Is there a God? The evidence for and against

There was a time, before Darwin, when a fairly good empirical case could be made for God’s existence. Anyone could appreciate the apparent finesse and purposefulness of the bodies of biological organisms. An eye looks like an ingeniously designed device whose purpose is to provide visual input to the brain; a heart looks like well-crafted pump whose function is to circulate blood.

The deist had an explanation for these observations: God had created the universe, including biological organisms, and because He was assumed to be infinitely intelligent and knowledgeable it was only to be expected that the creatures He created would be amazingly cleverly designed. An atheist, on the other hand, had no explanation for this fact. Using an inference to the best explanation, many concluded that the world must have been created by a deity of some sort.

Atheists had to contend themselves with general sceptical doubt. This doubt could be reinforced by pointing out certain internal difficulties with the deist’s explanation. For example, the problem of explaining why the Creator existed. Or the problem of evil: if the deity was not only all-powerful but also perfectly good then it is hard to see why there should be any evil in the world (such as young innocent children suffering from horrible diseases). Theologians wrestled with these objections, with varying degrees of success. Theologians also invented many other arguments for God’s existence of a more metaphysical character, none of which was found especially compelling by empirically-minded sceptics, and none of which is regarded as sound by contemporary analytic philosophers. But the argument from the apparent design of biological organisms was harder to dismiss.

Enter Darwin. Evolution theory gives a naturalistic explanation of how advanced biological organisms could come to exist. This explanation is superior in every way to the earlier deistic explanation. Not only does it not involve postulating some unobserved superhuman entity, but evolution theory is an infinitely richer source of successful empirical predictions than any rival creationist explanation.

At this stage of the debate, the atheist seems to have triumphed. Actually, that might be an overstatement, for the atheist has only refuted the deist’s argument for God’s existence; she has not proved that God does not exist. Still, considering the internal difficulties with deism referred to above, atheism, or at least agnosticism, would be the only reasonable conclusion at this point.

However, there is a further stage to the debate. It turns out that one can run a modern-day version of the old design argument. We can now explain how intelligent creatures could evolve in a universe such as ours. But how come the universe is such as to permit life to exist? Physics and cosmology reveal that the existence of life is dependent on various physical parameters having values within certain very narrow intervals. For example, if the expansion speed of the early universe had been very slightly less than it was, then the universe would have recollapsed within a fraction of a second, and no life could have evolved. If the expansion speed had been very slightly greater than it was, then the density of the universe would have been too low for galaxies, stars and planets to form – again no life would have evolved. There are a number of other parameters that appear in a similar manner to have been "fine-tuned" for the existence of intelligence life. (This is referred to as the so-called "anthropic coincidences".) If one uses any natural probability distribution over the possible values that these physical parameters could have, it turns out that there would only be an astronomically small probability that they would have values that permit the evolution of life.

Why do the parameters have life-permitting values? In other words, why are there these anthropic coincidence? The deist has an answer: Because God created the universe and He chose these values in order that intelligent life would exist. The balance of evidence seems to have shifted back to favor the deist like it did before Darwin. Or can the deist come up with an alternative explanation for the anthropic coincidences?

One purported explanation that one sometimes hears is this: "If the universe did not permit life to exist, we would not be here to observe it. Therefore we should not be surprised that it is life-permitting." This pseudo-explanation (which is sometimes mistakenly referred to as the anthropic principle) doesn’t work. The surprising thing is exactly that beings such as us are here, considering that that requires that various physical constants have very special values.

A more promising explanation, which may superficially look similar to the pseudo-explanation but is fundamentally different, goes as follows: "Suppose that there are many universes with randomly different physical parameters. Then, just by chance, it is to be expected that some of these universes will be life-permitting. Of course, we find ourselves in such a universe rather than in one which is not life-permitting. Hence we observe that the physical parameters of our universe have the very special values that allow intelligent life to evolve." This explanation does solve the apparent mystery of why our universe appears fine-tuned for life. It does this, however, only by presupposing the existence of an ensemble of universes with differing properties. The question we must therefore ask ourselves is whether we have any reason to believe that such an ensemble of universes exists.

We do in fact have fairly good reasons to suppose that. First of all, because some popular theories in modern cosmology – in particular, inflationary quantum cosmology – would have as a consequence that many universes with a wide variety of properties exist. (An open question, however, is whether inflation theory would itself require some fine-tuning in order to work. Early versions of inflation theory did require that, but more advanced versions may overcome this problem.) The second reason we have to believe in an ensemble of universes is that it would solve the problem of fine-tuning. This might look like circular reasoning, but it is not. It is only a form of inference to the best explanation: If you observe A, and the most plausible explanation of A is B, then that gives you some reason to think that B.

There are thus currently two possible explanations for the apparent fine-tuning of our universe. Either there is an ensemble of universes with varying properties or the universe was intentionally created to permit life to exist. Which of these explanations is most plausible?

If further physical evidence confirms inflationary quantum cosmology then the ensemble explanation is clearly superior, because there would then be independent reasons to believe in an ensemble of universes. If, on the other hand, cosmology and quantum physics fail to provide reasons to believe in an ensemble of universes then it would be more open to debate which explanation is more plausible. The ensemble explanation would still seem preferable, because it avoids the conceptual difficulties inherent in the God-explanation (for example, the problem of evil).

Conclusion: For the sake of brevity, some subtleties have been ignored: for example, the possibility that further developments in physics might show that it was inevitable that our universe would be life-permitting even without postulating an ensemble of universe. The main possibilities are nonetheless the following: either there is a large ensemble of universes with differing properties, or our universe was intentionally created. Currently, the ensemble hypothesis is the most plausible one.

Papers on anthropic reasoning -- Plenty of material here, both introductory and research papers.

Anthropic coincidences -- Al Shroeder. Lists and explains fifteen anthropic coincidences, ways in which the fundamental constants appear to be fine-tuned to allow life

The density parameter and the Anthropic Principle -- Jaume Garriga, Takahiro Tanaka, Alexander Vilenkin. An example of how anthropic coincidences can be explained in an ensemble-model. The authors predict the value of the density parameter Omega within the framework of inflation theory using the anthropic principle

Barrow and Tipler on the Anthropic Principle vs. Divine Design -- William Lane Craig. Argues that the anthropic principle does not refute the design argument

Is the Weak Anthropic Principle Compatible With Divine Design?: A Response to Craig -- Kyle Kelly. Criticizes Craig's paper

The Anthropic Coincidences, Evil and the Disconfirmation of Theism -- Quentin Smith. Attacks Swinburn's attempt to argue for God's existence on the basis of the anthropic coincidence

The Transhumanist FAQ -- A worldview that provides a viable alternative to the religious outlook?