What Is the Best Version of the Fine Tuning Argument?
- Dec 25, 2024
- 13 min read
There isn't only one formulation of the fine tuning argument. There are at least three.
This matters more than it might seem. Many people encounter one version, find a weakness in it, and conclude that they've disposed of the whole thing. But that's a mistake. The fine tuning argument can be formulated in several different ways, and not all of them share the same vulnerabilities.
In this essay, we'll compare three versions.
The first is the argument by elimination, associated with William Lane Craig. Fine tuning must be due to physical necessity, chance, or design. Once the first two are ruled out, design is the only explanation left.
The second is the probability argument, associated with Robin Collins, Luke Barnes, Richard Swinburne, and others. Fine tuning is far more likely if there is an intelligent cause than if there isn't.
The third is the approach we develop in Physics to God. It begins not with a process of elimination and not with a probability calculation, but with a mystery already present at the heart of physics: the mystery of the constants of nature. On this approach, fine tuning isn't the problem that needs solving. Fine tuning is the clue that helps solve a deeper problem that physicists already had. We think this version of the fine tuning argument is the clearest and strongest of the three.
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The Basic Discovery: What Fine Tuning Actually Is
To understand any version of the fine tuning argument, we first need to understand the basic physics. At the foundation of our best physical theories are the laws of nature: general relativity and quantum mechanics. These laws describe how the basic ingredients of the universe interact and develop over time.
But the laws of nature aren't purely qualitative. They also contain specific quantities. Gravity has a certain strength. The electron has a certain mass. The cosmological constant has a certain value. These fixed quantities are called the constants of nature, and there are roughly 25 of them built into the foundations of physics.
The strange thing is that physicists don't know why these constants have the values they do. While they can measure them with extraordinary precision, they can't derive their values from any deeper theory. The constants simply appear in the equations as unexplained numbers. They are measured, not explained.
That would already be strange enough. But modern physics has discovered something even more striking. Several of these constants appear to be fine tuned.
This means that if some of their values had been even slightly different, the universe as we know it would not exist. Not just life or human beings. The ordered structure required for atoms, molecules, stars, galaxies, planets, chemistry, and life would never have emerged.
This point deserves some emphasis. The fine tuning argument is often described as showing that the universe is fine tuned for life. That framing is common, but it can mislead. It makes the argument sound as if the only thing at stake is the emergence of biological organisms, and it needlessly exposes the argument to attacks about whether life could exist in radically different forms.
The more accurate point is that the universe appears to be fine tuned for a complex, ordered, and structured cosmos. Life is part of that picture, but so are atoms, chemistry, stellar fusion, galaxies, and the structured hierarchy of matter at every scale. Without the constants being what they are, there wouldn't be a rich universe filled with complexity. There might be only a thin soup of particles, or a universe that collapses too quickly, or one that expands too fast for any structure to form.
That's the discovery. The question is what explains it.
Version 1: The Argument by Elimination
One common way to present the fine tuning argument is by elimination.
William Lane Craig formulates it in roughly this form:
The fine tuning of the universe is due to either physical necessity, chance, or design. It is not due to physical necessity or chance. Therefore, it is due to design.
The structure is simple. There are three possible explanations. Two fail and one remains.
Physical necessity would mean the constants had to have the values they have — that some deeper law of physics forces them to be exactly what they are. But this doesn't seem to be the case. As far as physicists can tell, the constants could logically have been different. They appear in our best theories as unexplained numbers, not as derived consequences of deeper principles. And even if someone eventually found a theory that fixed the constants, the question would just move back one step: why does that deeper theory produce constants that allow for a complex, ordered universe?
Physical necessity doesn't seem like a strong explanation.
Chance would mean the constants just happened to land on the right values. But many of these constants appear to have structure-permitting ranges that are extremely narrow compared to the range of values they could have taken. In a single universe, getting such values by chance alone would be astonishing.
Chance becomes more plausible only if there are vastly many universes, each with different constants. Then it's not so surprising that at least one landed on the right values — and of course, we observe this one, because this is the kind of universe where observers can exist. This is the multiverse response.
