Modern physics has produced some amazing insights into the nature of the universe. One of the most astonishing is the discovery the fundamental forces of nature are balanced with a delicate precision.

Upset this delicate balance and the universe could have collapsed in on itself before getting started. No atoms could form, which means no stars, no planets, no carbon, and no life.

The size and mass of atoms, and the forces which link them, determine the chemistry in our world. The equations of the standard model of particle physics describe the fundamental building blocks of nature. Those equations contain 26 constants, ranging from the mass of the particles to the strength of the forces and their interactions.

Scientists have discovered the values of these constants are tuned to an astonishing precision. Change any of them even a small amount, and the universe would be very different. Most of the universes which would result couldn’t support life.

While there’s plenty of room for debate over the details, most scientists accept this general picture of fine-tuning.

The fact that fine-tuning exists doesn’t by itself mean there is a fine tuner or designer. But the fine-tuning is something which begs for an explanation. One of those possible explanations is design. The discovery of this fine-tuning has been formalized into a modern version of the design argument for God.

While the improbability of fine-tuning by itself doesn’t suggest design, combine that with other factors and design becomes more likely. If someone wins the lottery we don’t think there is anything which needs explaining. It’s an unlikely event, but someone will win.

There’s no reason to think the unlikely event needs any explanation other than chance. But what if the same person wins the lottery 3 times in a row, or 5 times, or 10 times?

The more times they win, the more we’re inclined to think luck or chance doesn’t explain why they won. The best explanation becomes they won because of intentional design. They’re cheating.

We don’t make this judgement solely on the probability involved in winning, but in combination with other information we have about lotteries and agents.

We know intelligent agents exist, we know they can manipulate the results of lotteries, we know they’re motivated by money to cheat. We also have a lot of experience cheating happens.

Is the fine-tuning of the universe like someone cheating at the lottery? Should we believe design is a better explanation than chance?

**Calculating probabilities**

In making this judgement, how we calculate the probability is important. It’s easy enough to calculate the probability of winning the lottery, but how can we calculate the probability the physical constants would have those values?

There’s only one universe, we don’t seem to have the necessary background knowledge. We have no physical theory that stands outside the constants to tell us how probable those values are.

But when it comes to making the judgement of the probability of design or chance, we aren’t using a frequency probability. A frequency method calculates the occurrence of something in repeated experiments. If we toss a coin multiple times, the frequency of it coming up heads will be 50%.

But as we’ve seen, we don’t make the lottery judgement based on a frequency probability. It’s not because we have no examples of anyone winning multiple times. The fact it’s unlikely doesn’t mean it’s designed. We use a different type of calculation.

**Bayesian probability**

Another approach to calculating probability is known as Bayesian. With a Bayesian approach we can compare how likely different theories are in the context of the evidence we already have available. In this sort of calculation, probabilities represent how plausible a particular result is.

This Bayesian approach is an extension of logic, it tells us the relationship between propositions. If we know how likely it is that B will happen if A happens, and we also know how plausible A is, we can calculate how plausible B is.

The fine-tuning argument uses Bayesian probability to show design is a more plausible explanation than chance. Recall our intuitive judgement the lottery winner is probably cheating. We come to this conclusion using an inference to the best explanation.

We have two possible explanations, chance and design, and we judged the winner was cheating after certain conditions changed. For the first lottery, there was no reason to think it was design. It could have been, but chance is the better explanation. Someone must win the lottery and there is no reason to be surprised if any particular ticket wins.

But as soon as someone wins repeatedly, design becomes a viable explanation, and every subsequent win makes design increasingly likely. By the 5th time, everyone is almost certain it isn’t happening by chance, it’s by design.

**Inference to the best explanation**

Notice that in the lottery example, we’re using a kind of Bayesian probability calculation to make an inference to the best explanation. If we judged by frequency probability, the chance of some ticket winning the 2nd, or even 10th lottery has the same probability as the first one.

But we’re not judging based on frequency probability, we’re judging the plausibility of it happening by chance considering other things we know. We’re judging how plausible winning by chance is in relationship to other facts we have available.

Facts about the human desire for money, the human willingness to cheat to get money, the human ability to design ways to cheat, the chance of the same human having the winning ticket repeatedly, and other things like this. As the chance hypothesis gets less likely, the design hypothesis becomes more likely because we’re comparing two possible explanations.

The fine-tuning argument uses the same kind of reasoning and calculation of plausibility. It asks us to compare two propositions, naturalism and theism, and how plausible it is those propositions would produce a universe which had the conditions to support life.

