lots of examples. Now, that's not what Darwin suggested.

Peter Adamson: So, it's like all the fish would get legs at the same time.

Richard Sorabji: Exactly. That would be the chance mutation. And then they, right, then natural selection would favor them. Well, Aristotle was able to refute Empedocles on that point because Empedocles had got it wrong because Aristotle has a most brilliant analysis of what chance is, or he was thinking really of coincidences. And he made it part of the definition of coincidence and of chance that it's something unusual, that it's something that's not normal. So, because Empedocles had coupled his wonderful discovery of natural selection with a serious mistake about the nature of chance or coincidence as something that could initially be very widespread, Aristotle dismissed him. And therefore, he didn't see that there was any good reason. He was perfectly aware of a reason because he replied to Empedocles, but he didn't see there was a good reason for supposing that the species had changed.

Peter Adamson: Right. Now, actually, Aristotle thinks something even stronger than this, because not only does he think that the cosmos as a whole and the species are eternal, but he also thinks that anything that is eternally present or eternally the case is necessary. So, this sort of makes sense, because if one plus one equals two, and if that's necessarily true, then it has to always be true. But you wouldn't think that the reverse would be the case. You wouldn't think that something that's eternal has to be necessary. For example, I don't have a sister. I've never had a sister. It doesn't look like I'm ever going to have a sister. So it's eternally the case that I don't have a sister. But you would have thought it was possible that I could have had a sister. It just didn't work out that way. So I guess I want to say that Aristotle looks right to suppose that something that's necessary would be eternal, but the reverse doesn't seem to be right. So can you explain why he thinks that, why the eternal should be necessary?

Richard Sorabji: Yes, he doesn't think that absolutely generally the eternal is necessary, because he carefully gives an example of how if my old cloak never gets cut up in the whole of eternal time, it's necessary that it never gets cut up. He denies that. It's only with things which are themselves eternal, unlike my cloak, that if they, in the whole of eternal time, never have something happen to them, then it's necessary that that thing never happens. It's only with eternal subjects that he applies this principle.

Peter Adamson: And here's where we'd come back to the stars, right? Because the stars are supposed to be eternally revolving around the earth, and therefore their motion is necessary.

Richard Sorabji: Yes, exactly. That's a very good example. There are temptations to think this way, a lot of people, if you ask them nowadays, about the monkeys randomly typewriting on a typewriter, as the example was originally. Now the common idea is that if you took monkeys - let them be eternally existing monkeys on an eternally existing typewriter - if they went on randomly typing for eternal time, they would eventually have to write out the works of Shakespeare. A lot of people think that's true, but it isn't actually true at all. Let's take something which Aristotle thought to be eternal, the physical universe. Now perhaps the physical universe will never contain in itself a golden mountain. Well, now, a golden mountain seems to be possible in various ways. For one thing, it seems to be a perfectly coherent, conceivable idea, unless when we understood the physics better, there's some contradiction we haven't noticed.

Peter Adamson: But there doesn't seem to be any conceptual problem with a mountain made of gold.

Richard Sorabji: Not so far as I know. Also, as far as I know, it would be physically possible. I don't know anything about the forces connected with the number of protons that you have in a gold atom that would make it impossible to have it massed up into a mountain. Let's suppose for a moment that it's both conceptually possible and even physically possible. Well, all right. Now on this mistaken view, which I'm afraid Aristotle does accept, given that in his view the universe lasts eternally, then since at least in two ways a golden mountain is possible, a golden mountain will have to be actual at some time or other. And yet that looks completely implausible. And so it is mistaken. But once again, the mistakes of a great philosopher are not stupid mistakes. In fact, as I've illustrated with the monkeys on the typewriter, they appeal to people nowadays when they first think about it.

Peter Adamson: Let me ask you one last question, which is also about this idea that the world has always existed and always will exist. That seems to be just a commitment to the idea that the world is, in a sense, infinite. It's temporally infinite. It's as it were infinite in both directions, the past and the future. Aristotle seems to think that. And yet he thinks something else, which is that the cosmos is finite spatially. He thinks it's a sphere that the heavens end with an outermost sphere. We live in the center of the sphere on the planet Earth - which for him isn't a planet, it's just this unmoving thing at the center. Why does he think that the cosmos is bounded spatially but not bounded temporally? In other words, why is it finitely small in size but infinitely big in time?

Richard Sorabji: Well, about time, we've already talked. The reason why he thinks time couldn't have had a beginning was the point about the supposed beginning would have needed an earlier motion to kick it off. But why does he treat space differently then? Why does he think that space is finite? Well, his answer is not the most obvious one, though it's understandable. The most obvious one had been given before his time by a Pythagorean: If you think there's an edge of the universe, well, could you stick your hand out when you got there? If you can't, there must be something beyond stopping you. So there's something beyond after all. But if you can, there's empty space beyond. But Aristotle doesn't use that. He knows of it, but he doesn't use it.

Peter Adamson: So just to clarify that for a second, maybe it's worth stressing that not only does Aristotle think that the cosmos is finitely sized, he also thinks there's no empty space around it. There's no void around it. All there is, is a finite magnitude.

Richard Sorabji: Absolutely. Yes, thank you. Well, he gives a different sort of answer as his main answer. He says that it's necessary that the stars and the matter which carries the stars moves round in a circle. After all, it eternally moves round in a circle around us. Of course, this is long before Copernicus said that it's we who are moving and not the stars which are moving around us. It's we who are spinning on our axis. There was a Greek a bit after Aristotle who thought that. And it's been canvassed even before Aristotle's times in one form or another. But he took the normal view that it is the stars which are circling around us and that they've done so eternally. And we've also seen that he thinks what happens eternally happens of necessity. Now, if they of necessity move and move in a circle, they can't fly off at a tangent. They can't fly off at a tangent and therefore they couldn't ever be received at some point beyond the furthermost star because they couldn't fly out there. Not even little bits of them could fly out there. That would be flying off at a tangent. They're confined to a circle. And then once again, he makes his mistake about the difference between capacity and opportunity. Because he says that when we talk of place, and we might put it in terms of space, we mean by that something which can receive matter. But if the furthermost stars are confined to moving in a circle and cannot fly off at a tangent, then matter cannot fly out in order to get received.

Peter Adamson: So it's almost like saying that there can be no empty space outside the universe because an empty space or place is just something where something could be. And if nothing can be there, then there's no place, no space.

Richard Sorabji: Right. But it's once again, not distinguishing between ability and opportunity. There might be something out there, empty space, I would say, which is defined as having the ability to receive matter. All that follows from the fact that matter can't fly off at a tangent and get received beyond the furthest star is that there'll never be an opportunity for matter to be received out there. But that doesn't stop there being something out there which has the ability to receive matter. And so I'm afraid, not for stupid reasons, but once again, his pioneering philosophy has got it wrong.