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by James J. Gibson (1979)
The affordance of the environment are what it offers the animal, what it provides or furnishes, either for good or ill. The verb afford is in the dictionary, but the noun affordance is not. I have made it up. I mean by it something that refers to both the environment and the animal in a way that no existing term does. It implies the complementarity of the animal and the environment. The antecedents of the term and the history of the concept will be treated later; for the present, let us consider examples of an affordance.
If a terrestrial surface is nearly horizontal (instead of slanted), nearly flat (instead of convex or concave), and sufficiently extended (relative to the size of the animal) and if its substance is rigid (relative to the weight of the animal), then the surface affords support…. It is stand-on-able, permitting an upright posture for quadrupeds and bipeds, It is therefore walk-on-able and run-over-able. It is not sink-into-able like a surface of water or a swamp, that is, not for heavy terrestrial animals. Support for water bugs is different.
Note that the four properties listed - horizontal, flat, extended, and rigid - would be physical properties of a surface if they were measured with the scales and standard units used in physics. As an affordance of support for a species of animals, however, they have to be measured relative to the animal. They are unique for that animal. They are not just abstract physical properties. They have unity relative to the posture and behavior of the animal being considered. So an affordance cannot be measured as we measure in physics.
… The human species in some cultures has the habit of sitting as distinguished from kneeling or squatting. If a surface of support with the four properties is also knee-high above the ground, it affords sitting on. We call it a seat in general, or a stool, bench, chair, and so on, in particular. It may be natural like a ledge of artificial like a couch. It may have various shapes, as long as its functional layout is that of a seat. The color and texture of the surface are irrelevant. Knee-high for child is not the same as knee-high for an adult, so the affordance is relative to the size of the individual. But if a surface is horizontal, flat, extended, rigid, and knee-high relative to a perceiver, it can in fact be sat upon. If it can be discriminated as having just these properties, it should look sit-on-able. If it does, the affordance is perceived visually. If the surface properties are seen relative to the body surfaces, the self, they constitute a seat and have meaning.
… The different objects of the environment have different affordances for manipulation. The other animals afford, above all, a rich and complex set of interactions, sexual, predatory, nurturing, fighting, playing, cooperating, and communicating. What other persons afford, comprises the whole realm of social significance for human beings. We pay the closest attention to the optical and acoustic information that specifies what the other person is, invites, threatens, and does.
Donald Norman (1988)
There already exists the start of a psychology of materials and of things, the study of affordances of objects. When used in this sense, the term affordance refers to the perceived and actual properties of the thing, primarily those fundamental properties that determine just how the thing could possibly be used…. Affordances provide strong clues to the operations of things. Plates are for pushing, Knobs are for turning. Slots are for inserting things into. Balls are for throwing or bouncing. When affordances are taken advantage of, the user knows what to do just by looking: no picture, label, or instruction is required….
(Note: The notion of affordance and the insights it provides originated with James J. Gibson, a psychologist interested in how people see the world. I believe that affordances result from the mental interpretation of things, based on our past knowledge and experience applied to our perception of the things about us. My view is somewhat in conflict with the views of many Gibsonian psychologists, but this internal debate within modern psychology is of little relevance here.)
Other clues to how things work come from their visible structure - in particular from affordances, constraints, and mappings. Consider a pair of scissors: even if you have never seen or used them before, you can see that the number of possible actions is limited. The holes are clearly there to put something into, and the only logical things that will fit are fingers. The holes are affordances: they allow the fingers to be inserted. The sizes of the holes provide constraints to limit the possible fingers; the big hole suggests several fingers, the small hole only one. The mapping between holes and fingers - the set of possible operations - is suggested and constrained by the holes. Moreover, the operation is not sensitive to finger placement: if you use the wrong fingers, the scissors still work. You can figure out the scissors because their operating parts are visible and the implications clear. The conceptual model is made obvious, and there is effective use of affordances and constraints.
—Chapter 1, The design of everyday things 2nd ed.
How can design signal the appropriate actions? … Another set of signals comes from the affordances of objects, which convey messages about their possible uses, actions, and functions…. Affordances can signal how an object can be moved, what it will support, and whether anything will fit into its crevices, over it, or under it…. Affordances suggest the range of possibilities, constraints limit the number of alternatives. The thoughtful use of affordances and constraints together in design lets a user determine readily the proper course of action, even in a novel situation. —Chapter 2, The design of everyday things 2nd ed.
More and more things have to be made invisible, in violation of all the principles of design. No constraints, no affordances; invisible, arbitrary mappings. —Chapter 6, The design of everyday things 2nd ed.
