Affordance. If ever a term separated the usability cognoscenti from everyone else, this is it. For a good, long time now, our trade's initiation rite has been the reading of Don Norman's The Psychology of Everyday Things (now published as The Design of Everyday Things). Norman's book introduced many of us (myself included) to the term affordance. As Norman's compelling arguments took hold, so did the term. It sprang up everywhere, moving from dusty journals to magazines and newspapers, and from there to the conceptual prime time of your local Starbucks.
Somewhere on the way from academia to Starbucks, however, something happened. The meaning of affordance became distorted and confused. At first it was subtle, but by now its meaning has bifurcated wildly. Whereas words become richer as they gain meanings, terms do not. Just as printing new money devalues existing money, the more new definitions the term affordance gains, the less value any one of them has.
This, then, is a story of the devaluation of the conceptual currency of our trade. It is also more than that. By itself, the fact that a term has lost meaning does not merit a loss of sleep. However, as this story will show, when the loss of a term's meaning signals a loss of much more, we do have a problem.
But that's the moral; first the story.
The term affordance was coined by the psychologist J.J. Gibson. Gibson had a growing dissatisfaction with information-processing theories of vision. He conducted a new type of research into perception and action as a response. Whereas information-processing theories limited themselves to considering stimuli (the object), receptors (the eyes), and processors (the brain), Gibson insisted that to understand vision one must understand the entire visual systemthe eyes, which are connected to the brain but also are part of a head attached to a body situated in an environment. This environment lies at the heart of Gibson's theories, and so he needed to describe it. The term affordance is one of the tools he used.
Affordances exist independently of perception and only as a relationship between an organism and an object.
This background is not just a theoretical curiosity. On the contrary, it is the key to understanding the concept of affordances. Gibson did not intend this term to describe what an organism perceives about the environment, but rather properties of the environment itself with respect to an organism. As he put it, "the affordances of the environment are what it offers the animal, what it provides or furnishes, either for good or for ill," and further that affordances "have to be measured relative to the animal" [emphasis in the original]. It is no coincidence that we speak of affordances in the environment with the suffix of "ability"sit-ability, stand-ability, or push-ability. Affordances are things the environment has the ability to furnish an animal. They are opportunities for action. So, the chair I am currently sitting on is sit-able for me, an adult male. Two things need to be stressed. First, affordances exist independently of perception. Even in a dark room where I could not see my chair, it would still be sit-able for me. Second, affordances exist only as a relationship between an organism and an object; they are not properties of an object by itself. The chair that has the affordance of sit-ability for me does not have that same affordance for my six-month-old daughter. She cannot sit, so the environment cannot have that affordance for herat least not yet.
This concept holds great value for the field of interface design, because it forces us to think more than screen deep. Affordances as many of us understand them are about the visual form that indicates that a button in a dialog box can be clicked. Affordances, according to Gibson, are all about the functionality behind that button and whether or not that functionality is appropriate for the target users. Gibson would argue that regions of a screen can't be clicked, but mouse buttons can. That click then makes use of many affordances of your computer, starting with its low-level ability to map a mouse location relative to a screen pointer and ending with the thing the user wanted to do (for example, deleting a file). Software may well be easier to use if its affordances are well advertised, but it also needs to have the right affordances in the first place.
In 1988, Don Norman incorporated the concept of affordance into his classic book The Psychology of Everyday Things . In the context of design, Norman defines affordances as "the perceived and actual properties of the thing, primarily those fundamental properties that determine just how the thing could properly be used."
Gibson labored to make affordances a characteristic of the environment that exists relative to an object but independent of perception.
Norman did the design world an important service by introducing them to this concept, but his definition missed the mark on two counts. First of all, it has only the object in view. The organism, though perhaps implicit in the surrounding discussion, does not receive clear mention here. Second, and more important, Norman makes affordances a combination of "perceived and actual" properties of an object. Gibson labored to make affordances a characteristic of the environment that exists relative to an object but independent of perception. In a subtle way, Norman's definition undermines both of these key aspects.
Why these differences? Norman's definition refers to an end note explaining that he disagreed with the Gibsonian definition. Norman was defining a different term. As he later clarified, he would have prevented a lot of misunderstanding if he had used that different termperceived affordances . Unfortunately for the growing field of usability and user interface design, Norman's definition of affordances became the received view. Practitioners ran with it.
