Creationists, Intelligent Design advocates, and theistic philosophers (one and the same?) argue the world is so complex it must have been designed. This is called the teleological argument. Analogy, as with most poor arguments, plays a big part in justifying this position.
Behe, an ID proponent testifying in the Dover trial, often talks about how a person coming across a watch, or seeing Mount Rushmore, can clearly tell someone designed each. And therefore, humans should be able to use the same reasoning we use to conclude that humans made the watch or Mt. Rushmore to detect design in nature: if nature, or a piece of nature, is designed it should have identifiable characteristics.
People usually argue against this analogy by pointing out that humans have a great deal of experience with what humans design and what humans' purposes are. We have seen people make human figures, and we have seen people make things that work mechanically. Thus identifying Rushmore and watches as designed is based on our past experience with human designed things. A person looking for God-design would have no examples of prior God-design to make a comparison.
Second, life forms can be acted upon by random mutation and natural selection. Watchs and rocky moutains cannot. Therefore, the appearance of design in a system that allows for random mutation and selection cannot be compared to an item that could not be formed in such a manner.
Thrid, we can identify human design because it is different than what would otherwise naturally form. If nature itself is designed, then you cannot evaluate if anything is designed.
I would add another argument that I have not seen previously:
The watch and Mt. Rushmore are the products of natural forces. Humans are not supernatural beings. Ants are not supernatural beings. Just as ants can create small anthills, humans modify our environment. If humans and ants exist as the result of the natural process of evolution -- as a function of the laws of physics -- then everything we create is also the result of the natural process of evolution and physical laws. The watch, and the anthill, were created by natural forces. Neurons fired, cells that form muscle tissue tightened, pieces of metal were placed, grains of sand were moved -- all in accordance with the natural laws of physics. Thus, the natural process of evolution did create the watch and Mt. Rushmore; evolution created things that are "designed" and also were created by natural forces.
If the watch, Mt. Rushmore, and an ant hill are considered naturally occurring phenomenon (proximately created by humans). Noticing something that appears very similar to what a human or ant might design is, therefore, no evidence at all for supernatural design, but fits into a pattern of natural design.
One might say I am begging the question. I first assume humans, and other lifeforms, are the result of non-supernatural events, and that this is the issue in dispute. However, I am not begging the question because I am addressing whether the “watchmaker” analogy is useful. It is not: under the theory of natural evolution, a comparison between human designed objects and other objects in nature is irrelevant. Watches, ant hills, along with irreducibly complex biological formations are entirely consistent with no supernatural “designer” existing.
In a sense I am arguing there is really no such thing as “intelligence” (or, possibly a soul or free-will), but instead intelligence is a natural process no different that how a Volcano decides to erupt.