Life’s restlessness

Why does life resist disorder? Because ever since the first replicating molecules, another kind of stability has beckoned

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Weight of numbers: if unchecked, self-replicators such as these monarch butterflies can multiply exponentially. Photo by Frans Lanting/Gallery Stock

Weight of numbers: if unchecked, self-replicators such as these monarch butterflies can multiply exponentially. Photo by Frans Lanting/Gallery Stock

Addy Pross is professor of chemistry at Ben-Gurion University of the Negev, Israel. His latest book is What is Life? How Chemistry Becomes Biology (2012).

Biology is wondrously strange – so familiar, yet so strikingly different to physics and chemistry. We know where we are with inanimate matter. Ever since Isaac Newton, it has answered to a basically mechanical view of nature, blindly following its laws without regard for purposes. But could there be, as Immanuel Kant put it, a Newton of the blade of grass? Living things might be made of the same fundamental stuff as the rest of the material world – ‘dead’ atoms and molecules – but they do not behave in the same way at all. In fact, they seem so purposeful as to defy the materialist philosophy on which the rest of modern science was built.

Even after Charles Darwin, we continue to struggle with that difference. As any biologist will acknowledge, function and purpose remain central themes in the life sciences, though they have long been banished from the physical sciences. How, then, can living things be reconciled with our mechanical-mechanistic universe? This is a conceptual question, of course, but it has a historical dimension: how did life on Earth actually come about? How could it have? Both at the abstract level and in the particular story of our world, there seems to be a chasm between the animate and inanimate realms.

I believe that it is now possible to bridge that gap. But before I explain how, it is worth mentioning how modern biology has generally dealt with it. Bluntly, it has dropped the problem into the ‘too hard’ basket and looked the other way. This has meant fencing off biology from physics and chemistry, and developing a separate philosophy of science. One of the leading evolutionary biologists of the 20th century, Ernst Mayr, openly argued for the ‘autonomy of biology’. Physics and chemistry deal with inanimate matter, he insisted, biology deals with living systems, and, at least for the time being, that’s that.

But this is not good enough. Nature is one. Science seeks to generalise, to unify. The purpose-driven character of life stands as a challenge to our understanding of the material nature of the universe. We can’t leave it there. And happily, we don’t have to.

I am a theoretical chemist drawn to a new field, systems chemistry. That means I’m interested in replicating molecules and the reaction networks they establish. Some recent research in this field appears to show us just how biology can be restored to the mechanical world. These replicators cross Mayr’s great disciplinary divide with impunity. In the laboratories of my colleagues, the living and dead realms bleed into one another.

And so the conceptual unification of biology with physics and chemistry is now underway. New insights are steadily coming into view. The first important one, in fact, concerns precisely that question of how life on Earth might have begun.

The name we give to the process by which simple life emerged from inanimate matter is ‘abiogenesis’. Evolution, on the other hand, is the biological mechanism by which life branched out into Darwin’s ‘endless forms most beautiful’. Traditionally, these are viewed as quite different things: the former, one of nature’s greatest mysteries; the latter, broadly understood, thanks to Darwin. Through systems chemistry, however, they stand revealed as a single continuous progression.

We now know that a mechanism akin to Darwinian evolution actually operates, in the first place, on nonliving matter – even on single molecules. Feed a population of RNA molecules the appropriate chemical building blocks and, under the right conditions, they will start to self-replicate. What’s more, over time, you will see the population evolve: slow replicators will give way to faster ones. RNA is not living material in any meaningful sense, yet it is subject to evolution. Thus we find our first bridge between living and dead matter.

The second insight is even more momentous. Evolution exhibits an identifiable driving force, a direction if you like, and this ‘teleological’ tendency acts at both the chemical and biological stages; that is, it operates both during, as well as after, what we think of as abiogenesis. Thus the purpose-driven character of life, the very thing that seemed to distinguish biology from the rest of nature, turns out not to be unique to life after all. Its beginnings are already discernible in certain inanimate systems, provided they are replicative and able to evolve. And this driving force can be described in strictly physical terms.

Put simply, it is nature’s drive towards greater stability – a drive that is as ubiquitous in physics as it is in biology.

