If you have taken a college biology class, or just watched Animal Planet, you may have been struck by the startling complexity of living organisms. From the grandest mammal to the lowliest cell, life displays intricacy and structure that would put a high-paid team of engineers to shame. How could such fantastically organized, complex structures arise blindly out of unintelligent matter? Speaking of matter, why is it the way it is? Though unimaginably vast, our universe has precise features, as does the matter in it. A glance at the inside back cover of a college physics textbook shows that there are extremely precise numbers describing the fundamental properties of matter. These include numbers for the speed of light in a vacuum, for the masses of fundamental particles like the electron, proton, and neutron, and for the strengths of forces like gravity and electromagnetism that act on those particles. These numbers seem utterly arbitrary. For all we know, they could have been completely different. Yet they turn out to be exactly what a universe needs in order for complex life to emerge in it. Likewise, the cosmology section of an astronomy course will teach you that there are very precise values for the temperature of the universe, for how much matter there is per cubic centimeter in the universe, for the rate at which the universe is expanding, and so on. How did those numbers get to be what they are? Were they just magically pulled out of a cosmic hat at the Big Bang?
If you’ve ever asked yourself these “Why is the world the way it is?” questions, you are not alone. Scientists, philosophers, and theologians throughout much of Western history have asked just such questions. And throughout that history, one of the most popular ways to answer these sorts of questions has been in terms of God. On this way of thinking, just as an arrow in a bull’s-eye requires a skilled archer, a universe with the just-right properties of ours requires an intelligence to pick them out. Just as a watch requires a watchmaker, goes the thinking, life requires a designer. This is the basic idea behind what philosophers call the teleological argument (from the Greek word ‘telos’, meaning ‘end’ or ‘purpose’). It is more widely known as the design argument for the existence of God. Proponents of the design argument say the universe and the intricate structures in it could not have arisen by chance. Chance making our universe would be like a magical tornado blowing through a dormitory – one that left every bed made and put every empty pizza box in a trashcan. Such an event is just far too improbable to happen by chance. If chance is not an option, then only intelligence is left as an explanation of all of the apparent design in the universe. This intelligence will have to exist outside of our universe in order to act upon it, and it will have to be immensely powerful and knowledgeable in order to create something as vast, complex, and orderly as our universe. Proponents of the design argument find it obvious that a supernatural, super-powerful, super-knowledgeable intelligence who created a world of tremendous richness and beauty would just be God.
II. PALEY’S ANALOGICAL VERSION OF THE DESIGN ARGUMENT
Variants of this line of thinking can be found as far back as the ancient Greek era, but the classic statement of the design argument was by William Paley in his book Natural Theology (1802). Paley spelled out the analogical version of the design argument. He drew an analogy between finding a watch out in the middle of nowhere and finding intricacies in nature such as eyes, wings, and circulatory systems. Think of an old-style wind-up watch with gears and springs. It is a finely calibrated, complex item that serves a function. Clearly complex, functional objects do not just pop into existence in the middle of nowhere naturally. If you find a watch, you can infer that an intelligent creator of the watch exists or existed. Furthermore, you should infer this even if there are some imperfections in the watch and even if the watch has some functions the purpose of which you cannot discern. If the watch does not keep perfect time, or if it has a knob on it the use for which you cannot discern, you would still think the watch was designed. And if it somehow happened that we discovered the watch was actually reproduced from a prior watch – Paley imagines watches with hatches in the back spitting out duplicate watches – that still would not explain the current watch, because we would still need an explanation of how the original watch got its structure. If anything, we would have even more to explain – not just how a watch got out in the middle of nowhere, but how it got to have the ability to reproduce itself.
Paley said analogous reasoning justifies us in concluding that there is or was a designer of the intricate functional objects found in nature. We can conclude this even if natural structures are imperfect (like the human back, which is not quite ideal for walking erect) or have no known function (like the human appendix). For much of the rest of the book, Paley presented anatomical drawings and microscopic pictures showing case after case of intricate, organized, and well-designed biological structures. To see what Paley was thinking, consider the human eye. From a design standpoint, it is just like a camera. Both have lenses, light sensors (rods and cones for the human eye vs. film for a camera), and so on. Paley saw all of this functional detail in biology as screaming out “Design!” He was not alone. Particularly because of the development of microscopes, the naturalists of the seventeenth and eighteenth centuries were stunned at the geometric regularity and apparent design of the parts of living creatures. They just could not see how these intricate, machine-like structures could arise by natural processes.
