Until 150 years ago, or so, the argument for God’s existence from design was probably one of the more commonly understood arguments of natural theology. (Natural theology consists of the things we can say about God by the light of our own reason and nature, in contrast to revealed theology, which are the things God has told us about himself.) After the rise of NeoDarwinism (by which term I refer to the Dawkinsian creation myth and not the scientific theory of evolution), the argument from design is still intuitively understood by many people, but it has generally become misunderstood formally. If you were to ask an atheist on the level of Richard Dawkins—who is among the best of the worst atheists—what the argument from design was, if you lucked into a calm and concise one you’d get something like this:
If you look at the natural world, many things in it are very simple, like rocks, but many of the things in it are far more complex than can reasonably be supposed to be assembled by blind chance. Things like plants and especially animals are too complex to be an accident, and so they must have been created by an intelligence more complex than they are. Since we, too, are part of the natural world, there must be something more intelligent than us which made us, and that thing is God.
This is not at all the classical argument from design, such as you can find in the Summa Theologica, though I will grant you that you can find something like it from young-earth creationists. It is, fundamentally, a god of the gaps argument. God of the gaps arguments are more repugnant to orthodox Christians than to atheists because they are an insult to God: they claim to show that God exists because the natural world doesn’t work and needs to be constantly fixed. This is a relatively new idea; it really only makes sense in the context of modern mathematical physics. Before that attempt to fit the workings of the universe into the human head, no one ever supposed that the universe didn’t actually work.
(At least next to nobody. There is probably some ancient Greek philosopher who argued that, because for pretty much any argument there is an ancient Greek philosopher who argued it. And technically (original) Buddhism is based on the idea that the universe doesn’t work, but at a higher and qualitatively different level than what I’m talking about here. Also, Buddhism is fundamentally atheistic. Since it holds that everything is an illusion, it holds that its gods are not real, and it certainly denies any uncreated creator. It’s much more akin to the zero-energy hypothesis.)
The classical argument from design is not based on probabilities and certainly does not depend on the idea that natural things do not fit together. It in fact contradicts the idea that the unfolding of nature couldn’t have been according to a natural process precisely because it argues from the fact that natural processes actually work. A fundamentally broken world would undermine the classical argument from design. So, without further introduction, here is a version of the classic argument from design (my words):
If you look at the world, it exists imperfectly according to a rational hierarchy of being. Things at lower levels work together to the advantage of better things, and these better things in turn order and improve the lower things. Quarks work together to form protons and neutrons. Protons, neutrons and electrons work together to form atoms. Atoms work together to form molecules. Molecules work together to form bodies. These bodies include plants, which turn sunlight into food, and animals, which eat the food the plants make. Some animals keep the other animals from over-eating the plant food. Other animals spread the seeds of the plants, as well as nutrients which the plants need. There are also less clever and more clever animals, with a rational animal at the apex, who directs the lower animals as well as the plants toward a harmonious function. In all of this there is a rational order where the parts fit into each other and work together to create a good whole. This rational design reflects and points to a rational mind which orders the natural world according to the good. Any such rational mind which is itself a part of nature, such as a super-intelligent space alien or a little-g god or extra-cosmic aliens in a universe that created our big bang, or whatever, would themselves be a higher step in this rational order, since they are a part of it by virtue of shared time and causality. There must, therefore, be some rational mind which is not part of it, which stands utterly apart from it, like how Shakespeare stands apart from Hamlet or the characters in The Mousetrap (the play within the play). This rational mind which is utterly apart from all of the rational creation with a shared causality is what all men call God.
(Where the natural world varies from the rational order, this constitutes is a rebellion of the rationally ordered creature against its creator, possibly very indirectly since things are supposed to receive their rational ordering according to the other things within their shared hierarchy. Thus we clearly live within a fallen world, but that means we live within a rationally ordered world that has partially broken, not within an irrational world that doesn’t work at all.)
To see the difference between this world and an irrational world, consider how any of the components could have gone wrong. Suppose up quarks weren’t compatible with down quarks: we’d have neither protons nor neutrons, and consequently neither atoms nor molecules nor bodies nor plants nor animals. All there would be is a vast sea of sub-atomic particles without any interesting organization. (And please bear in mind I’m only saying that world would be irrational; I’m not saying anything about how likely or unlikely it is—its probability is utterly irrelevant.) Or suppose electrons couldn’t orbit an atomic nucleus: the result would be an ever-dispersing gas of particles fleeing from each other since nothing held them together. And again, I don’t care whether that possible world is more or less likely than ours, I only care that it would be far less interesting, because that is just another way of saying it would not be rationally ordered. What interests us is intelligible order—no one is fascinated by noise.
The same can be seen if we look at evolution. Dawkinsian atheists love to talk about how order emerges from chaos because of simple rules, in this case the simple rule being natural selection. This is fair enough so far as it goes, but it doesn’t go very far because mere order is not the same thing as rational order. In science fiction one encounters stories of nan0bot catastrophes where self-replicating nanobots which can use their environment for raw materials get out of control and turn the entire world into unimaginably many copies of themselves. This is called the grey goo scenario. But we in fact already have self-replicating nanobots which can use their environment for raw materials. They’re called bacteria. So why isn’t there a bacteria-goo scenario, where some bacteria hit on the winning combination of genes/proteins and turned everything into a copy of itself? A single species that has smashed all others is very compatible with survival of the fittest. (Yes, I know that survival of the fittest is only an approximation of the biological theory of evolution, but the more proper theory doesn’t differ in this regard.) Perhaps domination might be bad for the bacteria, but that downside could only emerge once they’ve wiped everything else out, at which point there would be no other species left to balance things out again. This also applies to other layers in the hierarchy of being. Why aren’t all plants poisonous? It would not be hard for a plant to have eliminated the first herbivore, and to have made a herbivore-free world. If wolves ate every last prey animal, they would starve to death, but only after they ate the last one. Then there would be neither predator nor prey and just an animal-free world left. (And it’s no answer to say that the changes happen so gradually that balance is always maintained because we know that evolution often happens very quickly. The gradual accumulation of changes is more a just-so story for children than it is a description of how evolution has typically worked, and certainly is not a description of how it can work.)
