The scientific method looks something like this:
1. Make an observation: Things fall down.
2. Think up some explanation for that observation. This is a hypothesis: There is a force between objects which draws them together.
3. Make a prediction based on the hypothesis: The amount of force is based on mass.
4. Devise an experiment to test the the prediction: Build an instrument which would be able to measure the attractive force between objects. See if the attractive force - if it’s there! - correlates with mass.
If the predicted observation doesn’t happen, you reject the hypothesis and go back to the drawing board, using the new observations that you’ve collected to build a better hypothesis. If the predicted observation does happen, you see if the results are reproducible and consistent. When you’re really sure that that what you’ve got isn’t a chance anomaly but a repeatable, consistent observation, then you put a little checkmark next to the hypothesis and say, “We’re pretty sure this is a good explanation.” A scientific theory could be thought of as a group of hypotheses intended to explain related phenomena. A good scientific theory is one which has lots of supporting evidence - that is, it has been used to make lots of testable predictions which have led to repeatable, consistent observations that support the hypotheses. A bad scientific theory does not make accurate predictions.
It turns out that creationism (by which I mean things like Young Earth Creationism) has all the makings of a (bad) scientific theory while intelligent design (ID) does not, even though ID claims to be scientific. It’s all in how science is structured.
Creationism as scientific theory
When you structure creationism* like a scientific theory, it looks like this:
1. Observation: There is holy scripture which describes the origin of the universe, life and everything.
2. Hypothesis: The holy scripture is a literal description of the origin of the universe, life and everything.
3. Prediction: The holy scripture says that the Earth is 5000-10000 years old.
4. Experiment: Estimate the age of the Earth through the use of radiometric age dating or some other acceptable and technically sound method of dating**.
Structurally, everything fits. You can test creationism scientifically, and you can even modify the hypothesis to try again. Admittedly, this modification is usually done by asserting that the holy scripture in question isn’t a literal description of the origin of the universe, life and everything, which in turn is just a fancy way of saying, “The explanation isn’t in the holy scripture.”
ID as scientific theory
1. Observation: There are some really complex stuff in nature. That stuff is so complex that even removing one little bit makes it impossible for the complex part to do what it does (this is called irreducible complexity). A common, non-biological example is a mouse trap. Take away any part, and it ceases to function as a mouse trap.
2. Hypothesis: Some unknown designer created or designed the complex stuff in nature.
3. Prediction: ???
Let’s set aside for a minute the questionable nature of the originating observation (though go see this wikipedia article for a more thorough explanation of irreducible complexity and criticisms), ID proponents will try to tell you that they predict that there will be other irreducibly complex stuff. That’s not a prediction. That’s the originating observation. When you observe that stuff falls down, you can’t drop a ball and go “Hey, it fell down! So my theory of invisible spirits pulling stuff down is true!”
All right, what else? Some might argue that you can come up with some testable predictions of the unknown designer. The problem is that there’s virtually nothing to go on about this unknown designer. ID proponents have claimed that they are not affiliated with a religion, so you can’t rely on holy scripture, like creationists. In this situation, aliens, Sauron, the flying spaghetti monster, invisible pink unicorns and any other entity who could have some form of “designing” power are fair game. So, how do you choose which one to pursue? What other observation do you have which allows you to make any assertions about the unknown designer, especially when ID doesn’t require that the designer design more than once? “I have evidence of aliens!” is all well and good, but you need to connect aliens to the irreducibly complex thing.
What ID proponents want you to believe is that the prediction “The theory of evolution is wrong” is an acceptable prediction for ID. Yes, if ID is a correct explanation then the theory of evolution might be wrong, but this is not an acceptable prediction based on scientific method. Which is a pretty good thing for ID supporters, since if it was then it would be acceptable to say that if the theory of evolution is true, then ID is wrong. And since there is a lovely pile of evidence supported through the use of the scientific method, for the theory of evolution, this would mean that ID would be “proven” wrong.
Which is why ID is gaining momentum while creationism is sliding to the wayside. Creationism leads to testable predictions and can be subjected to the scientific method***. Intelligent Design can’t but that also means that you can’t use the scientific method to refute it.
* While I mean young earth creationism here, you could probably slide in any religion with holy scripture which is meant to be taken literally.
** For more on the methods used to date the Earth, here’s a Wikipedia article. As it turns out, the Earth is about 4.54 billion years old.
*** That the scientific method often shows that literal interpretation of holy scriptures is faulty is another story.