III. DAVID HUME’S CRITICISMS OF THE ANALOGICAL DESIGN ARGUMENT
Before we consider an alternative scientific explanation of design in nature (namely, evolution), we should see that, in terms of pure philosophy, there is plenty of room to criticize Paley’s analogical version of the design argument. These criticisms were spelled out by Scottish philosopher David Hume in his book Dialogues Concerning Natural Religion (1779). Note that Hume’s work was published nearly a quarter-century before Paley’s. Versions of the analogical design argument were quite popular even before Paley’s Natural Theology was published. It was these versions that Hume criticized. Many of Hume’s criticisms are instances of what we might call the identification problem. Why think the terms ‘the designer of the universe and the life within it’ and ‘the supreme eternal being who knows everything and can do anything’ refer to one and the same individual? In short, why identify the designer with God? The evidence of design does not rule out designers other than God. Indeed, when we consider the imperfections of the world, says Hume, the analogy should lead us to conclude that the designer of the world is an imperfect being, not a perfect one like God. Hume suggests in Dialogues Part V that the universe might even be the product of some "infant deity" or of "several deities [who] combine in contriving and framing the world.” To Hume, our universe looks like it could have been designed by a committee – and not the best one at that!
In this connection Hume imagines inspecting a building that is drafty, poorly lit, and poorly ventilated, with crooked steps and doors that do not fit into the doorways. If we had to guess, we might grudgingly admit someone or other designed the building. But we would never infer that the world’s greatest architect did the designing. The building is not nearly good enough. If we were told that I.M. Pei designed the building, well, maybe we could square that with what we observed. As one of the world’s greatest architects, Pei might have a subtle building plan we non-architects just cannot understand. Pei knows a lot more about architecture than we do. So if we had some independent evidence that Pei designed the building, such as a film of him drafting the architectural plans, maybe we could manage to believe that the building was designed by the world’s greatest architect after all. But if we did not have such independent evidence – if all we had to go on was the building itself – then there is no way we would say it was designed by one of the world’s greatest architects. Hume suggests the same point works against the analogical version of the design argument. People like Paley are not asking us to reconcile the observed world with belief in God. They are asking us to infer the existence of a perfect being from the observed world. It is not that people like Paley are showing us the universe – a universe full of diseases, tsunamis, and other horrors – and saying “It is not impossible that God created this.” They are showing us the universe, warts and all, and saying “God must have created this.” Hume insists we cannot draw this conclusion. He thinks a perfect being could and would have made something much closer to perfect.
Hume also suggests the design argument involves another mistake. To see what that mistake is, think first about a different mistake: egocentrism. Some time in your life – maybe after you acted in a particularly selfish way – you have probably heard someone say “You think the whole world revolves around you.” At that moment, you were just accused of egocentrism. A person is guilty of egocentrism if he overrates his own importance in the grand scheme of things. Likewise, a person is guilty of anthropocentrism if she overrates the importance of human beings in the grand scheme of things (‘anthropos’ is Greek for ‘human’). Hume alleges that proponents of the design argument mistakenly regard human beings, or human attributes such as intelligence and thought, as the be-all and end-all of the universe. Proponents of the design argument think they see design in the universe, but Hume suggests they are just projecting design upon the universe. They see intelligence and thought as the most important aspects of the universe, but in reality intelligence and thought are no more special than anything else – in the grand scheme of things. “What peculiar privilege has this little agitation of the brain which we call thought, that we must thus make it the model of the whole universe?” Hume asks in Dialogues Part II, paragraph 18. “Our partiality in our own favor does indeed present it on all occasions, but sound philosophy ought carefully to guard against so natural an illusion.” Hume illustrates this point at the end of Part VII with an analogy. If there were talking spiders, Hume suggests, they would say the universe was created for spiders. We would say those spiders would just be projecting their own petty biases onto the universe. Hume proposes that we humans do the same thing when we endorse the design argument.
Indeed, Hume goes so far as to suggest that the reasoning behind the design argument is theologically offensive. The basic idea behind the design argument is that the universe is very unlikely to be the way it is if chance created it, but extremely likely to be just the way it is if God created it. But who are we to say what God would or would not be likely to do? None of us is God, and so none of us can tell what God would or would not do. Hume suggests when a person tells us what God would do, he might only be telling us what he would do if he were God. That, Hume suggests, would be presumptuous – as believers in God should agree. The source of that presumptuousness is, once again, anthropocentrism. “By representing the Deity as so intelligible and comprehensible, and so similar to a human mind,” Hume writes at the end of Dialogues Part III, “we are guilty of the grossest and most narrow partiality, and make ourselves the model of the whole universe.” If we did not have such an inflated opinion of our own species, Hume suggests, we might never go along with the design argument.
IV. THE THEORY OF EVOLUTION
Many philosophers consider Hume’s objections to be decisive and to devastate any design argument like Paley’s. Yet however convincing Hume’s philosophical rebuttals may be, they still do not explain where biological complexity came from. Thus for many people Hume’s objections do not put the design argument to rest. Indeed, Richard Dawkins, author of The Blind Watchmaker (1986) and one of the world’s foremost public advocates of the theory of evolution by natural selection, said that he cannot imagine having been an atheist prior to the publication of Charles Darwin’s On the Origin of Species (1859). Whatever Hume said, organisms sure look like they were designed, says Dawkins. But Darwin showed us a mechanism for the development of life that made divine intervention unnecessary. All of the complexity of life could be understood in terms of a few basic principles operating over enormous periods of time. Eyes, wings, circulatory systems, and all of the other complex structures of life could now be explained as products of nature alone, unaided by God.