Here’s a little truism for you. Unchanging things don’t change, and changing things do change – until they change into things that don’t. That statement is, of course, true as a matter of logic: as true as ‘one plus one equals two’. And like ‘one plus one equals two’, it turns out to make surprisingly strong predictions about how the world works. If every changing thing really can change into an unchanging thing, then we should expect all changing things to change into unchanging things eventually.

things tend towards persistent forms because stability is the end of the road for all things – perhaps even the Universe as a whole

We talk a lot about stability both in science and in everyday life. It means pretty much the same thing in either context: long-lasting, persistent, unchanging over time. And true to the prediction of our little logical truism above, there is indeed a law of physics and chemistry that says that things, in general, become more stable over time. I’m talking about the Second Law of Thermodynamics, one of the most famous laws in all of science.

The Second Law explains the direction of change and why certain things remain unchanged over time. That explanation can be given in terms of energy. High energy is associated with instability. Low energy is associated with stability, and when a physical system reaches its lowest energy state (equilibrium), no further change takes place.

How pervasive is this law? Ever since the Austrian physicist Ludwig Boltzmann revealed its mathematical logic through the concept of entropy in the 1870s, a common view has been that the entire Universe is moving towards a terminal low-energy state, a so-called ‘heat death’ of infinitely persistent forms. But notice that the stability predicted by the Second Law is just a special case of our truism: things tend towards persistent forms because stability is the end of the road for all things – perhaps even the Universe as a whole.

Let’s look at the mathematics of entropy for a moment. It used to be thought that the Second Law was simply a brute fact about the universe. Energy flowed in certain ways, there were certain basic limitations that you ran up against when you tried to convert it into work, and that was that. Until, that is, Boltzmann’s entropy formulation explained how these empirical observations revealed a deep necessity. They emerged quite naturally from the mathematical theory of probability itself.

What is entropy? Simply put, it is a measure of the orderliness of a physical system. States of low entropy are ‘highly ordered’ in something very like the everyday sense of the phrase. Picture a tidy desk, papers neatly stacked, pens in the pot. As order decreases and the desk gets messier, entropy increases.

Now, suppose that every possible arrangement of the desk is equally probable (perhaps your way of cleaning up is to take the whole room and shake it). There are of course vastly more ways to be messy than there are to be tidy – so the chances are high that your desk is messy. After all, the messy states outnumber the tidy ones. The weight of numbers is on the side of messiness. What’s more, any random change to the arrangement is likely to make it even messier. Now, random changes, in the form of bumps and jiggles, happen constantly. So things get more stable (and messier) over time. As the Universe undergoes change, entropy increases.

High entropy and low energy, however, are just one manifestation of stability. Does nature offer others? It does. It turns out that stuff can be highly persistent even when it is highly unstable energetically. Indeed, that’s precisely what we find in the world of replicators.

Living things are low-entropy and energy-consuming, so they are unstable in the thermodynamic sense. Nevertheless, they can still be remarkably stable in the sense of persisting over time. Some replicating populations (certain bacterial strains, for example) have maintained themselves with little change over astonishing periods – millions, even a billion, years. They exhibit what we call dynamic kinetic stability (DKS). And, like entropy, DKS turns out to be driven by simple, powerful mathematics.

Change the environmental conditions and the winner of the replicative race can change

In fact, it rests on the mathematics of exponential growth. This is a pattern that we often see in self-replicating systems, and they don’t even have to be physical. Suppose you start with a dollar. Double it every week and, in well under a year, you’ll be the world’s richest person (assuming no one else discovers your secret). Keep going for another five years and you’ll have more dollars than there are atoms in the observable universe. Self-replicating molecular systems can, in the right circumstances, start off on the same explosive path. But there’s a twist: when they do, a new kind of chemistry emerges. Ultimately, it is this new chemistry that leads to what we term biology.

How could such a transformation come about? Why do replicating molecules give rise to replicating cells? In a word: evolution. Or, in four more: replication, variation, competition, selection.

Replicators do not always make perfect copies of themselves, and their variants have to compete with the originals for resources. Because both the originals and ‘bad’ copies share the same tendency towards exponential growth – because neither of them will stop unless they run out of resources – the more effective replicators end up driving the less effective ones into extinction. Accordingly, less persistent replicators tend to evolve into more persistent ones. Just like in the ‘regular’ chemical world, change in the replicative world is directed toward greater stability. Once again, it all comes down to the weight of numbers.

