Intelligent design cannot be taught as science because it is not based on the scientific method. Science is a systematic methodolgy for explaining how the universe works. The theory of evolution is based on that methodology and intelligent design is not.

I think that actually when teaching science, intelligent design could be used as a great example of a method for describing phenomena that is NOT based on the scientific method. Many people may find intelligent design or the literal creation story to be very compelling, and they are certainly free to believe what they want and teach to their own children what they want, but it is not appropriate to teach it as science.

Science procedes by systematically making measurements and observations. Objective measurements and observations are called "facts" --- they do not include any explanation of why or how something occurs, they are only measurements of what did occur.

Once facts are gathered, the scientific method provides that some educated guess about why or how the observed facts occur can be made. The proposed explanation is called a "hypothesis" --- it's an uproved explanation for the facts. The hypothesis is not a fact, and it is not a theory. A hypothesis should be consistent with all known facts. It should also be able to make predictions about other facts that have not yet been discovered. Using the hypothesis, experimenters should be able to propose new experiments to test the hypothesis. They should run those experiemtns and make new observations and measure new facts. The resulting facts might be consistent with the hypothesis or might contradict it. If they reinforce the hypothesis, then the hypothesis is given more weight. If they contradict it, then the hypothesis is proven false, and a new hypothesis must be proposed that is consistent with the new facts, or the phenomenon may remain unexplained.

Once a hypothesis has been tested through rigorous experimentation and observation, it can have enough weight to be considered a theory. A theory is an explanation that has been tested adequately to be generally accepted as true and consistent with all known facts. But the theory is never a fact. And science always holds open the possibility that new facts might be discovered that overturn the theory and require a new hypothesis.

Evolution is a theory that is consitent with all known facts regarding the change of species over time. New measuremtns and obeservations are made all the time that reinforce the weight of the theory of evolution. However the current theory of evolution cannot fully explain all observed facts regarding the change of species over time. It is consistent with all known facts, but it is not sufficient to explain them all. There are unexplained facts that do not contradict the theory, but are not explained by the theory.

This gap is where people who believe in intelligent design step in and say those facts that are not fully explained by evolution can be explained by an intelligent designer. But intelligent design is not a scientific hypothesis or theory. It does not propose new tests or experiments that could reinforce or refute it. It is not rooted in any other well accepted theory.

Scieince does not provide for explanations that invoke a supernatural power or any other unobservable or unmeasurable force. If every unexplained phenomenon were explained by simply saying, "God made it that way, " or "it was intelligently designed by a force we cannot measure," then science would never procede. Those kinds of explanations can never be proven or disproven. They also tend to discourage further scientific investigation by suggesting that the phenomenon has already been explained. The history of science has been a long march away from explanations based on mysterious forces we do not understand toward explanation based on measurable facts, hypotheses and rigorously tested theories.

The scientific way for dealing with the gap in current evolutionary theory is to honestly say that we have a theory that is consistent with observed facts but not complete enough to fully explain all facts. And to say that the theory needs to be refined through further hypotheses and observation.

In recent years there have been various efforts to teach evolution as a "theory not a fact." The phrase implies that there is still some doubt about evolution. Actually, if you understand the scientific method and know what these terms mean, it is true that evolution is a theory not a fact. It will always be a theory, not a fact. This is because it is an explanation for a phenomenon, not a measurable phenomenon itself. But it is not true that there is much scientific doubt about evolution. It is a very well supported theory. It might be incomplete, but it is consistent with observation.