To see how such evolutionary explanations work, consider an analogy. Suppose you are a con artist who wants to get an unsuspecting victim to write you a check for $10,000. What might your scam be? Here is a tried and true method. First, come up with the number of consecutive correct predictions about the daily overall performance of the Dow Jones Industrial Average (or just “the Dow”) that it would take to convince someone that you are a brilliant stock market analyst. Suppose you would convince someone you are a brilliant stock market analyst by correctly predicting for them, every day for two weeks, whether the Dow increased or decreased. Two weeks is ten business days. What could you do to ensure you made ten straight correct predictions about the Dow? Call up 210 people (1,024 people). Tell half of them the Dow will go up tomorrow and half of them it will go down. If it goes up, only call back people in the first group. If it goes down, only call back people in the second group. Repeat this procedure until you get to the tenth day. At the end of the ten days you are guaranteed to go from 1,024 people who have heard zero correct Dow predictions from you to one person who has heard ten correct Dow predictions from you! That person might very well conclude you are a brilliant stock analyst and thus might eagerly give you $10,000 to invest.
If you did this, would you, in fact, be a brilliant stock analyst? Absolutely not. What you would be is a run-of-the-mill scam artist who created the appearance of being a brilliant stock analyst. How did you create this appearance? By using three tools. First, you employed massive replication. By patiently making so many phone calls, you generated enough chances for you to predict correctly the Dow ten times in a row. Second, you introduced variation in your answers. Each day, you gave one answer to half of the people and a different answer to the other half. Third, you employed a selection mechanism. The actual performance of the Dow on a given day selected which people to call back. With these three tools, you generated the appearance of intelligence, even though in reality you were no better at predicting the Dow than a chipmunk.
Darwin pointed out that these three factors operate in nature. First, there is massive replication of organisms via reproduction. For example, you may have heard the figures about how, if every fly-egg survived to become an adult fly, in a few weeks the whole Earth would be covered with flies. This would happen because flies reproduce at a rate far greater than is necessary just to replace the dead flies. Second, in nature there is variation. Just as no two humans are exactly alike, no two flies are either. We now know that this variation is caused by random mutations in DNA, but even in Darwin’s time it was clear that offspring did display some variation and that some of these variations were more or less suited to the environments in which the offspring found themselves. A classic example from Darwin concerned the beak sizes of Galapagos finches. Longer beaks might be better suited to extracting food from certain plants than shorter beaks, while shorter beaks might be better suited for cracking open nuts than longer beaks. But since there is variation in beak size amongst finch offspring due to random mutations, both long beaks suitable for food extraction and short beaks suitable for nut cracking will be produced. Third, in nature resources are limited. Some animals are better suited to get those resources than others. Thus there is a selection mechanism in place – a natural selection – for organisms that are better suited to their environments. If disease leaves only nut-producing plants on an island, then the finches with beaks unsuited for cracking nuts will starve to death. The genes for short beaks will be passed on, while the genes for long beaks will not. Thus we will have a simple explanation for why the beaks of certain Galapagos finches are so perfect for cracking open nuts. If the beaks of their ancestors were not fit for cracking open nuts, those ancestors would not have lived long enough to reproduce. Just as you created the appearance of intelligence (in the form of Dow predictive ability) using massive replication, variation, and selection, so nature creates the appearance of intelligence (in the form of extremely well-adapted organisms) by massive reproduction, genetic variation, and natural selection. Organisms will look like they were designed by an extremely intelligent designer, but the appearance masks reality.
There is significant evidence favoring Darwin’s theory of evolution by natural selection, including the fossil record, the discovery of DNA, and observed cases of evolutionary adaptation. Evolution is the central organizing fact in contemporary biological science. Given this support for evolutionary theory from the sciences, it looks like Paley’s design argument for the existence of God fails. We cannot infer the existence of God from apparent biological design because in light of evolution we see that apparent design is really only that – apparent, not real.
V. IS THE THEORY OF EVOLUTION INCONSISTENT WITH BELIEF IN GOD?
Does this failure of Paley’s design argument show that God does not exist? No. All it shows is that the existence of apparent design in biology does not force us, intellectually, to believe that God does exist. The situation here is similar to that with a prosecutor who has failed to obtain a conviction. The failure of the prosecution just means the prosecutor’s case did not prove the defendant was guilty. But the lack of a compelling case for guilt is not the same thing as a compelling case for innocence. So for all that has been said so far, the truth of the theory of evolution is perfectly consistent with belief in God. The truth of evolutionary theory does not show that God does not exist. It only shows that one widespread argument for God’s existence – Paley’s analogical design argument – does not work.
