The representational theory of mind and the computational theory of cognitive processing are empirical hypotheses. However, they are empirical hypotheses whose truth has been pretty much assumed by just about everyone in cognitive science. (Wheeler, 2005, p. 8).Now, I suppose that if your aim is to undermine opposing views, it's easiest just to say that they are simply assuming something. But, when you have major, explicitly acknowledged empirical hypotheses--such as that cognition involves rule and representations--then it seems unlikely that such hypotheses will be mere assumptions. So, if you want to understand the major tenets of an opposing view, you should probably dig around and find out why they hold them.
So, I don't know why Gibsonians are so interested in the direct perception of affordances, but I figure there must be some experimental result or something that drives this. This is not just something they assume. And, I assume that there is some reason that Maturana and Varela (and it seems Evan Thompson following them) think that life and mind are very intimately related. I have no idea what that is, but I'm not going to go out on a limb and say they are just assuming that there is a connection.
Now, of course, finding out why some group holds a view takes a lot of time. I've been rooting around trying to find out what drives EP. I don't think reading things like Gibson, 1979, or TSRM are really doing it for me. I think I need to go back to some of the earlier experimental work. I've read some Maturana, Varela, and most of Mind in Life, but I still don't get it.
Wheeler, M. (2005) Reconstructing the Cognitive World. Cambridge, MA: MIT Press.