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That being said, we don’t want to get too far off track here dreaming up all these scenarios. Is it possible that a God might exist? Sure it is (or at least it’s conceivable, if nothing else). But these ideas are mostly just intended as interesting thought experiments. Just because it’s possible to imagine something doesn’t mean it should be considered likely. After all, it’s possible that the entire universe was created five minutes ago, and all the evidence that it’s older than that (including our own memories) is fake and was just planted to trick us. It’s possible that you’re just hallucinating this entire reality, and nothing in the universe actually exists except your brain. It’s possible that this whole universe is just an extremely vivid dream in the mind of a sleeping butterfly. All these things are possible – but that’s not a good reason to think that any of them are actually true.

And the same applies to religious beliefs. Although religious believers might try to justify their belief by saying that the existence of their God can’t be disproven – it’s always possible that he might exist – that’s not a good enough reason to think that he really does exist. Loftus elaborates:

In every case when it comes to my reasons for adopting my skeptical presumption, the Christian response is pretty much the same. Christians must continually retreat to the position that what they believe is “possible,” or that it’s “not impossible.” Robert M. Price tells us that for Christian apologists “the controlling presupposition seems to be, ‘If the traditional view cannot be absolutely debunked beyond the shadow of a doubt, if it still might possibly be true, then we are within our rights to continue to believe it.’” In my opinion, Christians must repeatedly assert this to defend their faith. Keep in mind that anything that is not logically impossible is, in fact, possible. It’s possible I’m dreaming right now. It’s possible I’m merely recalling an event in the year 2012 in a dream I’m having right now in the year 2020. It’s possible that Jim Carrey could’ve gotten the girl of his dreams in the movie Dumb and Dumber, too (remember, the girl said he had a “one in a million” chance of doing so).

However, the more Christians must constantly retreat to what is “possible” in order to defend their faith, then the more their faith is on shaky ground. Why? Because we want to know what is probable, not what is possible. If we ask Christians to defend a particular belief and they argue such a belief “isn’t impossible,” then this is a tacit admission that instead of the evidence supporting what they believe, they are actually trying to explain the evidence away. In [debates with Christians,] you will see them retreating to the “merely possible” defense far too many times. [I] argue that the more often Christians are forced into arguing their faith is merely possible in the face of contrary evidence, rather than probable, then the less likely it is that their faith is true. Every time they do this they are explaining the evidence away by admitting the evidence does not support what they believe. Probability is what matters.

So in the end, Christians demand that I prove their faith is nearly impossible before they will consider it to be improbable, since most of their arguments are possibility ones rather than probability ones. This demand of theirs allows them to believe in the midst of many powerful arguments to the contrary. But it’s an utterly unreasonable demand. So long as they can find any escape at all, no matter how improbable, they can continue believing. That’s the power of the delusion. All I can do is show that their faith is improbable, very improbable. That should be good enough. Just imagine a trusted banker telling his patrons that if they invest all their money into a particular fund it will bring financial ruin upon them. And then think what you would say if some of them invested all of their money in that fund anyway simply because their banker did not say that no other result is possible.

Barker gives another example to illustrate this point:

Even the staunchest skeptic admits that [finding] one natural explanation [for a phenomenon] does not completely rule out other possibilities. [For example,] perhaps there is a higher level of understanding that allows Santa to exist even though we are unable to prove it yet. The fact that kids have creative imaginations does not necessarily indicate that everything they imagine must be false. Even so, I can still claim that if there are adequate natural explanations that account for all the facts, then there is no driving need to search for supernatural explanations. This is just common sense. Without such a rational limit there would be no end to the fanciful layers that could be added to any hypothesis. (This is usually referred to as Occam’s Razor, the principle that suggests we should normally accept the explanation that requires the fewest assumptions.) The skeptic, slavishly honoring all the possibilities, could be forced to spend a lifetime running around trying to disprove an infinite number of fantastic theories.

For example, maybe Santa is an ambassador from a distant planetary outpost populated with red-and-white creatures who monitor the activities of specially chosen short people (elves and children?), seeking “conducive” humans as psychic vehicles for messages to holy reindeer that levitate when children dream during the winter solstice, with most adults being too hardened to believe. Can anyone prove that this scenario is untrue? (You read it here first.) Since I don’t have the means or the inclination to disprove such an idea, is this paragraph now allowed to count as evidence for such a theory?

A rational person would give the preceding paragraph an exceedingly low probability (virtually zero). However, if some natural explanation arises for the existence of the paragraph (such as an admission that I just made it up), then the probability can be safely dropped to zero point zero and the discussion shifts from the reaches of outer space to the reaches of my inner brain.

Just to drive the point home, Sagan provides his own (now-classic) analogy:

“A fire-breathing dragon lives in my garage.”

Suppose (I’m following a group therapy approach by the psychologist Richard Franklin) I seriously make such an assertion to you. Surely you’d want to check it out, see for yourself. There have been innumerable stories of dragons over the centuries, but no real evidence. What an opportunity!