Now once again, I’m not talking about what is more probable, but what is more rationally ordered. (That is why it’s irrelevant that one species could balance out against another’s recently gained advantage; that’s only a question of probability.) A world in which a super-bacteria ate everything else and so was the only thing left (and then died off if it wasn’t an autotroph) would be very orderly, but its extreme homogeneity would not be a rational hierarchy. It would be just as complex as the world we live in, since it would have just as many moving pieces, but it would be far less interesting. And as the way that every foreign animal introduced into Florida seems to kill off the native species shows, evolution does not of its nature tend to produce a more interesting world. It won’t for the same reason that the history of warfare shows weapons all converging on the same basic designs: optimizing for one thing rarely has more than one solution.
Now, the reason why probability does eventually enter the discussion is that for any configuration of matter, it is always possible that it got that way by sheer accident (“randomly”), and so a world organized according to a rational hierarchy of being must, of necessity, look like a possible accidental outcome of blind matter. (This is less true if one recognizes the existence of free will, but since people wish to entertain the notion that free will might actually be an illusion, the similarity is unavoidable.) Thus one must ask of a thing that is organized according to a rational hierarchy: how likely is this to really just be a pure accident rather than what it appears to be? But please note that this question is utterly different from a god-of-the-gaps argument. We are not asking whether this world could work without God. We’re asking whether this world that looks like it was made by God could in fact be an accidental similarity only. We’re asking whether the portrait of a man we’re looking at might have been the result of a canvas and some tubes of paint falling off of a table and the resulting mess just happened to look like a skillful portrait of the man. That could have happened; the right colors could have been on the table, and the dog might have carried the tubes of paint off back to its bed to chew on them. An excellent portrait of a man is not impossible without a painter. But between a skilled painter and a freak accident, my money is on the painter.
That being said, this is why the argument from contingency is much stronger than the argument from design: the argument from contingency shows that it is absolutely impossible that there is no God. This is also why Dawkinsian atheists value evolutionary anecdotes so much—vivid stories capture the imagination and make the whole thing seem more plausible. It’s also why Dawkins spends so much time angrily sneering. His alternative is to say, “Come on, guys, it’s not technically impossible!” and that would be poor salesmanship.
In closing, I would like to show the version of this argument which you can find in the Summa Theologica:
The fifth way is taken from the governance of the world. We see that things which lack intelligence, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that not fortuitously, but designedly, do they achieve their end. Now whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is shot to its mark by the archer. Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God.
There is a great difference of expression between my version of the argument and Saint Thomas’ version, but they mean roughly the same thing. Most of the differences arise from a very different standard education. We are not taught about goodness or what the relationship between different types of beings is, and very little about intelligence and less about rationality, so in a modern context such things must be explained at length, whereas in Saint Thomas’s time, anyone with even a tiny bit of education was familiar with those concepts. Our scientific knowledge, by contrast is far more advanced. Everyone has heard of protons and neutrons and electrons, and most people have heard of quarks.
There are two addenda which I should discuss briefly: the weak anthropic principle and the infinite multiverse.
The weak anthropic principle is, roughly, “if the universe weren’t configured in its present way, we wouldn’t be asking why it was configured this way.” Typically its phrasing is adapted to the needs of the moment, but it always means as little. Probably the strongest statement of it—and this isn’t saying much—it is technically possible that our evaluation of a thing is influenced by having grown up in a world where that thing having happened. Usually it’s said in a way to suggest that our evaluation most likely was so influenced, but this is pure showmanship, without any admixture of a reason to believe it’s true. “I believe it, so you should too if you want my respect,” intimates the Dawkinsian atheist, as if any self-respecting person wouldn’t question his life choices if a Dawkinsian atheist did respect him.
The infinite multiverse hypothesis family of hypothesis that claim, essentially, that every possible world exists in a parallel universe. Basically, take Occam’s Razor and reverse it: unnecessarily multiply entities. I think that this originated with the question of why our physical constants (the charge of basic particles, the gravitational constant, etc) were the way that they were, and so one answer proposed—presumably by someone who read too much science fiction—was that every possible world happened, and we’re just in one that turned out to produce life. How anyone gets past the instant destruction of science, I can’t imagine. If every possible world happens, then there are an infinite number of worlds where all scientific experiments came up with their results by accident. There are infinite number of worlds where some spiky demon-monster with amazing nano-technology to keep you alive whips you in a pit of fire until the heat-death of those universes for not believing in Jack T. Chick tracts. And so on. And there is precisely no way to tell which of these parallel universes you are in. Since there are infinitely many of the bad universes, there isn’t even a way to tell how likely any of these bad universes is. And all of this is relatively obvious with a few seconds of thinking about it, which should tell you how seriously any of the proponents of the infinite multiverse hypothesis actually take the idea.