There are, however, two big differences. The first is this. In the replicative world, stability can be unrelated to energy content. Provided there is a source of metabolic energy to keep the thermodynamic books balanced, anything goes. So this is genuinely a different kind of stability.

The second difference is a little harder to grasp. With entropy, the weight of numbers is always in the same direction. That keeps things simple: everything tends towards randomness and disorder. With DKS, on the other hand, stability is fickle. Some replicators are indeed astonishingly durable, but, crucially, DKS always remains circumstantial. Change the environmental conditions and the winner of the replicative race can change. In fact, that’s exactly what makes life so capricious and the evolutionary path largely unpredictable: the mathematics of replication forces it into a paradoxically restless search for rest.

Why are living things so complex? Here’s another seemingly eternal riddle that we’re now in a position to answer. As many a systems chemist has learnt to his or her sorrow, the simplest molecular replicators can be quite finicky. You need fancy labs, specialised equipment and dedicated researchers to get them to replicate and, even then, it can be hit or miss. By contrast, biological replicators – living things – are extraordinarily robust.

Consider some of the simplest life forms, bacteria. These highly complex entities can survive and prosper pretty well anywhere – some deep within the Earth, some high in the atmosphere, some in boiling water, some in nuclear reactors, no labs, equipment or human assistance required. The inordinate complexity of all living things has emerged for one reason alone – to facilitate the replicative function, thereby enhancing the stability of the replicating system.

How, then, did that extraordinary complexity come about? The answer, of course, is: one step at a time. Gerald Joyce, professor of chemistry at the Scripps Research Institute in La Jolla, California, recently demonstrated how a single replicating RNA molecule, on its own, is a relatively inefficient replicator. In contrast, a two-molecule RNA replicating network, in which each RNA molecule catalyses the formation of the other, is far more effective. The two-molecule system is more effective for the same reason that picking up an object with two fingers is a lot easier than with just one, and nature has exploited that fundamental design principle. So complexity and function go hand in hand. Joyce’s RNA experiment demonstrated the first (conceptual) step on a thousand-mile journey – toward that stupendously effective (and inordinately complex) replicator, the bacterial cell.

the reason for the teleological character of all living things becomes obvious: nature’s most fundamental drive is toward greater stability

Finally the secret of nature’s two material facets – animate and inanimate – is coming into focus. Animate and inanimate forms came about because there are two mathematical engines of stability. Boltzmann showed us the more familiar thermodynamic one, but the other is traceable to the English cleric Thomas Malthus. After all, it was Malthus, studying the economics of famine in the 1790s, who first recognised the profound consequences of exponential growth in a biological context. From Malthus, Darwin took his rise, and then on to the founding fathers of systems chemistry – Sol Spiegelman, Manfred Eigen, Leslie Orgel, Günter von Kiedrowski, and others.

Of course, once we recognise the existence of two distinct stability kinds, one based on probabilities and energy, the other on exponentially driven self-replication, the reason for the teleological character of all living things becomes obvious. Nature’s most fundamental drive, dictated by logic itself, is toward greater stability. That drive has a thermodynamic manifestation, as expressed through the ubiquitous Second Law, but it also has a kinetic manifestation – the drive toward increasingly persistent replicators. Two mathematics, two material forms. This distinction does not trace the dividing line between living and dead matter precisely – but it does explain it, and many of the other riddles of life into the bargain.

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  • Belisarius85

    This was a very interesting article. I've been reading some molecular biology textbooks in my spare time, and I found the RNA World Hypothesis fascinating, which the author touches upon tangentially.

  • Mike

    Very strange, why was my comment removed?

    • Clinton Weir

      It wasn't.

  • Psyclic

    "making a mountain out of a molehill" except that this seems to be a purveyance of a distinction without a difference. CPSnow's Two Cultures explored the cognitive difference between scientists and non-scientists and found a perceptual difference of attention and thought, and not a qualitative difference.
    The junction between animate (living) and non-living requires a better understanding of the physical (ie -chemical, mechanical and electrical) mechanisms which, ultimately, are the distinguishing features (at this level).
    The same organizational forces which seem to counter entropy of biologic systems, are the same which drive the growth of crystals.