“Show me,” you say. I lead you to my garage. You look inside and see a ladder, empty paint cans, an old tricycle – but no dragon.

“Where’s the dragon?” you ask.

“Oh, she’s right here,” I reply, waving vaguely. “I neglected to mention that she’s an invisible dragon.”

You propose spreading flour on the floor of the garage to capture the dragon’s footprints.

“Good idea,” I say, “but this dragon floats in the air.”

Then you’ll use an infrared sensor to detect the invisible fire.

“Good idea, but the invisible fire is also heatless.”

You’ll spray-paint the dragon and make her visible.

“Good idea, but she’s an incorporeal dragon and the paint won’t stick.”

And so on. I counter every physical test you propose with a special explanation of why it won’t work.

Now, what’s the difference between an invisible, incorporeal, floating dragon who spits heatless fire and no dragon at all? If there’s no way to disprove my contention, no conceivable experiment that would count against it, what does it mean to say that my dragon exists? Your inability to invalidate my hypothesis is not at all the same thing as proving it true. Claims that cannot be tested, assertions immune to disproof are veridically worthless, whatever value they may have in inspiring us or in exciting our sense of wonder. What I’m asking you to do comes down to believing, in the absence of evidence, on my say-so.

The only thing you’ve really learned from my insistence that there’s a dragon in my garage is that something funny is going on inside my head. You’d wonder, if no physical tests apply, what convinced me. The possibility that it was a dream or a hallucination would certainly enter your mind. But then, why am I taking it so seriously? Maybe I need help. At the least, maybe I’ve seriously underestimated human fallibility.

Imagine that, despite none of the tests being successful, you wish to be scrupulously open-minded. So you don’t outright reject the notion that there’s a fire-breathing dragon in my garage. You merely put it on hold. Present evidence is strongly against it, but if a new body of data emerge you’re prepared to examine it and see if it convinces you. Surely it’s unfair of me to be offended at not being believed; or to criticize you for being stodgy and unimaginative – merely because you rendered the Scottish verdict of “not proved.”

Imagine that things had gone otherwise. The dragon is invisible, all right, but footprints are being made in the flour as you watch. Your infrared detector reads off-scale. The spray paint reveals a jagged crest bobbing in the air before you. No matter how skeptical you might have been about the existence of dragons – to say nothing about invisible ones – you must now acknowledge that there’s something here, and that in a preliminary way it’s consistent with an invisible, fire-breathing dragon.

Now another scenario: Suppose it’s not just me. Suppose that several people of your acquaintance, including people who you’re pretty sure don’t know each other, all tell you that they have dragons in their garages – but in every case the evidence is maddeningly elusive. All of us admit we’re disturbed at being gripped by so odd a conviction so ill-supported by the physical evidence. None of us is a lunatic. We speculate about what it would mean if invisible dragons were really hiding out in garages all over the world, with us humans just catching on. I’d rather it not be true, I tell you. But maybe all those ancient European and Chinese myths about dragons weren’t myths at all…

Gratifyingly, some dragon-size footprints in the flour are now reported. But they’re never made when a skeptic is looking. An alternative explanation presents itself: On close examination it seems clear that the footprints could have been faked. Another dragon enthusiast shows up with a burnt finger and attributes it to a rare physical manifestation of the dragon’s fiery breath. But again, other possibilities exist. We understand that there are other ways to burn fingers besides the breath of invisible dragons. Such “evidence” – no matter how important the dragon advocates consider it – is far from compelling. Once again, the only sensible approach is tentatively to reject the dragon hypothesis, to be open to future physical data, and to wonder what the cause might be that so many apparently sane and sober people share the same strange delusion.

Barker sums up the issue like this:

Theists claim that there is a god; atheists do not. Religionists often challenge atheists to prove that there is no god, but this misses the point. In general, atheists claim that god is unproved, not disproved. In any argument, the burden of proof is on the one making the claim.

And that’s the key point to this whole debate: If someone is claiming that a thing exists, the burden of proof lies with them to support that claim. The thing isn’t just automatically assumed to exist by default; otherwise, we’d have no reason not to believe in dragons and Santa and so on. The person making the claim has to provide some kind of evidence that it exists. As Sagan famously put it, “Extraordinary claims require extraordinary evidence.” And as Hitchens added, “What can be asserted without evidence can also be dismissed without evidence.”

QualiaSoup once again addresses the topic beautifully:

In the absence of definitive proof one way or the other, then, the only truly reliable way of evaluating competing claims is simply to determine which one is more heavily favored by the balance of evidence. And the perfect tool for that job, as Carroll writes, is our old friend Bayes’s Rule – according to which “evidence that favors one alternative automatically disfavors others.” Carroll explains:

Imagine we are comparing two propositions, X and Y, and we observe an outcome that has a 90 percent chance of happening under X and a 99 percent chance of happening under Y. According to Bayes’s Theorem, after collecting that information, the credence we assign to X will go down.