Are there any other reasons for thinking that one cannot believe both that the theory of evolution is true and that God exists? That question has provoked intense discussion ever since the theory of evolution gained wide acceptance. Creationism is the view that the Biblical account of the six-day creation of the world presented in the Book of Genesis is literally true, with God directly creating each type of plant and animal and with the heavens and the Earth being less than ten thousand years old. Creationism is clearly inconsistent with the theory of evolution, simply because the theory of evolution says all living beings came to have the features they have by a process radically different than direct creation by God. However, creationism is also inconsistent with many other scientific findings. For example, our best geological theories tell us the Earth is around four billion years old, while the best current theory of the origin of the universe – the Big Bang theory – says the universe is somewhere between ten and twenty billion years old. Both geology and cosmology imply that the age creationists assign to the created world is off by a factor of nearly one million. If those scientific claims are correct, creationism is wrong, and vice versa. So evolutionary theory is not uniquely inconsistent with creationism. The creationist will have to see many theories of modern science as threatening, not just evolutionary theory.
Does belief in God require acceptance of creationism? If so, then the theory of evolution will be inconsistent with the belief that God exists. But whether this is so is a highly controversial matter. Theistic evolutionism is the view that God created the universe billions of years ago so that there would be enough time for a long evolutionary process to produce all life eventually without any special acts of creation by God. Theistic evolutionism obviously avoids the inconsistency with the theory of evolution that faces creationism. The question for theistic evolutionists, though, is whether their approach effectively abandons core commitments that any believer in God should have. For example, critics say theistic evolutionists simply ignore what religious scripture says. Theistic evolutionists typically respond that not all parts of scripture were meant to be interpreted literally. These disputes about what is required for genuine belief in God are matters of doctrine, of theology, and of faith. As such, they lie beyond the scope of a philosophy paper. The simple point to remember is that whether the theory of evolution is inconsistent with belief in God depends on a number of controversial religious presuppositions. The hasty assumption that no one can possibly believe in both God and evolution is a mistake.
VI. BAYESIAN INFERENCE
After Darwin, most philosophers, theologians, and scientists thought the theory of evolution removed any scientific basis whatsoever for belief in an intelligent designer. The design argument was dead, and Darwin buried it. In the past half-century, however, the design argument has undergone a tremendous resurgence. Before we look at the new scientific discoveries that are driving this resurgence, though, we must appreciate another way contemporary design arguments differ from older ones such as Paley’s. Modern design arguments typically are not arguments from analogy and are not qualitative. Paley tried to show that the detailed structures of organisms were analogous qualitatively – were very similar in appearance and plan – to various products of human design. Modern design arguments, on the other hand, typically are quantitative and are framed in terms of numerical probabilities. More specifically, modern design arguments are usually Bayesian inferences.
Bayes's rule is a formula in the mathematical theory of probability. It was discovered by Thomas Bayes, an eighteenth-century English clergyman. Bayes’s rule shows how we might revise, in the light of new evidence, the probabilities we initially assigned to competing hypotheses. Many contemporary philosophers think that we should (as much as we can) evaluate the impact of new evidence using Bayes's rule. Such philosophers are called "Bayesians."
To get a sense of how Bayesian reasoning works, imagine that you are following a professional golf tournament – the Bayes Open – in which there are a hundred golfers, one of whom you know is Tiger Woods. In golf tournaments there is always a “leader board” which tells you who is in the lead and by how much. Unlike the leader board of a typical golf tournament, however, imagine that the leader board for this tournament does not identify the players by name, but rather by number. Next, imagine that you just do not know Tiger’s number. Finally, suppose that the golfer at the top of the leader board is golfer 93, who is leading by twenty strokes. So imagine the first four lines on the leader board look like this:
WELCOME TO THE BAYES OPEN!
Tied for Second
Tied for Second
Now consider the following hypothesis (call it “T” for “Tiger”), the following bit of data (call it “L” for “leading”), and the following statement of what you know going in about golf (call it “G” for “golf”).
T = Golfer 93 is Tiger Woods.
L = Golfer 93 is leading by twenty strokes.
G = There are a hundred numbered professional golfers, all of them are extremely good, and one of them is Tiger Woods, the world’s best golfer.
What is the relationship between T, L, and G? To answer this, it will be helpful to use the following notation for conditional probability. Let P(x | y) stands for the probability of x given that ("conditional on") y. For example, P(you get an A on your final | you got As on your midterm and all your quizzes) is simply the probability that you get an A on the final, given that you got As on your midterm and on all of your quizzes.
