The deeper point is that much of fine-tuning concerns conditions that are more basic than the kind of chemistry life might use. Before there can be carbon-based life, silicon-based life, or any other kind of complex physical life, the universe first has to allow stable matter, complex chemistry, long-lived sources of usable energy, and large-scale structures such as stars and galaxies. Fine-tuning reaches that far upstream.
Here are a few examples that illustrate the point.
First, consider the cosmological constant, the energy density associated with empty space. Theoretical estimates can be about 10^{60} to 10^{120} times larger than the value we actually observe. That is 1 followed by 60 zeros up to 1 followed by 120 zeros. Yet if the other main features of our universe remained roughly the same, increasing the vacuum energy by only a few hundred times its observed value would be enough to suppress the formation of galaxies. Without galaxies, there would be no stars and no long-lived concentrations of matter and energy in which complex structures could develop.
Second, consider the proton and neutron. Each has a mass of about 939 MeV, but the neutron is heavier than the proton by only about 1.29 MeV, a difference of roughly 0.14%. That small difference is crucial. Reduce it by about 0.78 MeV, less than one-tenth of one percent of a nucleon’s total mass, and ordinary hydrogen becomes unstable. At that point, we are no longer asking whether life might use carbon, silicon, or some other chemistry. We are asking whether one of the most basic forms of stable atomic matter can exist at all.
Third, consider the tiny density variations in the early universe, which were only about one part in 100,000. Those slight irregularities became the seeds from which gravity formed galaxies and stars. If they had been much smaller, matter would have remained too evenly spread out for stars to form effectively. If they had been substantially larger, matter would have collapsed into increasingly dense and violent structures, and at sufficiently large values much of it could have collapsed into enormous black holes. The early universe needed to be uneven enough for stars and galaxies to form, but not so uneven that gravitational collapse overwhelmed the possibility of stable, long-lived structures.
There is also an even deeper example involving entropy. The early universe began in an extraordinarily special state of very low gravitational entropy. Roger Penrose famously estimated that an initial state like ours occupies only about one part in (10^{10^{123}}) of the relevant phase space of possible gravitational states. That is 10 followed by vastly more zeros than there are subatomic particles in the universe. At the least, this illustrates how extraordinarily special the universe’s initial condition was. That low-entropy beginning gave the universe the capacity to develop galaxies, stars, usable energy gradients, and increasing physical complexity over time.
Someone might ask whether several constants could change together in ways that compensate for one another. In some cases partial compensation is possible, and a few parameters allow more variation than older popular presentations sometimes claimed. Even so, the larger picture remains highly constrained. A universe capable of complex physical life must still satisfy many different requirements at the same time: stable matter, sufficiently rich chemistry, long-lived energy sources, cosmic structures, and initial conditions that permit complexity to develop. Where the constraints are independent, the probabilities multiply, and the joint requirement becomes far more restrictive than any single constraint on its own. Limited compensation between a few parameters softens the overall constraint but does not eliminate the need for this broader coordination.
That is why the force of the fine-tuning argument is cumulative. It is not simply that one number has to be exactly right for human beings to survive. Rather, as we move farther upstream from biology to chemistry, from chemistry to atomic and nuclear physics, and from there to cosmic structure and the initial conditions of the universe, we continue to find conditions that have to work together for any complex physical life to be possible.
Even if we grant that some other combination of constants could, in theory, permit life very different from ours, this does not explain the more fundamental fine-tuning that must already be in place. Before any kind of life, whether carbon-based or otherwise, can exist, the universe must allow stable matter, complex structures, usable energy, and the development of complexity.
That is also where the fine-tuning argument becomes philosophically significant. If the fundamental conditions of the universe are entirely unguided, this remarkable coordination of life-permitting conditions is surprising and calls for explanation. If, on the other hand, a Mind intended a universe capable of supporting complex physical life, then the existence of such a universe is much less surprising.
In other words, the question is not only whether a different kind of actor could live on the stage. Much of the fine-tuning concerns whether there could be a stable stage in the first place. And we think the remarkable suitability of that stage gives good reason to consider whether a Mind stands behind it.