The teleological argument is an inductive means reaching the conclusion that the universe was designed by an intelligent entity. Essentially, if were to find a complicated machine in the middle of nowhere, we would not suppose it were there by chance or by whim of nature. Rather, we would suppose that the complexity and form, and perhaps function it is apparent, inferred that it was created by an intelligent being. In the same sense, though, we find that complexity of equal and greater magnitude, and form far more impressive, is readily observable in the universe. The complexity of any human made machine pales in comparison to the complexity of the biological machinery of a single bacterium. So complex is nature that even today it never ceases to surprise us and evade our predictive and descriptive capacities.
Now the process of induction begins by claiming that like effects warrant the assumption of like causes. Within the complexity of every single human creation, we find evidence of intelligent human design. Thusly, in other incidences of complexity and form for which no designer is known, the only logical and consistent conclusion is that it was intelligently designed.
There are several strengths to the argument, such as its deceptively scientific approach to the issue. Through implying inductive logic to empirical data, as science does, the most probable and plausible conclusion is reached. Also, like any inductive conclusion, it admits the possibility of being wrong. Regardless, it is an attempt at the best possible explanation; an explanation consistent with the extent of human understanding and observation.
The argument, in my opinion, is not consistent with human understanding nor is it rational. This should be apparent when examining the impression that our human constructions are similar to nature's constructions, and nature itself. The first similarity, complexity, can be somewhat disputed. Our creations are not objectively complex, but rather relatively complex compared to some aspects of nature. On the other hand, the complexity of our designs is relatively simple when compared to the complexity of a single cell. The closest thing I can think of to describe complexity in objective terms is entropy. Still, though, the title of complexity can only be assigned relative to some other part aspect of nature. The teleological argument, in this context, can only be used to persuade one to believe that things more complex than our own creations are intelligently designed. The universe, though, is neither complex nor simple. The universe has both simple can complicated parts. In fact, the net entropy of the universe is increasing, and thusly net complexity is decreasing. At some point, far in the future, the degree of complexity in even our simplest creations will be impossible, and the argument would cease to be viable. Think of it in terms of temperature, where complexity would synonymous with hot or cold. The terms hot and cold only describe our impressions of the temperature, just as complexity only described our impressions of organization. To a far more intelligent being, the inner workings of a cell might seem simple. Similarly, the 25 degrees Celsius we find so comfortable might be fatally cold to an intelligent alien organism.
The next similarity worth examination is function. In examining a human creation, we can generally find some sort of means to an end, that end being of interest to the creator. An automobile, for example, produces motion, and that motion can be harnessed and controlled, which is evident from the presence of seats and a steering wheel. In a watch, we find the intention of knowing the time. In nature, too, we see function, especially in the context of life. Like a watch, the solar system turns and rotates. Unlike a watch though, there is no apparent intention in the orbiting of the planets, no evidence of an intended end. The motion of the planets is dictated by natural law, and so to are the motions of the watch. However, those parts of the watch were placed in such a way that indicates intention. We utilize the momentum of nature to produce a desired end, an end which can be derived from most of our constructions and our understanding of human need. The motions of celestial bodies, though, are dictated totally by natural law, namely gravity. Their placement and position, too, hold no apparent indication of strategy of a desired end. While the motions of the planet may serve some function to some outsider, our lack of understanding for such an outsider bars us from possibly being able to understand, describe, or even detect the intended function. Lest we presume the existence of a anthropomorphic creator, thereby begging the question, the desired function of anything but out own constructions is anything but apparent.
The function of something is relative to the creator. If we do not understand the nature of the creator we can not even begin to guess at the intended function. In this case, we are limited by our anthropomorphic tendencies. We can not pretend to know how a thing might serve a creator without presuming the creator exist and has certain needs (despite being "perfect"Eye-wink. It is speculation, at best, and gives no extra credibility to the argument as a whole until unless the argument is presumed to be true in the first place.
One might point to the complex laws of nature, and their origins. This presumes that the very fabric of the universe is complex. Though, in comparison to what is the universe complex? We have no such comparison and, even if we did, we still find ourselves troubled by intricacies of complexity previously discussed. In fact, upon closer examination, the laws of nature seem deceptively simple when compared to the extraordinary manifestations they lead to. Gravity gives solar systems, stars, galaxies, and yet is little more than a tendency of attraction. Within every part of nature observed thus far, despite its beauty and awe inspiring complexity, are nothing more than manifestations basic natural laws.
In summary, I do agree that like effects warrant the assumption of like causes. However, upon close examination, I see only few similarities between a painting and a nebula, a watch and the solar system, or an organism and city. The lack of apparent function and intention lead me to believe the universe is indifferent, serving no exterior need.