But the multiverse faces serious problems. It's speculative, not directly observed, and not a prediction of any well-confirmed physical theory. And, it runs into deep conceptual difficulties, including the measure problem and the Boltzmann brain problem. Therefore, it tends to function less like a scientific explanation and more like an escape from the appearance of design.
So once physical necessity and chance are both set aside, design is what remains.
The strength of this version of the fine tuning argument is its clarity. It lays the options on the table and asks which one actually holds up. Many people find it persuasive, and for good reason.
But it has two genuine weaknesses.
First, an argument by elimination only works if the list is complete. If necessity, chance, and design are truly the only possibilities, then eliminating the first two settles things. But a critic can always ask whether something has been left off the list. Lee Smolin, for example, proposed a theory called cosmological natural selection that doesn't fit neatly into any of the three categories. While we don't think it works as an explanation of fine tuning, it nevertheless illustrates the vulnerability of the argument by elimination. Someone can always wonder whether there's a fourth option that no one has imagined yet.
Second, this type of argument requires fully eliminating the alternatives. In practice, the strongest objections to necessity and chance show that those explanations are weak or deeply problematic — not that they're logically impossible. If you've shown only that the alternatives are unlikely, you haven't finished eliminating them. You've started comparing explanations. And once you're comparing explanations, you've moved toward the second formulation.
Version 2: The Probability Argument
The second version is based on probability. Rather than eliminating alternatives, it compares them directly.
The underlying principle is simple: when comparing two explanations, an observation counts as evidence for whichever one makes that observation more expected.
Robin Collins presents the argument as follows:
Premise 1. The existence of the fine tuning is not improbable under theism.
Premise 2. The existence of the fine tuning is very improbable under the atheistic single-universe hypothesis.
Conclusion: From premises (1) and (2) and the prime principle of confirmation, it follows that the fine tuning data provides strong evidence in favor of the design hypothesis over the atheistic single-universe hypothesis.
The reasoning is easy to follow. If an intelligent cause were setting up the constants with the goal of producing a complex universe, we'd expect the constants to be the kind of values that allow for atoms, stars, galaxies, chemistry, and life. That outcome wouldn't be shocking — it would be aimed at.
But if there's no intelligence behind the constants and only one universe, the constants could have landed almost anywhere. The fact that they landed in the narrow range that allows for a structured cosmos looks, at face value, wildly improbable.
This version has a real advantage. It gives us a framework for comparing explanations rather than simply ruling them out.
But it also has three notable weaknesses.
The first is about probability. This version depends on probability calculations, and those calculations are hard to ground. Since we've only observed one universe and one value for each constant, how do we establish a probability distribution over all possible values? What range should we consider? How do we know what "random" even means here?
Defenders of the probability argument have reasonable responses. They can use plausible upper and lower bounds. They can argue that the fine tuning ranges are so narrow that the conclusion holds across many different assumptions. They can also point out that multiverse scientists rely on similar probability reasoning when arguing that fine tuning is improbable without a multiverse — so the problem isn't unique to the design side. Still, framing the argument in probabilistic terms opens the door to disputes about the underlying assumptions.
The second problem is theological. This version typically supports its first premise by saying that fine tuning isn't improbable under theism because God would have reason to create intelligent, conscious, morally significant beings. That may be true. But a critic can reasonably ask: how confident can we really be about what God would choose to create? Aren't we speculating about the divine will — engaging in a kind of "divine psychology"?
The third problem follows from the second. Once we start asking what kind of universe God would probably create, we invite a familiar challenge. What about suffering? Natural disasters, disease, death, pain? Is this universe, with all its tragedy, really what we'd expect an all-good intelligence to design?
Of course, there are serious responses to the problem of evil. But the probability formulation raises this question in a way the other versions don't. Because it asks us to evaluate how probable our universe is under theism, it naturally brings in the likelihood of everything about our universe, not just the constants.
While none of these problems destroys the probability argument, they all complicate it. And there's a version of the fine tuning argument that avoids all three problems.
Version 3: Fine Tuning as the Solution to the Mystery of the Constants
The strongest version of the fine tuning argument, in our opinion, begins somewhere else — with a mystery formulated by physicist Richard Feynman before fine tuning even enters the discussion.
