At #hsse2012, practically pranked by @jimspohrer by question on my “Is that affordance essential?” presentation when Don Norman was sitting beside him.  I have read Norman’s work on affordances, e.g. http://www.jnd.org/dn.mss/affordances_and_design.html , but found Greeno (1994) http://dx.doi.org/10.1037/0033-295X.101.2.336 clearer on the relational perspective for varying levels of ability in service systems with multiple types of offerings.

In any interaction involving an agent with some other system, conditions that enable that interaction include some properties of the agent along with some properties of the other system. Consistent with his emphasis on understanding how the environment supports cognitive activity, Gibson focused on contributions of the physical system. The term affordance refers to whatever it is about the environment that contributes to the kind of interaction that occurs. One also needs a term that refers to whatever it is about the agent that contributes to the kind of interaction that occurs. I prefer the term ability ….

Affordances and abilities … are, in this view, inherently relational. An affordance relates attributes of something in the environment to an interactive activity by an agent who has some ability, and an ability relates attributes of an agent to an interactive activity with something in the environment that has some affordance. The relativity of affordances and abilities is fundamental. Neither an affordance nor an ability is specifiable in the absence of specifying the other. it does not go far enough to say that an ability depends on the context of environmental characteristics, or that an affordance depends on the context of an agent’s characteristics. The concepts are codefining, and neither of them is coherent, absent the other, any more than the physical concept of motion or frame of reference makes sense without both of them.

As Gibson’s idea of affordances has been developed in research, it seems most productive when it is treated as a graded property rather than as a property that is or is not present. (Greeno 1994, p.338)

Source: Greeno, James G. 1994. “Gibson’s Affordances.” Psychological Review 101 (2): 336–342. doi:10.1037/0033-295X.101.2.336. http://dx.doi.org/10.1037/0033-295X.101.2.336.