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An affordance is a property of an object, or a feature of the immediate environment, that indicates how to interface with that object or feature. The empty space within an open doorway, for instance, affords movement across that threshold. A couch affords the possibility of sitting down on it.

Workers in the field of perceptual psychology use this concept, as well as workers in the fields of cognitive psychology, environmental psychology, industrial design, human-computer interaction and artificial intelligence. Psychologist James J. Gibson introduced the term in 1966, then explored it more fully in his book The Ecological Approach to Visual Perception in 1979.

Gibson originally defined affordance to refer to all "action possibilities" latent in the environment, objectively measurable, and independent of the individual's ability to recognize those possibilities. Further, those action possibilities are dependent on the capabilities of the actor. For instance, a set of steps with risers four feet high does not afford the act of climbing, if the actor is a crawling infant. So affordances must be measured along with the relevant actors.

In 1988, Donald Norman used the term affordance in Human Machine Interaction which made it a very popular term in Interaction Design field. Later (see Norman, 1999) he had to admit he was not talking about 'normal' affordances at all: in The Design of Everyday Things he was actually referring to perceived affordance. This distinction makes the concept dependent not only on the physical capabilities of the actor, but their goals, plans, values, beliefs and interests. If an actor steps into a room with an armchair and a softball, Gibson's definition of affordance allows that the actor may toss the recliner and sit on the softball, because that's objectively possible. Norman's definition of perceived affordance captures the likelihood that the actor will sit on the recliner and toss the softball, because of their experience with them in the past.

Norman's definition makes the concept of affordance subjective rather than objective, and therefore out of the realm of hard science (but not social science). On the other hand, the concept of perceived affordance is much more pertinent to practical design problems.

Perceptual psychologists can ask, "What is it about this object that makes people want to use it this way?" The object must talk to us with some sort of language. If we can understand this language, then industrial designers can make tools that explain their own functions, and even tools that recommend themselves for some uses and discourage other uses.

Ecological cognition using neuroimaging studies suggests that when an actor either objectively or subjectively perceives an affordance, they will develop a plan to act on that affordance and will act on that plan unless they experience cognitive dissonance. Dissonance can lead to the actor using an object in a way other than the way it suggests through its affordances.

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