    • Ed Lake

      Hi Psyclic, I come bearing a reply from Addy Pross. He writes:

      "The driving force for crystal growth (entropy increase) is totally different to the entropy-reducing actions of all living things, which is only enabled through a highly sophisticated energy-gathering (metabolic) capability. It’s like comparing a ball rolling downhill with a car driving uphill (with gasoline in the tank) and saying that since both are physically allowed, both are natural processes. We understand the car’s action because it was designed to drive uphill. But how can the existence of a natural system that behaves like that car be explained? It is not by chance that leading physicists, biologists and philosophers of the 20th century (Bohr, Schrodinger, Wigner, Popper, Monod, to mention some) expressed bewilderment with the inexplicable physical behavior of living things. Regarding maths, yes, I agree: mathematics itself does not drive anything. But through the language of mathematics, in this case the math of exponential growth, the physicochemical behavior of living systems becomes explicable. The right maths enables us to characterize more explicitly the observed patterns of nature."

      • Psyclic

        Both 'life' and crystallization occur in open systems. From what I understand, the crystallization process results in a lower entropy level for the crystal by increasing the entropy of the existing solute. Thus it would appear to closely mirror the organizational utilization of environmental elements within the process of 'life'.
        Jeremy England's thesis ( seems to mirror this very behavior.
        And I do not think that Schrodinger (What is Life) was 'bewildered at the inexplicable behavior of living things', as I would suspect, neither were the others mentioned.

        • Addy Pross

          There is a very long list of great minds that found the life phenomenon inexplicable. Here are a few with some quotes:
          Jacques Monod: "...a profound epistemological contradiction. In fact the central problem of biology lies with this very contradiction, which....must be resolved"
          Erwin Schrodinger: "We must be prepared to find a new type of physical law prevailing."
          Niels Bohr: "Life is consistent with but undecidable or unknowable by human reasoning from physics and chemistry".

          • Psyclic

            I think that in writing science for the interested, there are some simplifications and literary tools (allusion, metaphor, hyperbole, etc) which are used with varying levels of subtlety for varying reasons - this is not, after all, a scientific paper in a peer-reviewed journal. That being said, I would refer you back to Schrodinger's introduction to What Is Life? as well to a rather excellent review of Schrodinger's original lectures in the Guardian (
            I would suggest that Schrodinger, and others, tread lightly into new realms, but tread they did, and without hesitation, befuddlement or qualms.

          • johnwerneken

            Yep. Possibly the origin of the universe might be explained, and even duplicated by humans. Yet there still possibly, maybe probably, would remain A mystery - how anything within the universe, or A universe, could know a thing about something outside that universe - whether there were such an outside, and whether the universe in question sprung in some quantum-mechanical way from it, or some being there created it intentionally.

            Nothing else seems mysterious, that is to say, potentially intrinsically unknowable. Including life, whether as to origin or condition or prospects.

          • Psyclic

            Not sure what your point is here.
            Postulate the existence of an unknowable entity.
            What is it? It is unknowable.
            Why is it? We invented it, but maybe it existed before us.
            That gets to the same place, doesn't it?

          • Roy Niles

            That a thing may never be known is far different from it therefore being an unknowable thing.

          • Roy Niles

            How do you know what's ultimately unknowable?

  • Lucas Mearian

    We have not a clue how life began. We have theories, not one of which explains "why" life began.

    • Stephen Kopil

      We do have ideas on how life began. It is not necessary to have a why.

      • Roy Niles

        All hows must have a necessary why.


    The fabric of reality is trying to see and know itself. That is what drives life to exist.

  • Howard

    Strikes me as a logic flaw to assume that because changeable things may change into an unchanging thing, then EVERY "changing thing really can change into an unchanging thing." Even if that were a possible outcome, there would still be no basis for expecting "all changing things to change into unchanging things eventually."

  • dude man

    "Science seeks to generalize, to unify." No. The idea that the ultimate purpose of scientific inquiry is to come up with the most general of statements is merely a reflection of hubris. Science has many purposes, one of which is to generalize, in the service of our needs. Generalization for its own sake is usually boring and tends to miss the point. Other purposes of science which are at least as important which this author "bluntly" pushes to the side include specificity, contingency, categorization, and deep, nuanced understanding. Generality cannot substitute for those, but it can work with them. It's a shame when we pit these different motivations and purposes against each other, rather than learn new, meaningful ways to combine them.