Now knowing nothing else about golfer 93 except that golfer 93 is one of a hundred professional golfers, you think golfer 93 has one chance in a hundred of being Tiger. So prior to getting any information about how golfer 93 is doing, you think
P(T | G) = 1% = 0.01
This is what Bayesians would call the prior probability – the probability you assigned to the proposition that golfer 93 is Tiger prior to acquiring the new information that golfer 93 has a twenty-stroke lead. But now you see on the leader board that golfer 93 is leading by twenty strokes. This is an extremely large lead for a professional golf tournament, so a golfer who could build such a lead would have to be much, much better even than the typical professional golfer. You think Tiger Woods could build such a lead because you think he is the best player in the world, and you also doubt that anyone other than Tiger could build such a lead. So you think the probability of a twenty-stroke lead’s being built by golfer 93, conditional on golfer 93’s being someone other than Tiger, is extremely low. Let’s say you think the odds of this are one in ten thousand. So you think
P(L | not-T & G) = .01% = 0.0001
On the other hand, you think the probability of a twenty-stroke lead's being built by golfer 93, conditional on golfer 93’s actually being Tiger, is fairly high. Let’s say you think the odds of this are one in a hundred. So you think
P(L | T & G) = 1% = 0.01
In light of looking up at the leader board and gathering evidence L, you realize you should assign a much higher probability to T than you did prior to looking at the leader board. In other words, you think finding out that golfer 93 has built a twenty-stroke lead is great evidence that golfer 93 is, in fact, Tiger. So while you used to think there was only a 1% chance that golfer 93 is Tiger Woods, now you think the chances are much greater that golfer 93 is Tiger. This new chance is what Bayesians call the posterior probability – the probability you assign after you get the new evidence about the identity of golfer 93.
But exactly how much higher should this posterior probability be? This is the crucial question that Bayes’s rule helps you answer. Call the probability that golfer 93 is Tiger and leads by twenty strokes the particular probability, and call the probability that any of the golfers lead by twenty strokes the total probability. Bayes’s rule says the posterior probability is simply equal to the particular probability divided by the total probability.
So now we have three probabilities in play. P(T | L & G) is the probability that golfer 93 is Tiger, given your background knowledge and the fact that golfer 93 has a twenty-stroke lead. P(T | G) x P(L | T & G) is the particular probability, which is the probability that golfer 93 is Tiger (given just your knowledge of the golf tournament) multiplied by the probability that Tiger would build a twenty-stroke lead. [P(L | not-T & G) x P(not-T | G)] + [P(L | T & G) x P(T | G)] is the total probability, which is the probability that any golfer would build a twenty-stroke lead. In this case, the total probability is just the sum of two probabilities: the probability that golfer 93 is Tiger and leads by twenty strokes, plus the probability that golfer 93 is not Tiger and leads by twenty strokes.
Using Bayes’s rule, we can now say what the new probability is.
the posterior probability = the particular probability divided by the total probability
= (0.01 x 0.01)/[(0.0001 x 0.99) + (0.01 x 0.01)]
= 0.5025 = around 50%
That is, in light of the fact that golfer 93 built a twenty-stroke lead, and given everything else you believe about the golf tournament, Bayes’s rule tells you there is a 50% chance that golfer 93 is Tiger Woods. By concluding there is a fifty-fifty chance golfer 93 is Tiger Woods, you would have just made a Bayesian inference.
VII. A GENERIC BAYESIAN DESIGN ARGUMENT
Now what do Bayesian inferences have to do with design arguments for God? Well, suppose something really improbable happened. For example, suppose that I, Neil A. Manson, had a twenty-stroke lead in the Bayes Open. I am really lousy at golf (though I am an avid player). So it would be extremely, extremely improbable that I would ever build a twenty-stroke lead. It would be so improbable, in fact, that I would conclude that God (or, at least, some minor golf deity) had performed a miracle on my behalf. My reasoning would be strictly Bayesian. The probability of my building a twenty-stroke lead, conditional on there not being any intervention by God, is unbelievably low. Practically speaking, it is zero. It just could never happen. On the other hand, the probability of my building a twenty-stroke lead, conditional on God’s miraculously intervening on my behalf, is very high. God is so powerful He could even make me win a golf tournament! Bayes’s rule tells me that the probability that there was an intervention by God is simply the particular probability – the probability of my building a twenty-stroke lead given that God miraculously intervened on my behalf, multiplied by the probability that God would miraculously intervene on my behalf – divided by the total probability of my building a twenty-stroke lead. Since the probability of my building a twenty-stroke lead, given that there was not any intervention by God, is practically zero (trust me, it is just impossible!), the particular probability and the total probability end up being nearly the same. Now if you divide a number by a number that is practically the same as it, you get a result very close to 1. And that means the probability that God performed a miracle on my behalf is very nearly 100% – that is, if I find myself leading a pro golf tournament by twenty strokes!