We take the information we have available about how plausible naturalism and theism are *in relation* to the fact of fine-tuning. And we use that relationship to decide whether chance or design is the best explanation for the fine-tuning of the universe.

**Comparing explanations**

If all we knew about the universe was that theism was true and God exists, we’d also know it was very likely the universe has the conditions needed to support life.

According to traditional theism God is omnipotent and omniscient so he is capable of creating and designing such a universe. God also desires to accommodate creatures with free will. Assuming theism is true, the probability the universe would have the necessary conditions for life is very high, close to 100%.

Contrast this with the alternative scenario. If all we knew about the universe was naturalism was true and God doesn’t exist, we’d also know it’s very unlikely that a universe which supports life would exist. Because the only mechanism which could produce the fine-tuning is chance.

And the chance of one constant being in the slim life permitting range is small, but the chance of multiple constants being in the narrow range required becomes increasingly improbable, very close to zero.

From these respective probabilities, we make an inference to the best explanation. Whichever explanation is most likely to produce the observation is the best one, because it’s most likely to be true.

This is the same method we use to judge the lottery win was designed. Because not only did the chance option become less probable with each successive win, but the design option also became more likely to produce the successive wins.

This is the general logic of the fine-tuning argument, so what are the possible objections?

**The values of the constants can’t be anything else**

How do we know the constants could have other values? Maybe there is no other value they could take so there is nothing which needs explanation. Changing numbers in theoretical models of possible universes is different to knowing if those values for the constants are actually possible.

For it to be impossible for the values to be different, the constants would be a mathematical or physical necessity. If that was true, they could be derived from the equations of physics. But this isn’t how physicists discover their values, the values aren’t derived from theory, they’re measured.

There is a range of possible values they can take given the parameters of the laws of physics. This range is the physically possible range. So it’s possible for the constants to be different.

It might be true that in the future science will discover equations that show the values are necessarily what they are. But this doesn’t affect the fine-tuning argument because it only pushes the question back a step.

We can still ask, why do we have those particular equations, with those particular values that create a universe which supports life. Is it due to chance or design?

Science can’t answer that question. The laws of physics describe how things function within a particular context. They don’t explain why we have that particular context.

**A universe with life is the only kind of universe we could observe**

The anthropic principle says if we didn’t live in a life permitting universe, we wouldn’t be here to observe it, so there is nothing to explain. But this objection misunderstands the probability involved.

We aren’t calculating the probability of *observing* a life permitting universe if we existed. We’re calculating the probability a life permitting universe would exist in the first place.

To illustrate the difference, the philosopher John Leslie uses the example of someone facing a firing squad of 50 sharp shooters. The shooters all miss. That’s so improbable we’d want an explanation.

It’s missing the point to say, if they didn’t miss we wouldn’t be here to wonder about it. What needs explaining is not that we can wonder about it if they miss, but why they all missed in the first place.

**There is a multiverse**

It’s tempting to think if we increase the number of universes, we can reduce the probability one of them will allow life. If our universe is part of a multiverse, it’s more plausible we just happen to be in one of the few that have the conditions necessary for life.

The theist should be pleased with this response. It not only concedes the fine-tuned constants are so improbable they need explanation, but the ontological extravagance of the solution is striking. We need millions, or billions of universes to increase the plausibility of the chance hypothesis in comparison to the plausibility of design.

But even then, it doesn’t do the explanatory work the naturalist needs. Because while it may increase the probability *some* universe will have conditions suitable for life, it doesn’t change the improbability *our* universe would. The logic of the fine-tuning argument is unaffected.

To see why, imagine the theist and the naturalist each have a bag containing millions of universes. The vast majority of the universes in the theist’s bag are life permitting. The vast majority of the universes in the naturalist’s bag don’t have the required conditions for life.

We observe our universe is life permitting. We also know our universe came from one of those bags.

*Which bag is it much more likely to have come from?*

The most likely explanation is the universe came from the theism bag.

**It’s God of the Gaps**

God of the gaps isn’t a serious objection but it’s so common I’ll include the obligatory denial. The argument isn’t that science can’t explain the fine-tuning therefore God-did-it. The argument doesn’t appeal to gaps in the science, or areas of scientific ignorance. It appeals to discoveries made by science.

The fine-tuning argument asks, given these discoveries of science about the values of the fundamental constants of nature, what is the best interpretation of the data? What is the best explanation of those discoveries?

Is our existence an exceedingly lucky series of coincidences, or has God left his fingerprints on this amazing creation?