The mystery is simply this: why do the constants of nature have the values they do?
This isn't a religious question forced onto physics from the outside. It's an internal question that arises from physics itself. How can physicists explain fundamental numbers?
The whole project of physics is to explain reality as deeply and simply as possible. Physicists look for unity. They look for elegance. They look for laws that explain many different phenomena from a few basic principles. And the history of physics has been one long series of successes in that direction: complicated phenomena reduced to simpler underlying laws, different forces unified, apparent complexity resolved into clean mathematical structure.
But the constants of nature sit oddly within that picture. At the deepest level of our current theories, we don't find only elegant laws. We find a list of roughly 25 specific numbers — the mass of the electron, the strength of gravity, the cosmological constant, and so on — that are simply measured and inserted into the equations. They aren't derived from anything deeper. Simply put, they look arbitrary.
Richard Feynman expressed this mystery vividly when discussing the fine-structure constant, a dimensionless number related to the strength of electromagnetism. Physicists can measure it with extraordinary precision, but they have no idea where it comes from. Feynman called it one of the greatest damn mysteries in physics — a "magic number" that comes to us with no deeper explanation.
That's the starting point. Before we ask why the constants are fine tuned, we should first ask why they have these particular values at all. This is what we call Feynman's mystery of the constants. It has nothing to do with religion and nothing to do with the design argument. It's the straightforward recognition that the constants resist the kind of explanation physics normally seeks.
Only now does fine tuning enter the picture. Fine tuning reveals that while the specific values of the constants may seem arbitrary, they are not arbitrary in their effects. Their values are precisely the kind needed to produce a complex, ordered, structured universe. They allow atoms to form, stars to ignite, galaxies to assemble, chemistry to flourish, planets to exist, and life to emerge.
That's the crucial reframing. Fine tuning is not the problem. Fine tuning is the key clue to the solution.
It's the clue that tells us how to think about the mystery of the constants. The constants initially seemed like a puzzling list of random numbers. Fine tuning reveals that their values are pointed toward a result — that they are, in effect, the values needed for a rich and structured universe to emerge. And that points directly to intelligence.
Consider what we mean by intelligence. At its most basic level, intelligence is the ability to select one option from among many possibilities for the sake of achieving a goal. An author who chooses the right words to express an idea is displaying intelligence. An engineer who selects the right materials to build something is displaying intelligence. In both cases, the common thread is purposeful selection: choosing just the right means necessary for a specific end.
The constants exhibit exactly this pattern. From among the many values they could have had, the constants are set for the emergence of a structured, ordered universe. They aren't random numbers. They're special numbers whose precise values are suited to producing a specific kind of result.
The inference to intelligence here functions much like other valid inferences: we look for the explanation that most naturally accounts for the pattern revealed by the evidence without introducing unnecessary complications.
We now see that the inference to an intelligent cause isn't a desperate move made after every other option fails. It arises directly from what fine tuning reveals: the constants bear the mark of purposeful selection.
Why This Version Is Stronger
This approach to the fine tuning argument has several advantages over the other two.
It doesn't depend on a complete elimination of alternatives. Of course, alternatives still need to be addressed — the multiverse, necessity, chance. But the core inference doesn't arise only after every other option has been ruled out. It arises from the positive content of fine tuning itself. The constants are set in a way that produces a specific, complex result. That kind of pattern points directly to intelligence.
It doesn't depend on probability calculations. We don't need to know the probability distribution over all possible values of the constants or assign a specific number to the improbability of fine tuning. The key point is simpler: the constants could logically have been different, yet the values they actually have are the ones needed for a complex universe. The argument rests on the indication of purposeful selection, not on our ability to compute an exact probability.
It also doesn't require speculating about what God would probably do. We don't need to say that God would probably want to create intelligent life, or that this particular kind of universe is exactly what a good God would choose.
The argument begins with physics, not theology. The mystery of the constants is not some obscure anomaly at the edge of physics. It sits at the foundation of our deepest theories of reality. Fine tuning reveals that their values are ordered toward a complex universe. Intelligence is the kind of cause that selects means for the sake of ends. That's the argument.