    • Psyclic

      The history of science produces generalizations; Science seeks to understand. Certain scientists realize that some restatements of observations can result in generalizations which unify (Newton: Laws of Motion, Maxwell: Electricity and magnetism, Einstein: space and time).

      • dude man

        Yes, AND... Why would you insist on narrowing scope? That's rather un-curious, to only push science in the direction of generality. Note your own Newton/Einstein examples - Newton had a description for everything until Einstein found glaring exceptions. Physics now needs different metaphors for different scales, despite everyone's best efforts to "unify" our description. If generality is the only direction, then we will take ourselves into blind alleys. New descriptions and multiple categories and theories are GOOD because we are not gods - we're mortals with perceptual systems and perspectival idiosyncrasies. I find it rather obnoxious when people condescend with narrow-mindedness. And if your response is to claim that regardless of disagreements and different paradigms in the course of scientific progress, that the end result is still generality, then I wholeheartedly wish you and the author of this piece all the best in a search for a formula that describes everything and nothing.

  • Derek Roche

    If you're serious about this, why not say that atoms also exhibit dynamic, kinetic stability; that, in fact, they are also homeostatic systems of subatomic particles? You might say Yes, but atoms don't evolve. And I might say No, but that's chemistry.

    • cuzzin

      agreed, the scope of this essay is too limited, i marvel at the "stability" of atoms that hold their form in what seems to be described as a wild and chaotic quantum universe. I might also add that "stability" and "unchanging equilibrium" are a complete misunderstanding of the never-ending flux in these systems - stable on the surface, constantly oscillating if you care to look a bit deeper.

      • Derek Roche

        Yeah. DKE (dynamic, kinetic equilibrium) might be a better acronym.

  • Mike

    My main issue is that this "truism of stability" is a special case of the second law of thermodynamics and not the other way around as the author puts forth.

    • Addy Pross

      If that were the case then, given the Second Law, the function and existence of living things would make sense. But it doesn't - read Monod's "Chance and Necessity" to see why. DKS as the second leg of the "truism of stability" offers a resolution of this thorny problem.

      • Mike

        Thank you for the suggestion, I'd like to learn more about that. What are your thoughts on the work of MIT's Jeremy England and the dissipation-driven adaptation theory?

        • Addy Pross

          Jeremy’s recent paper in J Chem Phys was very
          interesting though it primarily addressed the physics of replication, rather
          than the evolutionary process that replicating populations undergo. Basically
          Jeremy’s contribution belongs to the ‘life is a thermodynamic phenomenon’ school, whereas I believe that life and evolution are fundamentally kinetic phenomena, with
          kinetic stability (DKS) as its underlying organizational principle.

          • Psyclic

            In what way do you distinguish the mechanisms of replication from the mechanisms of evolution?

          • Addy Pross

            These are quite different. The mechanism of replication is that by which 1 entity becomes 2 (template for a molecule, more complex for bacterium or crab). The mechanism of evolution is based on differential replicating kinetics within a heterogeneous replicating population. Eigen and Schuster did great work on the mechanism of evolution with their quasispecies model.

          • Psyclic

            This is an 'in vitro' distinction. It would happen in a Xerox copier. This distinction vanishes in the wild - the mechanism of 'replication' contains the mechanism for 'evolution'.

          • Roy Niles

            Any mechanism that purports to accidentally create an intelligently operated form from scratch is unexplainable.

          • otakucode

            Accidentally? Intelligence is a very effective means of ensuring adaptation, replication, and survival. Or at least it seems to be thus far. It's no more 'accidental' than the evolution of cell walls.

  • Dr. Shekelstein

    Just ask a Christian.
    "God did it."

    • Allani

      The unknown is always "explainable" with mysticism.

  • Psyclic

    "the drive toward increasingly persistent replicators" - not sure what this is supposed to mean.
    Cockroach and many 'lower' life-forms (horseshoe crab, ginko) seem to have persisted in a fairly constant and consistent evolutionary state for a period of time longer than man's existence. Many life forms have more chromosomes than primates, yet these, too, have remained relative unchanged genetically over a much longer evolutionary time-frame than primates.
    Life goes on. Until it doesn't.

    • Addy Pross

      Exactly. Your last line said it all. A recent life form isn't necessarily a 'better' life form. Time is the only true judge.

