The affordances of the environment are what it offers the animal, what it provides or furnishes, either for good or ill. The verb to afford is found 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.Chairs, for example, afford sitting. Not just any chair, though, and not just any person, as you'll realize if you ever try to cram yourself into a desk built for a second grader. A chair affords sitting if its seat is the right height from the ground (about the same as the distance from your heels to the backs of your knees), if it's wide enough for your posterior, if it's strong enough to hold your weight, and so forth.
We can look downward in size, to ask whether a coffee cup affords being held in one hand. (Some large cups have very small handles and when filled are too heavy to hold with just a thumb and forefinger.) We can think bigger, to ask whether a given architectural design affords the kinds of interactions between people that we might like to see. (Cabrini Green is one famous failure.)
And, as Don Norman points out in The Psychology of Everyday Things, it's critical that we're able to identify what can be done with objects and environments, to recognize their "perceived affordances." Which brings me to this:
That's not artistic purple shading around my eyes. Here's how it happened. I was heading to a meeting at the new Hunt Library on Centennial Campus, to give a presentation for our industrial advisory council. I saw one of my students in the lobby, just outside the main area of the library.
We talked for a minute or so about a new project, and then I turned to go to my meeting...
...and walked into the glass wall that flanks the turnstiles. This raised an enormous knot on my forehead, noticeable enough that the staff called a team of paramedics to look me over. (I checked out fine.) A few days later I look as if I've lost a fist fight.
And of course, being introspective, I think about the affordances of glass walls. They're great--you can see more of what's around you than with ordinary walls. You have a sense of openness, connected at a distance even with the outside. But for a specific person in a specific state of mind, it may not be immediately perceived that glass walls do not afford passage.