And it avoids the God-of-the-gaps objection. That objection says: you're inserting God into a gap in scientific knowledge, and as science advances, the gap will close and God will retreat. But that's not what this argument does.
Fine tuning isn't a gap in knowledge. It's a piece of positive scientific knowledge.
Before the discovery of fine tuning, the constants looked arbitrary — unexplained numbers with no apparent pattern. Fine tuning reveals that they're not arbitrary in their effects. Their values are coordinated toward the emergence of complexity, order, and structure. The argument is based on what physics has discovered, not on what physics hasn't yet explained.
Of course, we don’t want to overstate what the argument demonstrates. “Indicates” does not mean “logically proves.” Evidence often points toward an explanation even while alternatives remain logically possible. Fingerprints at a crime scene indicate a suspect even though an elaborate conspiracy could still explain the evidence differently. In the same way, fine tuning indicates intelligence because the constants display the pattern we associate with purposeful selection. That does not make alternatives impossible. But it does mean that any alternative must compete against an explanation already naturally suggested by the evidence itself.
What About the Multiverse?
Our version of the fine tuning argument, like the other two versions, still has to contend with the multiverse, so it’s worth addressing briefly.
The multiverse is the main naturalistic alternative to design. The idea is that if there are enormously many, perhaps infinitely many, universes, each with different values for the constants, then it’s not surprising that at least one universe has the right values. And naturally, we find ourselves in that universe because only such a universe could contain observers.
On our formulation, the multiverse enters as a proposed counter-explanation. Since the inference to intelligence arises directly from the pattern revealed by fine tuning, the multiverse cannot simply be assumed in order to avoid that conclusion. It needs to earn its place as a better, or at least independently grounded, explanation.
That's a heavy burden. The multiverse has no direct observational support. The claim that constants vary across universes is an unjustified assumption, not a demonstrated feature of confirmed physics. The devastating measure problem — how to assign meaningful probabilities across an infinite collection of universes — remains unsolved. And in many multiverse scenarios, the Boltzmann brain problem undermines our ability to trust our own observations.
These aren't peripheral technical issues. They go to the heart of whether the multiverse can function as a genuine explanation of fine tuning.
We think a strong case can be made that the multiverse fails on its own terms. But even without developing that argument fully here, the key point stands: fine tuning gives us a positive reason to infer intelligence. The multiverse is offered to remove that inference. To do so, it needs to be a better explanation than the one fine tuning naturally suggests. Simply existing as a logical possibility isn't enough.
Conclusion: Begin Where Physics Begins
So what's the best version of the fine tuning argument?
The argument by elimination is useful. It maps the terrain and shows that the main naturalistic alternatives — necessity and chance — face serious problems. But it can feel incomplete, and it depends on showing both that the list of alternatives is exhaustive and that the other options have been fully eliminated.
The probability argument gives us a framework for comparing explanations and indicates that fine tuning is far more expected if there's an intelligent cause than if there's only blind chance. But it invites disputes about probability distributions, raises questions about divine psychology, and needlessly pulls the discussion toward the problem of evil.
The strongest version begins where the physics itself begins: with the mystery of the constants.
The constants of nature are specific, unexplained numbers built into the foundations of our best theories of the universe. Physicists can measure them, but they don’t know why they have the values they do. That is Feynman’s mystery: the honest recognition that the constants resist the kind of explanation physics ordinarily seeks.
Fine tuning is the key clue to solving that mystery. It reveals that the constants aren’t random in their effects. They’re specific numbers whose values are precisely suited to the emergence of a complex, ordered, structured universe. That’s not what we’d expect from brute, arbitrary numbers. It’s what we’d expect from purposeful selection.
And intelligence, at its most basic, is the ability to select the right means to produce an intended result.
The fine tuning argument, on this reading, isn’t a process of elimination, a probability trick, or a gap-filler. It’s a direct inference from one of the most surprising discoveries in modern physics. The constants of nature appear to be set for a purpose. And a cause that selects the right means to produce a purpose is, by definition, intelligent.
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