Intentionality, Function and Description

I don't think Dennett ever realy tells us how a bag of molecules can take an intentional stance, but his hierarchy has some appeal.

I offer the following version:

Physical
System (signalling)
System (function)
Intentional
Reflexive

And, as before, while we must attribute intentionality to an interlocutor, we may attribute otherwise if it 'works' (though always fallibly, from Kripke).

What I hadn't thought about properly until a talk with BT on Monday, is that there are circumstances where we must practically, as opposed to 'logically' (or to avoid undermining our own intelligibility), attribute intentionality (and function).  This is where lower level descriptions are just too complex.

Higher level biological systems are a good example:  a physical description of them woud be hopelessly complicated, and only a functional description is tractable.

With some computer systems, it is possible (depending upon the level selected) that even a system level descriptions would be too complex.  And it looks like brains might be like this too ....

These are still different reasons from the ones we have for treating interlocutors as intentional.  What I'm wondering, though, is whether there is some relationship here that I'm missing.

One of the reasons I'm wondering is that there isn't such a big difference between treating a system as law bound and treating it as functional.  Maybe.

And if we cannot maintain intelligibility without attributing intentionality on practical grounds, what then?