One person who ran with Norman's definition was Alan Cooper. In 1995, just as the seeds of the DotCom boom were being sown, he published About Face , his practical volume on interface design. In its early pages, Cooper deals with the concept of affordances and their usefulness to design. He writes:
- I would alter Norman's definition by omitting the phrase "and actual." By doing this, affordance becomes a purely cognitive term, referring to what we think the object can do rather than what it can actually do.
Cooper goes on to explain that if a button looks like a doorbell, its affordance is "100 percent doorbell," even if pushing the button sounds no bell but rather opens a trapdoor beneath the user's feet.
With Cooper, the definition of affordance is reversed. In inventing the term, Gibson intended to describe the actual environment relative to an organism and independent of perception. According to Cooper, affordances don't even bother themselves with actualities of the environment. Rather, they are all about perception, because for interface design, Cooper believes that perception is where all the action is. (As long as Mr. Cooper persists in this view, I am available to install a doorbell on his home.)
We need to reclaim the original meaning of affordance. Many in the usability field have objected to my view, arguing that language moves on. Although the original definition of affordance might have been useful in its context, they posit, the new meaning has greater use in our context.
This seems reasonable enough, until you dig deeper. First, this argument assumes that usability as a discipline owns the term affordance. This is just not true. Gibsonian psychology originated this concept and continues to develop it even as we argue. But whether theyor weown this term makes no difference. We usability specialists have a continuing need for basic and applied research to inform our work, and Gibsonian psychology can provide some of this. If we are glibly content to redefine Gibsonian terms, we will erode our ability to communicate with its researchers and to use their findings in our work. We will be far poorer (and more insular) as a result.
Second, this reasoning assumes that the concept of affordance currently in use by our community is useful. Two researchers investigated this claim by reviewing the proceedings of CHI conferences. In 19 papers that used the term affordance, they found eight using Gibson's original definition, six following more closely to Norman's ideas in The Psychology of Everyday Things, one using a different concept altogether, and four in which "the use of [this term] is unclear" . For building a solid base of theoretical knowledge, a term with such an unclear meaning is worse than useless. It spreads confusion in its path.
Finally, some who object to my argument say that even if the current meaning of affordance is unclear, the original meaning was useless. This is an uninformed and unfortunate objection. The continuing work of Kim Vicente and his colleagues on ecological interface design shows that affordancesproperly construedhave great use in design for complex, safety-critical systems such as nuclear power plants and health care (see www.mie.utoronto.ca/labs/cel/).
One might argue that Vicente's conclusions are atypical, but in fact they echo many textbooks dealing with human-computer interaction (HCI): Human needs are an important consideration during all stages of the product development life cycle, but especially when making the early and large decisions about the problem to be solved and how to solve it. Practice bears this out: Usability engineering delivers much more value when we help to produce requirements documents than when we work in response to them. The original meaning of affordance speaks precisely to this point. Affordances are about not the forms on a screen but the functions behind them. Affordances don't ask how to make something look like a door, but instead ask if a door should even be there. When properly understood, affordances direct usability practice back to its point of prime influence and give us a framework to use when working there.
Influencing requirements is hard work. Organizations can be quite content to let an HCI professional work on use cases and user architectures, but they are careful to guard the seats at the requirements table. Even finding a place there, let alone being heard, takes a long time. This is why I find the redefinition of affordances so unfortunate. As a discipline, usability engineering certainly does need to develop a vocabulary that deals with the "perceived" properties of a thing. (Norman's clarification of affordances  is a great contribution here.) If, however, we are willing to do this at the expense of a term that deals with the actual properties of a thing in relation to a user, it may signal that we are adapting inappropriately to not having a place at the requirements table.
Affordances go much more than screen deep. As a discipline, do we?
I would like to thank Roy Ballantine, Roger Chang, Michael Cooper, Kipp Lynch, and Kim Vicente for comments on earlier versions of this article.
About the Author
Gerard Torenvliet has spent the last three years establishing and growing the user-centered design process at Watchfire Corporation. He has found a place at the requirements table; his next challenge lies in maintaining and growing it. In his spare time, he can be found with his wife helping their two children to learn the affordances of their world. If his stock options pan out, he plans on pursuing a doctorate.
Elizabeth Buie, Senior Principal Engineer,
Computer Sciences Corporation
15245 Shady Grove Road, Rockville, MD 20850,
301-921-3326, fax: 301-921-2069, email@example.com
©2003 ACM 1072-5220/03/0700 $5.00
Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee.
The Digital Library is published by the Association for Computing Machinery. Copyright © 2003 ACM, Inc.