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An affordance is a quality of an object, or an environment, which allows an individual to perform an action. For example, a knob affords twisting, and perhaps pushing, while a cord affords pulling.
Different definitions of the term have developed, as explained in the following sections. The original definition described all actions that are physically possible. This was later adapted to describe action possibilities of which an actor is aware. The term has further evolved for use in the context of HCI to indicate the easy discoverability of possible actions.
The term is used in a variety of fields: perceptual psychology, cognitive psychology, environmental psychology, industrial design, human–computer interaction (HCI), interaction design, instructional design, science, technology and society (STS) and artificial intelligence.
As action possibilities 
Psychologist James J. Gibson originally introduced the term in his 1977 article "The Theory of Affordances" and explored it more fully in his book The Ecological Approach to Visual Perception in 1979. He defined affordances as all "action possibilities" latent in the environment, objectively measurable and independent of the individual's ability to recognize them, but always in relation to agents and therefore dependent on their capabilities. For instance, a set of steps which rises four feet high does not afford the act of climbing if the actor is a crawling infant. Gibson's is the prevalent definition in cognitive psychology.
Affordances were further studied by James Gibson's wife, Eleanor J. Gibson, who created her theory of perceptual learning around this concept. Eleanor Gibson's book, An Ecological Approach to Perceptual Learning and Development explores affordances further.
As perceived action possibilities 
In 1988, Donald Norman appropriated the term affordances in the context of human–machine interaction to refer to just those action possibilities that are readily perceivable by an actor. Through his book The Design of Everyday Things, this interpretation was popularized within the fields of HCI and interaction design. It makes the concept dependent not only on the physical capabilities of an actor, but also the actor's goals, plans, values, beliefs, and past experiences. If an actor steps into a room with an armchair and a softball, Gibson's original definition of affordances allows that the actor may throw the recliner and sit on the softball, because that is objectively possible. Norman's definition of (perceived) affordances captures the likelihood that the actor will sit on the recliner and throw the softball. Effectively, Norman's affordances "suggest" how an object may be interacted with. For example, the size and shape of a softball obviously fits nicely in the average human hand, and its density and texture make it perfect for throwing. The user may also bring past experiences to bear with similar objects (baseballs, perhaps) when evaluating a new affordance.
Norman's 1988 definition makes the concept of affordance relational rather than subjective or intrinsic. This he deemed an "ecological approach," which is related to systems-theoretic approaches in the natural and social sciences. The focus on perceived affordances is much more pertinent to practical design problems from a human-factors approach, which may explain its widespread adoption.
Norman later explained that this restriction in meaning of the term had been unintended, and that he would replace the term by "perceived affordance" in any future revision of the book. However, the definition from his book has become established enough in HCI that both uses have to be accepted as convention in this field.
Further shift of meaning 
Norman's adaptation of the concept has seen a further shift of meaning, in which the term affordance is used as an uncountable noun, referring to the property of an object or system's action possibilities being easily discoverable, as in "this web page has good affordance," or "this button needs more affordance."
This has then in turn given rise to a use of the verb afford – from which Gibson's original term was derived – in a way that is not consistent with its dictionary definition. Rather than "to provide" or "to make available", designers and those in the field of HCI often use afford as meaning "to suggest" or "to invite".
The different meanings now associated with the word, although closely related, can be a source of confusion in writing and conversation if the intended meaning is not made explicit and if it is not used consistently. Even authoritative textbooks can be inconsistent in their use of the term.
False affordance 
William Gaver divided affordances into three categories: perceptible, hidden, and false. A false affordance is an apparent affordance that does not have any real function, meaning that the actor perceives nonexistent possibilities for action. A good example of a false affordance is a placebo button. In his article, Gaver explains that a hidden affordance indicates that there are possibilities for action, but these are not perceived by the actor. At a perceptible affordance, there is perceptible information available for an existing affordance, meaning that the actor perceives and can then act upon the existing affordance. This means that when affordances are perceptible they offer a direct link between perception and action, and when affordances are hidden or false, they can lead to mistakes and misunderstandings.
See also 
- James J. Gibson (1977), The Theory of Affordances. In Perceiving, Acting, and Knowing, Eds. Robert Shaw and John Bransford, ISBN 0-470-99014-7.
- James J. Gibson (1979), The Ecological Approach to Visual Perception, ISBN 0-89859-959-8.
- Uexküll, Jakob von (1980 [1920 etc.]): Kompositionslehre der Natur. (Edited by Thure von Uexküll). Frankfurt am Main.
- Donald Norman, The Design of Everyday Things, ISBN 0-465-06710-7. Originally published under the title The Psychology of Everyday Things, often abbreviated to POET.
- Donald A. Norman (1999). Affordance, Conventions and Design. Interactions 6(3):38-43, May 1999, ACM Press.
- Affordance, Conventions and Design (Part 2)
- In Human–Computer Interaction, Preece et al. (1994, p. 6) explicitly define perceived affordances as being a subset of all affordances, but the meanings are intermingled later in the same paragraph by talking about "good affordance"; In Universal Principles of Design, Lidwell, Holden & Butler (2003, p. 20) first explain that round wheels are better suited for rolling than square ones and therefore better afford (i.e. allow) rolling, but later state that a door handle "affords" (i.e., suggests) pulling, but not pushing.
- Gaver, W. 1991. Technology affordances. In Proceedings of the CHI 1991, ACM Press: New York, 79 – 84.
- [http://www.interaction-design.org/encyclopedia/affordances.html "Affordances"
- "Placebo buttons, false affordances and habit-forming"