On the topic of 'intelligent design' and 'irreducible complexity, Behe talks about the criterion being the inter-component necessity of all to produce a functional capability.
I think we can go just a little further than this observational summation.
From a systems perspective, the complexity is built up from the non-deterministic grouping of parts that have parsimonious close-coupled interfaces with each other.
The 'non-deterministic' is important. I mean it to convey that the final function is not pre-determined in any of the parts in themselves. This itself tends to point to 'design', indeed from a systems engineering perspective, I would say it lies at the heart of design.
That is, the parts have no in-built teleology that necessitates any particular function, but are brought together a-teleologically (unless by a designer, where the telos would lay) to do something which independently they are unable to do.