      • Psyclic

        Of course, then this becomes a tautology: The older it is the older it is. What is the 'judgment', (ie 'value') of persistence? And if the 'drive towards increasing persistence' means that cockroaches, sponges, ginkos and horseshoe crabs have attained this persistence, why bother with anything additional? Where is 'time's arrow' in evolution?

        • amphiox

          The article already explains this. Persistence is dependent on the environment. Cockroaches, sponges, gingkoes and horseshoe crabs have persisted (incidentally, the modern forms are NOT identical to the ancient forms - they HAVE changed, indeed in many ways, over time) because the environments in which they are successful have persisted. Should those environments change, then they will cease to persist (ginkoes are already an endangered species today).

          More recent forms have not persisted as long, because the environments in which they are successful arose more recently.

          We do not know how long any current environment will persist. That is random. Very recent forms might well become the "living fossils" of the distant future.

    • polisny

      Isn't your question answered by the part of the article that says "They exhibit what we call dynamic kinetic stability (DKS). And, like entropy, DKS turns out to be driven by simple, powerful mathematics."

  • Roy Niles

    "Unchanging things don’t change, and changing things do change – until they change into things that don’t. "

    Except that there are no things that don't change and there never were.

  • seneca

    is stability, when understood within context (ie. stability within what system?), actually more akin to fittedness? systems change, evolve etc and we know organisms must adapt or fail. does the metaphor of fittedness less easily sit within the unifying project of the author to equate increasing physical disorder with organic persistence over time?

    • Addy Pross

      Yes, stability and fitness are indeed related. But 'fitness' is a biological concept whereas 'stability' is a more general physical concept. By thinking about biology in stability terms it helps place biological systems within a general physical context.

  • ChrisinCT

    Excellent article (even for non-scientists). I've often wondered at the paradox of the tendency towards entropy in the physical universe versus the tendency towards order in evolution. I've always thought of entropy as tending towards disorder and evolution as tending towards order. The idea that entropy tends towards a state of "stability" is interesting, as is the idea that evolution can tend towards rest or stability.

    Funny though, when you look out into the physical universe you don't see much disorder. You see an orderly system of constellations, galaxies and solar systems which are tending towards disorder. But what is the final resting state? Disordered stability or something else?

    • Addy Pross

      The Second Law and its applicability within a cosmological context is one that remains highly controversial so I'll pass on that one - it's not my field. But it is interesting nonetheless, that, since Boltzmann, much of the cosmological debate continues to revolve around the basic concepts of stability and the Second Law.

      • Thorfinn Axelson

        Dr. Adrian Bejan in his 2012 book, Design in Nature, written for a general audience, is a step towards a better layman's understanding of entrophy.
        This is Bejan's field, thermodynamics. His constructal law predicts flow systems behavior, regardless of animate, or inanimate distinction.

    • rsanchez1

      Constellations are a human construct. We see them as patterns because that is something our brains are good at doing.

      As for stars and galaxies, you can look at it this way. Entropy tends towards stability as the final state, but what really drives physical processes is the immediate tendency towards the lowest energy state.

      One of the physical forces is gravity, and the lowest energy state for two objects under mutual gravitation is to fall in towards each other, to attract each other. For interstellar gas, this cascades until the matter collected reaches critical mass and forms a star. The surrounding gas also coalesces but does not reach critical mass, so they form planets and other bodies that orbit stars.

      On a galactic scale, what scientists believes gives the galaxies their shapes are density waves. Galaxies typically have more visible matter in the center and the matter density goes down away from the center. The gravitational attraction between stars at different distances away from the center organizes the stars into structures of different densities, and these density waves are the spiral arms we see. Stars can go in and out of a density wave, but the density wave maintains itself through gravity and that's why we see spiral arms today.

      Of course, since entropy tends towards stability, eventually the stars will fizzle out, to put it simply, and the galaxies will also fade away. The end state does remain controversial, as Addy says, but the "heat death" is a popular theory for the end.

  • mbri
  • Luke Schray

    The crux of the argument all comes back to the unsupported truism: "Unchanging things don’t change, and changing things do change – until they change into things that don’t." Whence the unchanging thing in the real world?

    it all sounds a little too much like Aristotle, in Book 8 of the Physics and Book 12 of the Metaphysics, "there must be an immortal, unchanging being, ultimately responsible for all wholeness and orderliness in the sensible world."