Donald A. Norman is a professor and author of many books that deal primarily with usability and cognitive psychology.
James J. Gibson was an American psychologist who is considered one of the most important scientist to deal with visual perception.
Both of these psychologists have similar but specifically different views about affordances are. Gibson believes that affordances are governed by three laws. One, An affordance Exists relative to the action capabilities of a particular actor. Two, The existence of an affordance is independent of the actors ability to perceive it. Three, An affordance does not chance as the needs and goals of the actor change. Norman thinks that an affordance refers directly to the core features of an object. "I believe that affordances result from the mental interpretation of things, based on our past knowledge and experience applied to our perception of the things about us." Norman does not address an 'actor' at all in his view on affordances.
I was confused with all of the laws and different aspects of affordances, but when the article gave examples it became much more clear what Norman and Gibson were talking about. While Norman keeps his definition of an affordance simple, I feel as though Gibson's ideas about it make more sense. I felt that his ideas about nested affordances keep things in line. It's a simple way of branching out and categorizing different types of affordances without getting too specific right off the bat.
I was really interested in the example given about sitting in a chair. Using Norman's way of thinking about affordances he says the chair affords support, therefore affords sitting. It's kind of a backwards way of thinking about it, but it makes sense.
The Design of Everyday Things - Donald A. Norman
Affordances: Clarifying and Evolving a Concept - Joanna McGrenere