The fight over teaching Intelligent Design in science classrooms has been based primarily on the issue of whether ID is a valid scientific theory. Those favoring the teaching of ID claim that it is science and those against it say it is not.
The proponents of ID take on the Theory of Evolution by saying it's just a theory and attempt to disprove it by pointing to what they see as flaws or gaps in the theory. Those against the teaching of ID say that, although ID may have a place in the curriclum, that place is not in the science classroom. They argue that the criticisms of the theory of evolution by ID proponents are distortions of the data and that, because ID is not subject to analysis of the scientific method, cannot be science.
A more sophisticated argument against ID is based on the notion that one of the hallmarks of a scientific theory is its falsifiability. In other words, to be scientific, a theory must have the potential to be proven wrong. Note that this view differs from the common assumption that science is in the business of "proving" things right.
From a practical point of view, one way to distinguish a scientic from a non-scientific theory is to see if it's possible to design an experiment, either hypothetical or real, to test it. From this perspective, ID fails miserably. What kind of experiment can you design that would test whether a "higher intelligence" was behind how life, or for that matter, the universe works? To my knowledge, proponents of ID have yet to come up with such an experiment, whereas evolution has been so tested repeatedly.
One would hope that the failure to meet this criteria for a scientific theory would have put an end to the argument that ID is a valid scientific pursuit and should be taught as a counterpoint to evolution in the biology classrooms. But of course this has not been the case.
If an appeal to the scientific method and the nature of scientific knowledge doesn't work, is there another way to argue that ID is not a valid theory, whether scientific one or not? I think there is and it goes back to a fundatmental assumption behind the basic tenent of ID, which is that the universe and living things in particular are so complex that only some intellgence greater than that of man could have designed them.
The assumption to focus on is this: complexity of design is a reliable sign of intelligence. Even without going into theories of what constitutes intelligence, it's easy to see the problems with this.
For example, there are numerous cases wherein complexity is a sign of stupidity. Imagine a Rube Goldberg solution for just about anything. Although it might be humorous, few would see anything but stupidity in such complex solutions to simple problems. How long would a Rube Goldberg type last in a engineering or design firm?
For one, who can presume to know what an intelligence superior to that of humans is capable of? Who is to say that complex design is such a capability. In fact, complexity is often a sign of fuzzy thinking. Scientists, matheticians, economists, historians - essentially anyone involved in a discipline that seeks to explain something - prize simple over complex explanations or solutions. The simpler, the better. The simpler an explanation, the more elegant it is. From this point of view, the complexity of living things would reflect the operation of a less than superior intelligence, unless that intelligence is of a fun-seeking Rube Goldberg type.
Presumably, if a superior intelligence designed us, then why would humans have come up with the concept of Occam's Razor, the idea that the simplest solution is the best - the most elegant - solution. If we, like other living things, are so complex, it thus seems more likely that it happened as a result of random, willy-nilly alterations to our biology - including those to insert patches for "bugs" resulting from earlier random occurences - than it would from some superior intelligence putting it all together in a perfect, super-elegant way from the start.