The Teleological Argument for The Existence of God
Aquinas and Paley
This argument was used successfully until the mid-19th century and is recently gaining new traction amongst Creationist who reject the theory of Evolution. It was presented by St. Thomas Aquinas, a 13th century Christian Philosopher who included it in his work, Five Proofs. It was used most notably though by William Paley in his Natural Theology, in which he presents us with his well known watchmaker analogy. The argument goes something like this;
If an aborigine finds a pocket watch on a beach he has no life experiences that would indicate to him what it's purpose is, but still he can tell it's not a part of the naturally surrounding order, that it's not an odd shaped rock or seashell, that it indeed had an intelligent designer.
The analogy is to the apparent order found within the flora and fauna in nature. To attribute this order to an intelligent designer was an intellectually respectable position until 1859 when Darwin's naturalistic observations from the Galapagos Islands while traveling on board, "The Beagle," where published in On The Origin of Species.
The Death of An Ancient Argument...?
The Teleological argument is an a posteriori (Latin for after the fact) argument based on the premise that the organisms in nature are so ordered that they must have been designed. However once this premise is superseded by a naturalist explanation the argument crumbles.
Interestingly enough this argument far predates Christianity. Plato more than three hundred years before Christ had a disagreement regarding the argument from design with his student Aristotle. Plato postulated a "divine artificer," while Aristotle believed there to be some as yet unknown natural explanation.
The question wasn't settled until a theory that accounted for the illusion of apparent intelligent design was rendered by Darwin. Darwin's theory of natural selection posits that amongst populations with small differences (genetic variations, genes were not known of in Darwin's time) some differences would be more advantageous to survival and subsequent procreation than others would be. The organisms less suited for survival would either die out along with their genetic variations or drift toward a different survival niche for which they were better suited. Once this drift has proceeded far enough so as to leave the two populations too genetically different to mate with one another and produce fertile offspring (this is the trait that by definition differentiates one species from another in Biological Taxonomy) speciation has occurred and now there are two separate populations of two separate species where before there was only one.
There are still some erudite points of contention regarding the exact mechanisms of evolution. For instance genetic mutations were at one time thought sufficient to account for all the requisite variation required for speciation. Now an emerging field called Epigenetics which studies how vestigial strands of DNA are toggled off and on thus effecting genetic expression is being used to refine that supposition. However, there is no doubt within the scientific community that all species both extant and extinct are cousins upon a great tree of life which traces itself back to a common ancestor. This common ancestor would be something akin to a simple bacterial Prokaryotic cell.
While there may be no debate in the scientific community, there debate in society, particularly stemming from the Christian Right Wing of the United States. A large contingency, gallop has been coming up with a figure roughly at about 45% since the 80's, of the country believe that all species were made roughly as they are now sometime within the last 10,000 years. This stance is referred to as young Earth creationism. It is important to note that this postulate is not a scientific theory in that it neither unifies nor explains any observable facts which is the function by definition of a scientific theory. It is actually a tenet of faith derived from the Genesis account of the Bible.
Unfortunately, these 40% want this tenet of their faith taught as science in public schools. If we expand the question to, "Should the controversy be taught in Schools?" (that is both evolution and creationism) the number bloats to something around 70%. And this pseudoscientific agenda is gathering momentum. Two States, Ohio and Tennessee, have passed bills into law requiring public school teachers to include creationism in their Biology curriculum.
While this is troubling as an index of the ignorance surrounding the theory of Evolution within the American Zeitgeist, creationism, which has been rebrand, "Intelligent Design," has no empirical evidence and thus carries no teachable material besides, "This is an article of faith amongst Christians." Once this statement has been dispensed with, Evolution, which is as strongly supported a theory as Gravity or Heliocentricity with around a dozen different physical sciences all converging on it's validity, can be taught. It can be examined alongside it's mountain of evidence including not only the fossil record but experiments performed with bacteria and predictions about the mutability of pathogens which have thus far always been validated.
One last point I'd like to make is that Theism and Evolution do not actually seem to be mutually exclusive, most high ranking Catholics, including most of last 20th and 21st century Popes, support evolution as well as almost the entirety of the Anglican Church.
What's more, a number of notable scientists are Theists. Francis Collins, for example, the head of the Human Genome Project which demonstrated our genetic relation to all other organisms is a devout Christian. Many Theists seem to believe that Evolution is the mechanism by which god created many species and they take the Genesis account in the Hebrew Bible as metaphoric instead of literal.
Returning to The Argument
And so with the apparent design of nature being accepted as a given the argument held for so long thusly;
Using, "if...then" reasoning (called Modus Ponens in Symbolic Logic) The first given is the conditional claim that implies the consequent claim.
Premises; If Nature is designed then god designed it N---->D
Nature is designed D
Therefor, God designed it (G), or in purely Symbolic Logic form
However, conditional proofs rely on their premises being correct to reach a valid consequent. Our first premise is not true thus the Teleological proof for the existence of god fails.
If nature is designed then god designed it N--->D
Nature is not designed (~N)
Therefor, Then God did not design it (~D) or in purely Symbolic Logic form
Notice I did not say therefor god does not exist, the negation of the antecedent that nature is designed only logically leads to god not designing it which is not the same thing as saying god does not exist. It is logically not possible to prove a negative in this way, but we have shown that the Teleological proof is invalid as a positive proof for god.