That can seem counterintuitive. After all, if X were true, we would have a 90 percent chance of obtaining that outcome – how can observing it count as evidence against this theory? The answer is just that it’s even more likely under the other theory. The shift in credences might not be large, but it will always be there. As a result, the fact that you can come up with an explanation for some event within some theory doesn’t mean that event doesn’t lower the credence you have for the theory. The converse is also true: if some observation would have favored one theory, but we obtained the opposite of that observation, that result necessarily decreases our credence for the theory.

Consider two theories: theism (God exists) and atheism (God doesn’t exist). And imagine we lived in a world where the religious texts from different societies across the globe and throughout history were all perfectly compatible with one another – they all told essentially the same stories and promulgated consistent doctrine, even though there was no way for the authors of those texts to have ever communicated.

Everyone would, sensibly, count that as evidence in favor of theism. You could cook up some convoluted explanation for the widespread consistency even under atheism: maybe there is a universal drive toward telling certain kinds of stories, implanted in us by our evolutionary history. But we can’t deny that theism provides a more straightforward explanation: God spread his word to many different sets of people.

If that’s true, it follows as a matter of inescapable logic that the absence of consistency across sacred texts counts as evidence against theism. If data D would increase our credence in theory X, then not-D necessarily decreases it. It might not be hard to explain such inconsistency, even if theism is true: maybe God plays favorites, or not everyone was listening very carefully. That is part of estimating our likelihoods, but it doesn’t change the qualitative result. In an honest accounting, the credence we assign to a theory should go down every time we make observations that are more probable in competing theories. The shift might be small, but it is there.

He continues:

What we’re supposed to do [under Bayes’s Rule] is to imagine, as fairly as possible, what the world would probably look like according to either of our two possibilities, and then compare it to what it actually is like. That’s really hard. Neither “theism” nor “atheism,” by itself, is an extremely predictive or specific framework. We can imagine many possible universes that would be compatible with either idea. And our considerations are contaminated by the fact that we actually do know quite a bit about the world. That’s a considerable bias to try to overcome.

Take the problem of evil. Why would a powerful and benevolent God, who presumably could simply stop humans from being evil, nevertheless allow it in the world? There are many possible responses to this question. A common one relies on free will: perhaps to God, it is more important that humans be free to choose according to their own volition – even if they end up choosing evil – than to coerce them into being uniformly good.

Our job, however, isn’t simply to reconcile the data (the existence of evil) with the theory (theism). It’s to ask how the data changes our credences for each of the two competing theories (theism and atheism).

So imagine a world that is very much like ours, except that evil does not exist. People in this world are much like us, and seem able to make their own choices, but they always end up choosing to do good rather than evil. In that world, the relevant data is the absence of evil. How would that be construed, as far as theism is concerned?

It’s hard to doubt that the absence of evil would be taken as very strong evidence in favor of the existence of God. If humanity simply evolved according to natural selection, without any divine guidance or interference, we would expect to inherit a wide variety of natural impulses – some for good, some for not so good. The absence of evil in the world would be hard to explain under atheism, but relatively easy under theism, so it would count as evidence for the existence of God.

But if that’s true, the fact that we do experience evil is unambiguously evidence against the existence of God. If the likelihood of no evil is larger under theism, then the likelihood of evil is larger under atheism, so evil’s existence increases our credence that atheism is correct.

Put in those terms, it’s easy to come up with features of our universe that provide evidence for atheism over theism. Imagine a world in which miracles happened frequently, rather than rarely or not at all. Imagine a world in which all of the religious traditions from around the globe independently came up with precisely the same doctrines and stories about God. Imagine a universe that was relatively small, with just the sun and moon and Earth, no other stars or galaxies. Imagine a world in which religious texts consistently provided specific, true, nonintuitive pieces of scientific information. Imagine a world in which human beings were completely separate from the rest of biological history. Imagine a world in which souls survived after death, and frequently visited and interacted with the world of the living, telling compelling stories of life in heaven. Imagine a world that was free of random suffering. Imagine a world that was perfectly just, in which the relative state of happiness of each person was precisely proportional to their virtue.

In any of those worlds, diligent seekers of true ontology would quite rightly take those aspects of reality as evidence for God’s existence. It follows, as the night the day, that the absence of these features is evidence in favor of atheism.

How strong that evidence is, is another question entirely. We could try to quantify the overall effect, but we’re faced with a very difficult obstacle: theism isn’t very well defined. There have been many attempts, along the lines of “God is the most perfect being conceivable,” or “God is the grounding of all existence, the universal condition of possibility.” Those sound crisp and unambiguous, but they don’t lead to precise likelihoods along the lines of “the probability that God, if he exists, would give clear instructions on how to find grace to people of all times and cultures.” Even if one claims that the notion of God itself is well defined, the connection between that concept and the actuality of our world remains obscure.

One could try to avoid the problem by denying that theism makes any predictions at all for what the world should be like – God’s essence is mysterious and impenetrable to our minds. That doesn’t solve the problem – as long as atheism does make predictions, evidence can still accumulate one way or the other – but it does ameliorate it somewhat. Only at a significant cost, however: if an ontology predicts almost nothing, it ends up explaining almost nothing, and there’s no reason to believe it.

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