Many contemporary proponents of the design argument say that new scientific evidence shows the universe and the life in it have very special features. These features, they say, are extremely, extremely improbable (my-leading-a-pro-golf-tournament-by-twenty-strokes improbable) unless an intelligent designer like God built them into the universe. In light of this new scientific data, they say, we should revise upwards our prior probability estimates for God’s existence. The general structure of their arguments is exactly like that of the argument for thinking golfer 93 is Tiger Woods. As with the Tiger Woods argument, these arguments are framed in terms of three statements. The first is some statement of our background scientific knowledge (call it “K”). This is some statement regarding our best overall scientific understanding of the world, including physics, biology, astronomy, and so on. The second is a statement of the evidence of design (call it “E”). This is some statement about how some feature of the universe overall, or conditions on or near Earth, had to be just right in order for there to be intelligent, conscious living beings such as ourselves. We will look at this alleged new evidence of design in section VIII. The last is some design hypothesis (call it “D). This is some statement about the nature, powers, and purposes of some supernatural intelligent being such as God. There are many possible design hypotheses. Perhaps a malevolent demon created the world. Perhaps a super-scientist in some alternate reality made a new universe come into existence. For ease of discussion, however, I will simply assume that the design hypothesis in question is just that God exists.
The proponent of a Bayesian design argument for the existence of God would then make the following three claims. (1) The probability of the evidence of design, given our background scientific knowledge, and assuming that God does not exist – expressed symbolically as P(E| K & not-D) – is extremely low. It is exceptionally unlikely that the conditions of the universe and the life within it turned out right just by chance. Note that this is analogous to the claim that it is extremely unlikely that any golfer other than Tiger would build a twenty-stroke lead. (2) The probability of the evidence of design, given our background scientific knowledge, and assuming that God does exist – expressed symbolically as P(E| K & D) – is quite high. In other words, it is pretty likely that things would turn out just right in the universe if God directed things things. This is analogous to the claim that it is a lot more likely that Tiger would build a twenty-stroke lead. (3) It is more likely that God exists than that things in the universe turned out just right just by chance; this is expressed symbolically as saying that P(D| K) is considerably greater than P(E| K & not-D). This is analogous to the claim that it is more likely that golfer 93 is Tiger (1% chance) than that some golfer other than Tiger built a twenty-stroke lead (.01% chance). Claims (1), (2), and (3) together are the premises in a generic Bayesian design argument.
THE GENERIC BAYESIAN DESIGN ARGUMENT
(1) P(E| K & not-D) is extremely low,
(2) P(E| K & D) is quite high, and
(3) P(D| K) is considerably greater than P(E| K & not-D);
so, by Bayes’s rule, P(D| E & K) is quite high.
In plain English, the basic conclusion of the argument is that the new scientific evidence of design strongly confirms the belief that God exists. So, whatever one’s prior probability for the belief that God exists, the posterior probability – the probability one should have after hearing about the new scientific evidence – should be much higher.
Each of the three claims essential to this argument are open to criticism. Here we will briefly consider a few objections to (2) and (3); in section VIII we will turn to claim (1). (3) is the claim that, even if we consider it only in light of what we knew prior to getting the new evidence of design, it is still more probable that God exists than that, by pure luck, the conditions in the universe are just right. Now in the golfing case, we knew the probability that golfer 93 was Tiger (1%), and we knew that probability was greater than the probability that someone other than Tiger built an enormous lead (.01%). But in this case, what reason do we have for thinking it is more likely that God exists than that, by pure luck, the conditions in the universe are just right? Just what is the prior probability that God exists? Well, most people think the prior probability that God exists is not extremely low. Even those who think God does not exist usually admit it is possible that God exists. They are open to the idea that God exists, even if they do not believe themselves. Thus it seems that, for most people, (3) is a pretty reasonable claim. Note that if it were not - if it were not even possible that God exists, so that there was a 0% chance that God exists – then the Bayesian design argument would just a waste of time. If God is impossible, we could not explain anything in terms of God’s existence, including apparent cases of design. Here is an analogy. Suppose you leave your wallet on your dresser with $40 in it, and return to find only $20 in it. You confront your roommate about this. Your roommate denies taking the money and offers an alternative explanation of the missing $20. Some explanations might be plausible, e.g. “You miscounted your money.” Some explanations might be less plausible, e.g. “A thief broke in and took $20 out of your wallet.” Some explanations might be extremely implausible, e.g. “The CIA collected it as evidence in a counterfeiting case against you.” Yet however good or bad these explanations are, all of them are at least possibly true. Here is one that is not: “$20 equals $40.” That’s just nonsense! Since it is a logical contradiction that $20 equals $40, the probability that $20 equals $40 is zero, and the claim that $20 equals $40 could never explain anything. But most people do not see the hypothesis that God exists this way. They see no logical contradiction in saying God exists, and so they see no reason to think it is impossible that God exists.
