Confusing Affordances

When I began the weekly reading, I had absolutely no clue what an affordance was.  At first I thought that perhaps it was related to finances; how much a product costs and how that could be relayed in the design. But to my great surprise, affordances had nothing to do with money or being able to "afford" something, but the design of an object and visual clues that design gives as to its use. 

In Joanna McGrenere's article "Affordances: Clarifying and Evolving a Concept ",  the reader is given two definitions of what an affordance can be.  The first definition belongs to James Gibson, the creator of the concept of affordance. He states that "... affordances of the environment are what it offers the animal, what it provides or furnishes, either for good or ill." The second definition  belongs to  Donald Norman, the author of "The Design of Every day Things." He states that "…the term affordance refers to the perceived and actual properties of the thing, primarily those fundamental properties that determine just how the thing could possibly be used."

But this is where I become confused. Which one is which? Is one better than the other?  To me, Gibson's theory suggests that an affordance is only available in one way despite the creature's knowledge of the object.  In other words, it's there, it does this and that's all it's going to do. An physical example would be stairs. We go up the stairs to get somewhere; that's their purpose. But Norman's theory provides a different view on the topic. He suggests that an affordance doesn't do just one thing, but provides clues as to what it does despite the creatures knowledge.  Using the stair example, we know stairs go up due to the fact they continue in an upward movement; we climb them and we get higher and higher in elevation. But we can also gather that we don't have to use stair just for climbing. We can also sit on them and store objects on them. 

In conclusion, it does not seem that one is better than the other. They both seem to be saying the same thing.  An affordance is part of a design that relates back to our brains and tells our brain what the object can do and what we can use the object for. That's what I've got so far..