What is the fine-tuning argument for God’s existence, what are two of the strongest objections to it, and how would you respond to those objections?

The fine-tuning argument notes that human existence in the universe depends on a wide array of natural constants existing within very narrow ranges, and that it does not seem that those constants necessarily had to be within those ranges, and infers from these two observations that the universe has been fine-tuned for the existence of human life. For example, the balance between the strong and weak nuclear forces is just such that matter forms, and not only matter but the wide variety of light and heavy elements found throughout the universe (not just something like hydrogen). The strength of the gravitational constant is just such that the universe neither inflated so rapidly that no stars could form, nor so slowly that it collapsed back in on itself under the influence of its own mass, during the Big Bang at its origin. The electromagnetic force is of just the right strength so that electrons are bound to their molecules, but not so tightly that they cannot be exchanged between particles, with all the extraordinary effects on exchange of energy that enables in a wide variety of systems.

Many more examples could be multiplied; the point is that if any of these or many other factors were only very slightly different, the universe would not exist as we know it, and nothing remotely like human life could exist at all. Moreover, there seem to be very many (possibly infinite) other possible values for these constants. There is no apparent reason the gravitational field could not be twice as strong or a third as strong as it is, for example. So the universe seems to have been designed—its basic “settings” fine-tuned—to be a place inhabitable by creatures like human beings.

Two of the stronger and more common arguments advanced against the fine-tuning hypothesis are the anthropic principle, and the many worlds hypothesis. Proponents of the anthropic principle as an explanation of the fine-tuning note that the only way anyone could exist to observe the universe is if it had these constants. Since people are here and do observe the universe, it is unsurprising that the universe has these constants. On the one hand, this seems patently obvious: of course the only kind of universe which could be observed by creatures like humans is one suitable for them to live in. On the other hand, it seems to miss the point entirely. If a firing squad of a dozen crack marksman line up to carry out an execution, all with live ammunition, all at close range, and after they all fire the person to be executed still lives, it does not serve as an explanation to him to say, “Well, obviously you could only be here to talk with us about this if you lived; if you had died you wouldn’t be here.” The improbability of surviving such an event demands explanation. So, too, the improbability of the one universe that exists having these properties demands explanation. After all, its existence does not depend on the people observing it (not least since there were no people around to observe it for most of its very long history). These factors exist entirely independent of the observers, and as such the fine-tuning argument is well-warranted.

A strong form of the argument might take into account the problems that come from observation selection effect. If one noted that all amoebas which have ever been observed were within an inch of a microscope, and then inferred that therefore all amoebas which exist are within an inch of a microscope, this would be a bad observer selection inference. The limitations of the observation prejudice the availability of the data. Likewise, if one had a net which was capable of catching only ten-inch-long or larger fish, and inferred from catching only ten-inch-long or larger first that there were not smaller fish in the pond, the inference would be a bad one. However, the fine-tuning argument is not like this. Rather, it is more like an argument which says: (1) Offspring are usually fairly like their parents. (2) This fish is just over ten-inches long. (H3) The fish might have parents which are only one inch long each. (H4) The fish might have parents which are nine to eleven inches long. (5) Observation 2 is much more likely on H4 than on H3 given Observation 1. Therefore (6) this fish’s parents were probably nine to eleven inches long. This argument might be wrong, but it is nothing like the earlier, obviously silly examples. The fine-tuning argument is of the same basic form: (1) The physical constants of the universe could have had any of a very wide, possibly infinite array of values. (2) The universe’s physical constants are just such that life is possible. (H3) The universe was created by God. (H4) The universe arose by chance. (5) Observation 2 is much likelier on H3 than on H4 given Observation 1. Therefore, more probably the universe was created by God.

Because of the severe problems with the argument from the anthropic principle, some atheists have instead advanced a many-worlds hypothesis. In this view, there are many universes—trillions, perhaps an infinite number. Universes constantly bubble into existence from the quantum foam, expanding with their own randomly chosen variations on the constants. Given such a plethora of universes, it is no longer improbable that a universe such as this one exists; in fact, its seems very probable. There are several serious problems with this idea, however. The first is that while it may make more probable the existence of some universe with a given set of constants, it does not make more probable the fine-tuning of any specific universe. The same basic problem faced by the anthropic principle explanation therefore still applies: out of all the universes out there, why is this one well-tuned for existence? Turning the earlier example slightly: imagine that an execution would be stayed only if every die in a set of 20 20-sided dice came up with a one. If an execution were stayed on that basis, the event would not be explained by the existence of many worlds such that it is likely that such a thing would happen in some world. It would still seem (and be!) a miraculous survival. Second, and more seriously, the many-worlds hypothesis introduces an infinite regress, but of something which is not apparently necessary: the universe. But if the problem is to be solved by introducing either infinite regress, or by treating something as necessary, the multiplication of worlds seems to radically reduce simplicity relative to the idea that God exists. Moreover, incidentally, on the Christian God’s existence it seems far more probable that universes would be fine-tuned for life even on the many-worlds hypothesis, for the Christian God is interested in knowing and being known by creatures; no such constraint is operative in a non-theistic many-worlds scenario.

Neither of these arguments against the fine-tuning argument is very telling. Both are very nearly question-begging, in fact, and in each case it still looks far more likely that the universe is fine-tuned because someone wanted it to support life, than that it just happened that way by chance.