An affordance is a quality of an object, or an environment, that allows an individual to perform an action. The term is used in a variety of fields: perceptual psychology, cognitive psychology, environmental psychology, industrial design, human–computer interaction (HCI), interaction design and artificial intelligence.
As explained below, two different definitions have developed. The original definition describes all action possibilities that are physically possible; a refinement to that definition describes action possibilities of which the actor is aware.
Affordances 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 the actor 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.
Jakob von Uexküll had already discussed the concept in the early twentieth century, calling it the "functional colouring" (funktionale Tönung) of objects.
Affordances 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 which 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 the actor, but also their goals, plans, values, beliefs and past experience. 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 experience with similar objects (baseballs, perhaps) to bear 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 adaptation of the term had been unintended. However, the definition from his book has become established enough in HCI that both uses have to be accepted as convention in this field.
Consequences of the duality of the term
Norman's adaptation of the concept has brought about the use of affordance
as an uncountable noun, referring to the property of an object or system having easily discoverable action possibilities, as in "this web page has good affordance," or "this button needs more affordance."
It has also caused many to use the verb afford, which Gibson's original term was derived from, 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.