Ah, affordance, that now-much-misused word.
Like many misused words, it means something very particular and very useful, which is sadly right next to something easier to understand (and also useful).
An affordance is “what an object tells you it is for”. (It is often misused to mean a device, like a handle or button or knob, which has the affordance we wish to provide.)
Watch a child of two or three at play: there, you will discover the affordances of an object. Does the child grab it, or twist it? Its affordances include grabbing and twisting. Does the child put things on top of it? It affords supporting. Does the child try throwing it? Because it affords throwing — it is liftable, holdable, seems like it might be heavy enough.
A tinier child is still testing affordances, and will throw things to learn how their arm works, to learn how physics works. They are learning that throwing is a thing you can do; that objects fly through the air, and land again. They're learning the rules — what actions are, what they do. This helps learn what objects afford a particular action.
An affordance is something that you perceive in a situation that tells you that an action is possible or appropriate. Bars afford grasping, plates afford pushing, flat horizontal surfaces afford supporting and resting.
That toddler is also learning their arm affords throwing, after learning it affords grasping and banging. So much banging.
But an arm does not afford ripping something apart; it has no rip affordance. You can pull, but not rip, with a single arm. (Tearing—pulling something off something else—can be done with one hand; but that’s different.)
And so, to rip, we learn to use both hands. The tool matters.
Handles afford twisting, but to the human hand. They don’t afford twisting to the foot; they’re not called footles. But pedals are for feet. Context matters. Pedals afford stomping: though you could push on them with a hand, they're usually positioned such that this affordance is muted. When we can push on them with a hand, we call the levers, or plates, or buttons.
The object matters.
Not all things can be ripped. A sheet of paper rips nicely. Thin fabric, likewise. Thick cardboard does not afford ripping, not for a small child. A sheet of steel does not rip.
(Unless you're a humaniform combat robot, of course.)
A working definition
An affordance is a function, then:
the observer's understanding of their capabilities; plus
the observer's understanding of the rules; plus
the observer's perception of the object in context
an observer’s conclusion as to what actions apply to the thing.
Let’s look at each of those in turn.
Capabilities: the seven-foot-tall combat robot (let's call her Bubbles) sees more things as rippable than the infant. The infant, in turn, sees many more things as climbable and enterable — the space under a big chair, for example. Bubbles sees that chair as a weapon; it affords hitting-with for her. The kid would, too, if the chair were smaller, or if she were freakishly strong.
Understanding of the Rules: We build a model of the physics of the world, which includes the capabilities we have — what will typically happen to a thing, and to us, when we do an action. When the rules change, we are lost, and have a very hard time not applying the old actions. This is seen as children get larger and heavier — a fall that used to be a minor thing, resulting in a bump or a cut or a bruise, now becomes a major one involving broken bones and wired jaws. People adapt at different speeds.
(Bubbles sees a fall as a weapon; so does Massachusetts law, which considers an attack that throws an opponent to the ground to be assault with deadly force. The ground can kill. For adults, the rules have changed, and must be relearned. More on this later, when we talk about new social media.)
Perception of an object: And here is where people usually think ‘affordance' lives: solely in the object. This is because in the real world, generally, do not think about changing the capabilities of the observer, or changing the rules; we consider those either outside our control, or fixed.
And for physics, this is true. The laws of physics don’t change, not over a human lifetime.
For capabilities, this is not true. Tools can change the capabilities of a person. An adult who carries a hammer can drive nails; for a hammered person, nails afford driving, and become a good way for joining pieces of wood. For an unhammered person, nails afford throwing (poorly), poking, scratching, and so forth, but there’s no good way to use them to join pieces of wood together.
The flip side is the canard “When all you have is a hammer, everything looks like a nail.” This is true, but it's about capabilities and perceived affordances. A hammer changes how you perceive the world.
Because a hammer is so good at striking, it makes you assign the “being hammered” affordance to things that aren’t really suitable. Walls can be hammered, certainly — but it makes holes in them, rather than fastening them to something. (Driving nails into a wall is driving nails, not hammering walls.) People can be hit with a hammer, as a mechanism for ending up in jail. Nearly anything can be hit with a hammer.
And that's the point: the hammer has changed what you believe you can do, and in doing so it literally changes the way you see the world. It assigns new affordances to the things around you, through your hammer-wielding eyes.
A now-classic experiment demonstrates how capability affects perception. Participants are asked to rate how far away targets were, and whether they could touch them, until they have achieved some level of competence. Then they’re given a reaching-stick, and asked to touch the targets with that. After becoming proficient with it, they routinely rank things as closer to them — even though all that has changed is their “arm length”. The yardstick they used to measure the world had changed, literally. And rather than updated their yardstick, they updated the WORLD.
(But only when they used the stick for reaching. Without that, it was just a stick.)
Interaction as action
I've used hammers and knobs and chairs so far because they're easy to understand for nearly everyone. But some of our most important social interactions happen in the realm of communication exchanged with others. And affordances matter.
Because the affordances in online communication SUCK. More in part 2!
References (a partial list)
Clark, A. (1999) Being there: Putting brain, body, and world together again. Cambridge, MA: MIT Press.
Gibson, J. J. (1977) The Theory of Affordances. In R. E. Shaw & J. Bransford (Eds.), Perceiving, acting, and knowing.
Gibson, J. J. (1979) The ecological approach to visual perception. Hillsdale, NJ: Erlbaum.
Jacques, J. (2003-) Questionable Content.
Norman, D. (1988) The (Psychology/Design) of Everyday Things.
Witt, J., Proffitt, D., and Epstein, W. (2005) “Tool use affects perceived distance, but only when you intend to use it.” In Journal of experimental psychology: Human Perception and Performance, Vol 31(5), Oct 2005.