An Intro to the Philosophy of Religion, Ch. 5 Part 3 (Design arguments)

The analogy argument
Rea and Murray don't mention the origin of this argument, but Hume summarizes it well through the words of Cleanthes.

Look round the world: Contemplate the whole and every part of it: You will find it to be nothing but one great machine, subdivided into an infinite number of lesser machines, which again admit of subdivisions, to a degree beyond what human senses and faculties can trace and explain. All these various machines, and even their most minute parts, are adjusted to each other with an accuracy, which ravishes into admiration all men, who have ever contemplated them. The curious adapting of means to ends, throughout all nature, resembles exactly, though it much exceeds, the productions of human contrivance; of human design, thought, wisdom and intelligence. Since therefore the effects resemble each other, we are led to infer, by all the rules of analogy, that the causes also resemble; and that the Author of nature is somewhat similar to the mind of man; though possessed of much larger faculties, proportioned to the grandeur of the work, which he has executed. (Dialogues Concerning Natural Religion, Oxford World's Classics Ed., p.45)

Rea and Murray reconstruct a basic argument and then proceed to a more advanced version:

5.34 The universe is like a machine.
5.35 Machines are typically caused by designers.
5.36 Therefore: the universe is likely caused by a designer.

More advanced version:

5.37 There is some property P such that (a) some natural object N (or perhaps the cosmos as a whole) has P, (b) many artifacts (watches, for example) have P, and (c) artifacts that have P do so because they are products of design.
5.38 Things that are alike generally have causes or explanations that are alike as well.
5.39 Therefore: it is reasonable to conclude that N has P because it is likewise the product of design.

Analogy arguments are weak because they either define "machine-ness" in a way that begs the question or else have trouble substantiating the claim that all instances of such machine-ness imply a designer.

Inference to the best explanation
"In drawing this conclusion you make an inference to the best among a number of possible available explanations. Arguments of this sort work only when there are no competing explanations." Under this criterion, Rea and Murray do not find biological design arguments very compelling. Evolutionary theory is a competing explanation that explains apparent design by reference to "workings of an algorithmic natural process, namely, variation and natural selection."

However, the contemporary debate has shifted from biological design to cosmological design. As cosmologist and atheist Fred Hoyle remarks, "A common-sense interpretation of the facts suggests that a superintellect has monkeyed with physics as well as with chemistry and biology [to create suitable conditions for life]..."[1] Rea and Murray reconstruct the cosmic fine-tuning argument as follows:

5.40 The universe exhibits fine-tuning of a sort that makes it suitable for life.
5.41 The existence of fine-tuning is probable under theism.
5.42 The existence of fine-tuning is highly improbable under atheism.
5.43 Therefore: fine-tuning provides strong evidence in favor of theism over atheism.

Some examples of fine-tuning are listed: the rate of cosmic expansion after the Big Bang, the strength of the nuclear strong force, and the electromagnetic force (particularly its delicate balance with the strong force).

Some objections can be raised against premise 5.42. An example is given that attempts to show that it isn't highly improbable that the configuration of constants and forces is fine-tuned for life as opposed to some other life prohibiting configuration. We are told to imagine a dial with ten numbers on it, each taking on a value between zero and nine. For life to be possible, all ten dials must have the value 5; the odds of this are one in 10 billion. But what about any other random configuration of values on the dials? The probability is still one in 10 billion, and thus life permitting values are no less probable (indeed, equally as likely) than any other set of values.

Rea and Murray give a counterexample (William Lane Craig has used it before as well). Suppose a friendly poker game is disrupted after one of the players draws royal flushes for ten straight hands. To his friends' dismay, John insists that the sequence of hands he drew was no less probable than any other sequence of hands. Why does John's response fail to satisfy his companions? Because there are only a few hands that beat all others, and his friends want to know "why he drew an improbable and special series of hands." In other words, though the probability of drawing the same hand ten times is equi-probable with drawing any other series of hands, the probability of drawing an unbeatable hand (as opposed to a garbage hand) ten times is not. Likewise, the probability of the constants and forces having life permitting values is not equi-probable with a life prohibiting series of constants and forces.

A formidable objection to premise 5.42, the multi-verse objection, asserts that a finely tuned universe is improbable on atheism only if there is a single universe. Perhaps there are many universes with varying natural laws, and so it isn't so unlikely that one of them is life permitting. Unfortunately there isn't a any empirical evidence for this claim. Secondly, it isn't clear (in these speculative multi-verse theories) that the other universes would take on different constants and forces. Thirdly, whatever the mechanism for generating universes is, that mechanism itself would have to be fine-tuned; indeed, a universe-making machine might require a higher-level of fine-tuning than modern physics can presently appreciate.

[1] Fred Hoyle, "The Universe: Past and Present Reflections," Engineering and Science (November 1981), p. 12.