Yet numerous philosophical atheists think that not only does God not exist, but that God could not possibly exist. Why? Well, to pick just one reason as an example, some people think nothing could be all-powerful. They bring out the old Paradox of the Stone. Can God create a stone so heavy that God cannot lift it? If so, there is something God cannot do: lift the stone. If not, there is something God cannot do: create the stone. So either way, there is something God cannot do. But then, contrary to what believers in God tell us, nothing could be genuinely all-powerful. Thus if God is, by definition, all-powerful, then God could not exist. Now the point here is not to assess whether the Paradox of the Stone shows it is impossible that God exists. That is an issue to be explored in philosophical theology. The point is simply that claim (3) of the Bayesian design argument for the existence of God is not incontestable. Skeptics of the Bayesian design argument may argue that it is impossible that God exists. Barring that, they may argue there is no reason to think the prior probability that God exists is any greater than the probability that the conditions in the universe are, by pure chance, just right. Awareness of these possible maneuvers by the skeptic reinforces a general lesson that you should draw from philosophy class. Even the most obvious premises of an argument usually need defense.
A similar point applies to claim (2). It may appear obvious to everyone that of course God would be likely to create the sort of wondrous, complex universe to which the evidence of design calls our attention. But what reason do we have to think this? We have a pretty good idea of how likely it is that Tiger Woods builds a huge lead in a golf tournament, since we have seen him do it before. What basis do we have for thinking God is likely to create a universe like this one? What insight do we have into the mind of God? Here we should remember Hume’s complaints about the design argument, specifically his suggestion that it rests on anthropocentrism. When you say that of course God would create a world like this one, are you simply projecting your own human biases? Perhaps you would create a world like this if you were super-knowledgeable and super-powerful, but why think God would do the same? Again, these questions are not meant to show that God would not create a world like this one, but only to show that skeptics of the Bayesian design argument are going to demand defense of claim (2). All three basic premises of the Bayesian design argument are open to debate, not just the first one.
VIII. THE NEW EVIDENCE OF DESIGN
What about claim (1)? What are the special features of the universe that allegedly are so unlikely if the universe was not designed by God? Let us begin with features of the universe as a whole. In the twentieth century a series of breakthroughs in physics and observational astronomy led to the development of the Big Bang model of the universe. Scientists also discovered that the Universe is highly structured, with precisely defined parameters such as age, mass, curvature, temperature, density, and rate of expansion. Looking at the very precise numbers of these parameters, some scientists asked “How would the universe have been if the values of these parameters had been slightly different?” The answer – to the surprise of many – was that the universe would not have been the sort of place in which life could eventually emerge. The numbers describing the universe, scientists discovered, were like the just-right spot on a radio dial: if you turned the knob just a bit, the clear signal would turn to static. As a result, many physicists started describing the values of the parameters as fine-tuned for life.
To give just one of many, many possible examples, the cosmological constant (symbolized by the Greek letter ‘‘) is a crucial term in Einstein’s equations for the General Theory of Relativity. When is positive, it acts as a repulsive force, causing space to expand. When is negative, it acts as an attractive force, causing space to contract. If were not exactly right, either space would expand at such an enormous rate that almost every object in the universe would fly apart, or the universe would collapse back in on itself immediately after the Big Bang. Either way, life could not possibly emerge anywhere in the universe. Some calculations put the odds that has just the right value at well below one chance in a trillion trillion trillion trillion. Similar calculations have been made showing that the odds of the universe’s having carbon-producing stars (carbon is essential to life), or of not being millions of degrees hotter than it is, or of not being shot through with deadly radiation, are likewise astronomically small. Given this extremely improbable fine-tuning for life, say some proponents of the Bayesian design argument, we should think it much more likely that God exists than we did initially. After all, if we believe in God, we will have an explanation of cosmic fine-tuning, whereas if we just say the universe is fine-tuned by chance, we are stuck believing something incredibly improbable.
In the second half of the twentieth-century, developments in the field of biology – particularly in the fields of molecular and cell biology – showed that life is incredibly complex, even at the level of the cell. The typical cell, packed with mitochondria, DNA, RNA, and all the rest, displays more organized, functional complexity than a television or a computer – much less Paley’s watch! Does evolutionary theory imply that these cellular systems arose by chance? If so, does the universe provide enough chances? Note that the Big Bang model implies that the universe is finite in space and time. By the most recent calculations it is about 13.5 billion years old and about 13.5 billion light years in diameter. Proponents of what has come to be known as Intelligent Design Theory say “Yes” to the first question and “No” to the second. They say some of these parts of cells are irreducibly complex. A system is irreducibly complex if the removal of any one of its parts makes the system completely nonfunctional. The standard example of an irreducibly complex system is the common household mousetrap. A mousetrap has five parts: platform, catch, spring, hammer, and holding bar. If any one of those parts is missing, the mousetrap will not just be less good at catching mice. It will be no good at all.
