The Teleological Argument
"The universe shows signs of purpose and intelligent design. It's so orderly, it can't be the product of random chance. To use the old analogy, if you found a watch on the beach, wouldn't you think that it was made by some intelligence? Well, the universe and the life forms in it are like a watch. Consider the harmony of the ecosystem: plants breathe in carbon dioxide and expel oxygen, while animals conversely breathe in oxygen and expel carbon dioxide; plants feed animals while decomposed animals feed plants... There is a regular cycle of life... Moreover, consider how complexly organized a single organism is... Each organ and tissue has purpose, all of which shows signs of an intelligent creator; there is no other possible explanation for it."
Objection 1: Bad Analogy
The Teleological Argument is based on analogy which, if taken seriously, actually yields pagan conclusions.
If we extrapolate from the nature of the universe to the nature of its creator then we should infer from the finitude of the universe that the Creator is finite.
In human experience, complex enterprises require teamwork. This suggests that creation is a joint enterprise. There is no single God, then, but rather a family or committee of gods.
The universe contains inefficiencies and imperfections. This suggests that the divine committee is staffed by bunglers.
On earth, creation rarely results from human ingenuity, engineering, and manufacturing. Most of what is created happens in the natural world of reproduction. By analogy, then, the universe is the unintentional result of sexual activity by the gods.
Analogies are very helpful as illustrations, but as arguments they are always weak, and the Teleological Argument is no exception.
Objection 2: Bad Induction
If we found a watch on a beach then we would have reason to regard it as a human artifact because we have a great deal of experience with watches and other industrial products. (If savages found a watch and had no experience of metalwork or glasswork, they would not have reason to regard it as a human artifact. They would compare it to an unusually impressive pearly seashell. Conversely, if city-dwellers discovered an unbroken conch shell in a museum, and if they knew nothing about shells either directly or indirectly, they would mistakenly regard it as artifactual.)
In order to legitimately judge the provenance of the universe, we need to know whether other universe-like things are created mostly by nature or mostly by design. Since we cannot do this, the Teleological Argument is invalid. To use the terminology of those who classify fallacies, it rests "on an induction of one".
Objection 3: Evolution
It's true that in some sense the heart has a purpose, but it's a lie to say that there is only one possible explanation for how it came about. In addition to the Creationist explanation there is also the Darwinian explanation.
Because of political pressure from the religious right, American schools often short-change evolutionary theory. Therefore certain misunderstandings are common, and they need to be cleared away.
Objection 3: Regress
Anti-evolutionists hold this principle (P), that the complex features of the universe cannot emerge from the simple features of the universe because complexity cannot emerge from simplicity. Therefore the cosmos (C) must be a result of a cosmos-creator (CC).
However, if P is true then CC must be even more complex than C, in which case CC must be a result of a cosmos-creator creator (CCC). The reasoning iterates to infinity, and there is no last stage that can be called "God".