Thinking of affordances as a relationship between two properties is an interesting point that Norman mentions. Prior to reading The Design of Everyday Things, I have thought of objects and their affordances only within the context of one particular user. For example, a car affords a person with the ability to drive. But thinking of it as a relationship between two properties forces me to think of the user as more than one person. What is the relationship between someone who is shorter and the car compared to the relationship between a person who is taller and a car? The affordances change.