The alleged existence of irreducible complexity in biology is the second special feature of the universe that is supposedly too unlikely to exist by chance. Proponents of Intelligent Design Theory say many cellular systems are irreducibly complex. They think such systems cannot be produced by an evolutionary process, because the ancestors of the organisms with such systems would have crucial parts that just would not function. Yet in order for a system to have been produced by an evolutionary process, the ancestors of the organisms having that system would have to have been functional. If the ancestors were not functional – if the ancestors were incapable of surviving and reproducing – then evolution could not even get off the ground. So proponents of Intelligent Design Theory say evolutionary theory cannot explain the existence of such irreducibly complex biological structures. Neither can chance. According to their calculations, the probability that even one of these irreducibly complex biological structures arose anywhere in the entire history of the universe is still so low that the possibility can be dismissed. According to the Intelligent Design theorists, an irreducibly complex system’s arising by chance would be just like that magical dorm-cleaning tornado mentioned earlier, but this time blowing through an airplane parts warehouse and spitting out a Boeing 747. Since evolution is ruled out and the probability that such systems arise by chance is just too low, we must revise upward the probability we assign to the belief that an intelligent designer exists (though Intelligent Design theorists are generally careful not to identify explicitly that intelligent designer with God).
Both cosmic fine-tuning and Intelligent Design Theory are highly controversial topics. [Intelligent Design Theory especially has drawn much fire since it is at the center of recent political fights over what should be taught in high school biology classes; its critics often deride Intelligent Design Theory as “warmed-over creationism.”] One line of criticism is purely scientific: that the scientific data just do not support the claims of fine-tuning in physics and irreducible complexity in biology. Other critics focus on philosophical objections of the sort that were mentioned in parts III and VII of this paper. In addition to these routes for blocking a Bayesian design argument, some physicists have proposed the multiverse theory. The multiverse theory, they tell us, is a natural consequence of much of contemporary physics, particularly the field of quantum cosmology. According to the multiverse theory, there is some mechanism for the production of a vast multitude of universes. These universes vary randomly in their basic characteristics and there are enough of them to make it likely that at least one has just the right conditions for life. If the multiverse theory is true, the fact that the universe appears to us to be fine-tuned for life can be explained in terms of the anthropic principle. The anthropic principle says that of course we will find ourselves observing a universe that is fine-tuned for life. After all, if the universe were just right for life, we would not be around to observe it! Notice what we have here is an explanatory strategy that very much resembles Darwin’s. The multiverse plays the role of the massive replicator with random variations – but of universes rather than organisms! Meanwhile, the anthropic principle introduces the selection mechanism – in this case, a selection effect that explains why only fine-tuned universes get observed. The appearance of design in universes thus gets explained away, without bringing in any supernatural intelligence like God to engineer the whole thing.
As things currently stand, neither the proponents of Bayesian design arguments nor their critics have achieved decisive intellectual (or political) victory. As this article has shown, through history the design argument has captured the attention of some of the world’s great scientists and philosophers. Assuming it continues to do so, the design argument promises to be a live issue for the foreseeable future.
IX. FOR FURTHER READING
Students interested in finding out more about the history of the design argument might start with some primary sources. William Paley’s Natural Theology and Charles Darwin’s On the Origin of Species are widely reprinted and should be available in any college library. In The Blind Watchmaker: why the evidence of evolution reveals a universe without design (W.W. Norton and Company, 1986), Richard Dawkins presents a powerful case that evolutionary theory upends the design argument. God and Design: The Teleological Argument and Modern Science, edited by Neil A. Manson (Routledge 2003), is a comprehensive, up-to-date anthology covering the science and the philosophy behind both the cosmological and biological design arguments. It also includes a separate section on the multiverse theory. In Universes (Routledge, 1989), John Leslie exhaustively presents the fine-tuning data from physics and gives clever arguments for both the design hypothesis and the multiverse theory. In Darwin’s Black Box: The Biochemical Challenge to Evolution (Simon & Schuster, 1996), Michael Behe articulates the irreducible complexity concept, argues that biochemistry shows there are irreducibly complex biological structures, and claims this is evidence of the existence of an intelligent designer. Both critics and supporters of Behe’s position can be found in Intelligent Design Creationism and Its Critics: Philosophical, Theological, and Scientific Perspectives, edited by Robert T. Pennock (MIT Press, 2001). In Finding Darwin’s God: A Scientist’s Search for Common Ground between God and Evolution (HarperCollins 1999), Kenneth R. Miller opposes Intelligent Design Theory and defends theistic evolutionism as an acceptable alternative for theists. The issue of whether belief in the truth of evolutionary theory is compatible with belief in God is also pursued in Michael Ruse’s Can a Darwinian Be a Christian? (Cambridge University Press, 2000). Lastly, students interested in finding out more about Bayesian reasoning might start with Colin Howson and Peter Urbach, Scientific Reasoning: The Bayesian Approach (Open Court Press, 1989).