A Preliminary Description and Classification of Affordances
J. J. Gibson, Cornell University; February 1971
This memo has been published in E.S. Reed & R. Jones (Eds.) (1982). Reasons for Realism. Chapter 4.9, Part II. pp. 403 - 406. Hillsdale, NJ: Lawrence Erlbaum Associates, Inc.
The hypothesis that things have affordances, and that we
perceive or learn to perceive them, is very promising, radical, but not yet
elaborated (Perceptual Systems, p. 285). Roughly, the affordances of
things are what they furnish, for good or ill, that is, what they afford
the observer. A list of examples and a classification is needed; the reader
is invited to make his own list, or to supplement the tentative list given below.
Not only objects but also substances, places, events, other animals, and artifacts
have affordances. We might begin with the easy-to-perceive components of the
environment consisting of surfaces and surface layouts. And we should assume
a human animal as observer, to start with, since the list of affordances will
be somewhat different for different animals.
I assume that affordances are not simply phenomenal qualities of subjective
experience (tertiary qualities, dynamic and physiognomic properties, exc.).
I also assume that they are not simply the physical properties of things as
now conceived by physical science. Instead, they are ecological,
in the sense that they are properties of the environment relative
to an animal. These assumptions are novel, and need to be discussed.
In a theory of information - based perception, learning to perceive affordances
is only one kind of perceptual learning or perceptual development. (For other
kinds, see Perceptual Systems, Ch. 13, esp. p. 283 ff. and Principles,
Ch. 5.)
The examples that follow are intended to be only suggestive.
I. Surfaces and surface-layouts related to posture and locomotion
- a stand-on-able surface of support; a place that affords rest.
- a walk-on-able surface, one that affords "footing." (For terrestrial locomotion
the substratum must be nearly level and rigid; a water surface is excluded.)
- a vertical rigid surface, an obstacle, affording collision and barring locomotion.
- an interspace or opening between obstacles, affording locomotion.
- a falling-off place, the brink of a cliff, affording injury by collision with
the ground.
- a gap between the cliff-edges which (depending on its width) may afford jumping.
- a stepping-down (or stepping-up) place, affording descent (or ascent).
- a sit-on-able surface (affording sitting).
- a stand-on-able object, stool, affording a high reach.
- a climbable layout (tree, ladder, stairway).
- a get-underneath-able surface, affording shelter (roof).
II. Surfaces that reveal or conceal; transparent or opaque
- an occluding surface, with its occluding edges (screen, wall, lid, clothing).
An opaque surface.
- a revealing surface (glass).
- a place affording concealment of oneself from others (hiding place, "private"
place).
- a place or layout affording concealment of an object from others. (Note that
children are deeply interested in the possibility of occlusion, an in peek-a-boo
and hide-and-seek and other games of concealment.)
III. Objects affording manipulation and related activities
(We distinguish portable from immovable solid
objects, and graspable from non-graspable
solid objects.)
- a handle (a graspable object attached to a portable object).
- a hand-hold (a graspable object attached to an immovable layout).
- a stick (or rake). An elongated rigid object affording a long reach (or a
long grasp).
- a tree branch (affording arboreal support to a primate).
- a throwable object, missile (rigid, graspable, moveable, of moderate weight).
- an object that affords hitting; a club, hammer.
- an object that affords cutting; a knife, axe (having an edge
with an acute dihedral angle).
- an object that affords piercing; needle, spear.
- an object that affords knotting, binding, lashing; string,
thong, rope, thread.
- an object that affords plunging; a convexity that fits into
and fills a concavity.
- a surface that affords support for useful objects; a bench, shelf, table.
- an object that affords rolling (sphere or cylinder) as distinguished
from one that has a flat base and affords sliding.
IV. Substances with affordances
- a substance that affords pouring, dripping, dabbling. A liquid.
- a substance that affords smearing, painting, trace-making. A viscous
substance.
- a substance that affords being shaped by manipulation. A plastic or malleable
substance.
-a substance that resists change of shape. A solid with persisting shape and
size. An object.
(Note that the properties of a substance are not the same as the properties of an object; this fact is recognized in speech, if not in physics, by the use of a "mass noun" instead of a "count noun.")
- a substance or object affording nutrition. Food.
- a substance or object affording illness. Poison. (But note that both food-objects
and poisonous objects afford ingestion and that they are sometimes
hard to distinguish by optical information. Occasionally, if rarely, they are
not even distinguishable by taste and smell, i.e., by the chemical value system.)
V. The affordance of injury or benefit
We now come to a consideration of the positive or negative "valences" of things.
Phenomenologists maintain that they are facts of "immediate experience" but
have not analyzed the biophysical basis of this perception. Nevertheless the
perception at a distance of what something affords if
encountered is said to be the great virtue of vision (cf. Berkeley's
New Theory of Vision, 1709, on "the damage or benefit which is like to
ensue." Also Gibson on The Problem of Safety, reprint). A mechanical
encounter or other energy-exchange may cause tissue damage. But the object,
place , substance, event, or animal that affords injury need not be encountered;
it can be avoided, escaped or averted, if perceived.
- The edge of a cliff affords falling.
- a wall affords collision (but may afford climbing).
- an approaching missile ("looming") affords injury.
- a knife-edge affords being cut (but also affords cutting).
- a fire affords being burned but also affords warmth.
- a snake affords being bitten.
- a surface of deep water affords drowning, but a surface of shallow water affords
bathing.
Those places, events, or animate objects are all specified in an ambient optic
array. In each case the affordance can be seen, I suggest,
and this is not the same thing as saying that the injury can be foreseen.
The argument is reinforced by the evidence to show that the imminence
of collision is optically given, as in Schiff's experiment. But there are other
places, substances, and events in an environment that are either not
specified in an optic array, or not obviously, or whose affordances
are not visible, or not without specific training.
- a potential rock-fall or avalanche is hard to see.
- a lightning-bolt is not seen until it occurs.
the danger of sunburn or gamma rays is not indicated.
VI. The detecting of affordances by young animals
The human young must learn to perceive these affordances, in some degree at
least, but the young of some animals do not have time to learn the ones that
are crucial for survival. Ethologists therefore are interested in what they
call "sign-stimuli" and "releasers." If the foregoing is correct, however, the
behavior in question should be reconsidered in terms of stimulus information,
not of stimuli. A listing of releasers in these terms would be interesting.
Top
If you are in need of consulting services, we recommend Atlantic Human Factors
for human factors consulting and
usability consulting.
Or Search for Human Factors and Usability Consultants
Copyright © 2003 - 2007 by HuWI.org