"In crossing a heath, suppose I pitched my foot against a stone, and were asked how the stone came to be there; I might possibly answer, that, for anything I knew to the contrary, it had lain there forever: nor would it perhaps be very easy to show the absurdity of this answer. But suppose I had found a watch upon the ground, and it should be inquired how the watch happened to be in that place; I should hardly think of the answer I had before given, that for anything I knew, the watch might have always been there. (...) There must have existed, at some time, and at some place or other, an artificer or artificers, who formed [the watch] for the purpose which we find it actually to answer; who comprehended its construction, and designed its use. (...) Every indication of contrivance, every manifestation of design, which existed in the watch, exists in the works of nature; with the difference, on the side of nature, of being greater or more, and that in a degree which exceeds all computation."
--William Paley, Natural Theology (1802)

In modern English, Paley is saying that we can see design in a watch, and can thus conclude there is a designer - he then suggests that we can perform analogous actions with animals - we can see design in the way their eyes work, their limbs move, etc., and thus, they had a designer - which Paley claimed was God. Specifically, the God of his particularly brand of Christianity (Although, to be perfectly fair, Paley was pretty ecuminical when it came to doctrinal disputes).

This is he essence of the teleological argument - the claim that the universe is too beautiful, complex, good, or, most often, well-designed to have come about without some sort of designer. In my experience, it is one of the most common arguments put forward for the existence of a god of some sort - when someone says "But look at how well the universe is designed for us! God must exist!", they are presenting a teleological argument.

Obviously, I think it's wrong.

Why? Well, for a number of reasons. To start with, a man who was particularly enthusiastic about Paley's books later produced a scientific theory that would destroy the basis of Paley's argument.

"As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form."

I am, of course, referring to Charles Darwin.

Darwin made immediately obvious one of the main flaws in the teleological argument as presented by Paley - Paley assumed that the appearance of design implies design - Darwin demonstrated, quite effectively, that things could appear designed, and could do much better than anything any human designed, without a designer. Darwin provided a way for watches to construct themselves.

Expressed in more general terms, one of the issues I have with the teleological argument is that the argument jumps from "X appears designed" to "X is designed" - I don't think that's valid.

Of course, some people still contest the concept of evolution. There's no real need for consideration there - evolution happened, and is still happening, get used to it. You may as well contest gravity. At the very least, the concept works in a number of well-demonstrated ways (such as genetic programming), which is all it needs to be an example.

Another major issue I have is that the case for the appearance of design may often be overstated. Consider, for example, the claim that the universe is perfectly designed for life. The Earth is at just the right distance from the Sun for liquid water to stick around on the surface, the Sun is at just the right temperature to not fry us, etc. etc.

Never mind that Earth is the only place we know of where life exists - a little strange if the universe is so well designed for it. Never mind that 99.99999999% (approximately) of the universe is either vacuum, at 3 degrees kelvin, plasma, at six million degrees celsius, or maybe just bathed in high-energy radiation. The vast majority of the universe is incredibly hostile to life. Sure, we wouldn't exist if some physical constants were varied, but something else might. And always beware the principle Darwin introduced - maybe there's some good reason for the properties of the universe to prescribe something stable enough for life to exist - some sort of multiverse would certainly do the trick (Thanks to the Anthropic Principle - life would hardly show up in a universe that doesn't allow it).

Finally, there is always this particular old chestnut - surely something capable of designing a universe would be a little complicated themselves. Does the teleological argument apply to them, then? Do we need a super-supernatural designer? (Or would the term by 'hypernatural'?)

In short, the appearance of design does not imply design, the appearance of design may rely on our limited viewpoint, and the argument implies a ridiculous infinite regress.