Donald Norman (2008)
The concept for “affordance” has captured the imagination of designers. The term was originally invented by the perceptual psychologist James J. Gibson to refer to a relationship: the actions possible by a specific agent on a specific environment. To Gibson affordances did not have to be perceivable or even knowable - they simply existed. When I introduced the term into design in 1988 I was referring to perceivable affordances. Since then, the term has been widely used and misused…. Forget affordances: What people need, and what design must provide, are signifiers…. Designers of the world: Forget affordances. Provide signifiers. —Signifiers, not affordances
In the world of design, the term “affordance” has taken on a life far beyond the original meaning. It might help if we return to the original definition…. I introduced the term affordance to design in my book, “The Psychology of Everyday Things” (POET: also published as “The Design of …”). The concept has caught on, but not always with true understanding. Part of the blame lies with me: I should have used the term “perceived affordance,” for in design, we care much more about what the user perceives than what is actually true. —Affordances and design
Donald Norman (2013)
The term affordance refers to the relationship between a physical object and a person (or for that matter, any interacting agent, whether animal or human, or even machines and robots). An affordance is a relationship between the properties of an object and the capabilities of the agent that determine just how the object could possibly be used….
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.
… To be effective, affordances and anti-affordances have to be discoverable - perceivable…. If an affordance or anti-affordance cannot be perceived, some means of signaling its presence is required. I call this property a signifier….
Affordances exists even if they are not visible. For designers, their visibility is critical: visible affordances provide strong clues to the operations of things…. Perceived affordances help people figure out what actions are possible without the need for labels or instructions. I call the signaling component of affordances signifiers….
Designers have practical problems. They need to know how to design things to make them understandable…. How could designers describe what they were doing? There was no word that fit, so they took the closest existing word - affordance. Soon designers were saying such things as, “I put an affordance there,” to describe why they displayed a circle on a screen to indicate where the person should touch, whether by mouse or finger. “No,” I said, “that is not an affordance. That is a way of communicating where the touch should be. You are communicating where todo the touching: the affordance of touching exists on the entire screen: you are trying to signify where the touch should take place. That’s not the same thing as saying what action is possible.”
… Designers needed a word to describe what they were doing, so they chose affordance. What alternative did they have? I decided to provide a better answer: signifiers. Affordances determine what actions are possible. Signifiers communicate where the action should take place. We need both.
… The term signifier has had a long and illustrious career in the exotic field of semiotics, the study of signs and symbols. But just as I appropriated affordance to use in design in a manner somewhat different than its inventor had intended, I use signifier in a somewhat different way than it is used in semiotics. For me, the term signifier refers to any mark or sound, any perceivable indicator that communicates appropriate behavior to a person….
Affordances represent the possibilities in the world for how an agent (a person, animal, or machine) can interact with something. Some affordances are perceivable, others are invisible. Signifiers are signals. Some signifiers are signs, labels, and drawings placed in the world, such as the signs labeled “push,” “pull,” or “exit” on doors, or arrows and diagrams indicating what is to be acted upon or in which direction to gesture, or other instructions. Some signifiers are simply the perceived affordances, such as the handle of a door or the physical structure of a switch. Note that some perceived affordances may not be real: they may look like doors or places to push, or an impediment to entry, when in fact they are not. These are misleading signifiers, oftentimes accidental but sometimes purposeful, as when trying to keep people from doing actions for which they are not qualified, or in games, where one of the challenges is to figure out what is real and what is not….
To summarize:
- Affordances are the possible interactions between people and the environment. Some affordances are perceivable, others are not.
- Perceived affordances often act as signifiers, but they can be ambiguous.
- Signifiers signal things, in particular what actions are possible and how they should be done. Signifiers must be perceivable, else they fail to function.
—The design of everyday things, revised and expanded edition
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Work on the predictive brain already accounts for multiple interlocking features of lived human experience. It offers — we saw — a principled account of how inner and outer sensing work together to put us in touch with a structured world populated by meaningful possibilities for action. … As my bodily state alters, the salience of various worldly opportunities (to eat, for example) alters too. That means I will also act differently, harvesting different streams of information. Philosophers and psychologists talk here of “affordances,” where these are the opportunities for action that arise when a certain type of creature encounters a certain kind of situation — a hungry green sea turtle encountering a nice patch of algae discovers an affordance for eating, whereas a human diver encounters a different affordance — perhaps it is an opportunity to photograph the turtle having its lunch. —p117-118, The experience machine: How our minds predict and shape reality