Donald weighs in on affordances applied against UI design – in a nutshell, they don’t really apply (because they are most relevant in the physical world):
“The most important design tool is that of coherence and understandability which comes through an explicit, perceivable conceptual model. Affordances specify the range of possible activities, but affordances are of little use if they are not visible to the users. Hence, the art of the designer is to ensure that the desired, relevant actions are readily perceivable.
Today we do much of our design on computer screens, where the range of possible actions are limited to typing on a keyboard, pointing, with a mouse, and clicking on mouse and keyboard switches. Soon we will add spoken words and visual gestures to the list of interactions. All of these actions are abstract and arbitrary compared to the real, physical manipulation of objects, which is where the power of real and perceived affordances lie. Today’s design often lies in the virtual world, where depiction stands in for reality. Many aspects of physical affordances are denied the designer: the alternatives are constraints and conventions. These are powerful when used well. Personally, I believe that our reliance on abstract representations and actions is a mistake and that people would be better served if we would return to control through physical objects, to real knobs, sliders, buttons, to simpler, more concrete objects and actions. But that is a different story for a different time. Moreover, control of our artifacts through abstract commands implemented via typed and spoken items, pointing, and clicking will be with us for a very long time, so we do need to adapt.
Please don’t confuse affordance with perceived affordances. Don’t confuse affordances with conventions. Affordances reflect the possible relationships among actors and objects: they are properties of the world. Conventions, on the other hand, are arbitrary, artificial and learned. Once learned, they help us master the intricacies of daily life, whether they be conventions for courtesy, for writing style, or for operating a word processor. Designers can invent new real and perceived affordances, but they cannot so readily change established social conventions. Know the difference and exploit that knowledge. Skilled design makes use of all.”