I know you have been chomping at the bit for my answer to this question. In Darwin’s Black Box, Behe defines IC:
a single system composed of several well-matched, interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning.
Now I know you can’t wait to provide the “other” definition for me…
So this is one definition (I will call it IC1).
And I agree that the flagellum is IC1 by this definition. That is demonstrable directly with experiments. We can define the system, the function, and the parts. Then start removing parts one at a time, to see if the function is retained. For one example of flagella at least, removing some of the parts does not obliterate function, but removing many of them does obliterate function. Therefore (because removing some of the parts kills the function) it is an IC1 system. This [is an] objective [definition, directly observable and measureable, and that is its brilliant and strength. Everyone using the IC1 definition will agree what is and isn’t IC1.]
Now, Behe’s ID hypothesis is that IC1 systems cannot be evolved. We can test that hypothesis. We have, and we find that this hypothesis is false. There are several examples of IC1 systems evolving in laboratory systems. So that hypothesis, (that IC1 reliably identifies evolvable systems) has been clearly falsified. It was falsified even before Behe first made the argument in 1996.
One simple example is the labmbda phage that Venema discusses in his blog: http://biologos.org/blogs/dennis-venema-letters-to-the-duchess/the-evolutionary-origins-of-irreducible-complexity-part-4. The new species of virus is an irreducibly complex system that requires OmpF (on its prey), OmpF-binding proteins, genetic material and capsid proteins, etc. Remove anyone of these things, and it stops working, therefore the new phage is by definition an IC1 system. However, we observed directly that it has been produced by evolution. There are literally thousands of examples like this. IC1 does not reliably identify un-evolvable systems.
If the flagellum is not evolvable, this has nothing to do with it being an IC1 system. Evolution has no problem with IC1 systems, and this is demonstrable in experiments.
This is one reason that Behe himself has abandoned this argument. He does not make it anymore, because it fails.
So tell me now, what is the next definition of IC that was proposed (IC2 if you will)? You have read ID more than most people, you say. So tell me. What was the next definition? If you don’t know what it is, then why exactly do you think the IC argument is so strong? Maybe flagella are not evolvable, but without doubt the IC1 definition you use does not prove this so. Given overwhelming evidence, the argument fails basic logic.