The presence of an affordance is jointly determined by the qualities of the object and the abilities of the agent that is interacting.
This relational definition of affordance gives considerable difficulty to many people. We are used to thinking that properties are associated with objects. But affordance is not a property. An affordance is a relationship. Whether an affordance exists depends upon the properties of both the object and the agent. And the agent is a person, and people change or are in different states of mind or accessibility constantly. So affordances will change for the same person over time.
The Design of Everyday Things: Revised and Expanded Edition
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