The Theory of Affordances (Gibson, 1979, Chapter 8)

The most influential part of this book is the theory of affordances. We still argue a lot about what these are and how they work, because they are slightly weird (at least from the traditional point of view). So it was interesting to go back and read Gibson's original approach. I was struck by how much sense it makes in the context of the previous chapters; the book really builds and develops and this chapter should be discussed in that context I think.

Environments are the surfaces that separate substances from the medium. But these configurations of surfaces aren't simply there; they offer possibilities for action. They afford things to the animal. Gibson asks 

How do we go from surfaces to affordances? And if there is information in light for the perception of surfaces, is there information for the perception of what they afford? Perhaps the composition and layout of surfaces constitute what they afford. If so, to perceive them is to perceive what they afford. This is a radical hypothesis...

pg 119

In this chapter, Gibson does not offer a single straight-forward definition; that's not really how this works. Instead he lays out examples, and uses those to constrain the concept. As usual, ecological categories have fuzzy boundaries and this is ok!

While most people use the 'the affordances of the environment are what it offers the animal' quote, I quite like this bit - it's Gibson trying to illustrate all the key features of the concept.  

If a terrestrial surface is nearly horizontal (instead of slanted), nearly flat (instead of convex or concave), and sufficiently extended (relative to the size of the animal) and if its substance is rigid (relative to the weight of the animal), then the surface affords support. It is a surface of support, and we call it a substratum, ground, or floor. It is stand-on-able, permit ting an upright posture for quadrupeds and bipeds. It is therefore walk-on-able and run-over-able.

Note that the four properties listed—horizontal, flat, extended, and rigid—would be physical properties of a surface if they were measured with the scales and standard units used in physics. As an affordance of support for a species of animal, however, they have to be measured relative to the animal. They are unique for that animal. They are not just abstract physical properties. They have unity relative to the posture and behavior of the animal being considered. So an affordance cannot be measured as we measure in physics.

pg. 119-120

Note that by 'physics' and 'physical' he is just contrasting an ecological approach to that of the science of physics. This is one potential ambiguity made very clear by the broader context of the earlier chapters.

Affordances are therefore higher-order things (walk-on-able is composed of lower order things like being horizontal, flat, extends and rigid) but they are experienced as the higher-order thing, not as the sum of the lower-order things (they 'have a unity'). This is analogous to information variables as higher-order invariants in transforming arrays. Affordances are also relative to the observer; they and the organism are complementary. But environments and therefore affordances do not depend on the presence or even existence of an animal for their existence; they have to pre-date the organism, because they are why the organism evolved the way that it did (Gibson connects affordances to the concept of a niche). This means they are also not phenomenal or mental things in a private world of an organism. They are also neither objective properties nor subjective properties; they are a different kind of thing than normally features in theories of perception. 

Humans change our environments (many animals do, but of course we do it the most and to the greatest degree). What we are doing is altering the layout of affordances; promoting the good and reducing the bad (for us). Because affordances are organism-relative things, this means that we often alter the layout of affordances to the detriment of other organisms who don't share our needs. 

Gibson then lists and discusses affordances of the terrestrial environment. The medium (air) affords breathing and locomotion through. Substances can afford nutrition, while some are toxic, and some can be manufactured to make objects. Surfaces afford locomotion on, or (if it's a wall) afford no locomotion. Then there are of course objects, and these afford a dizzying array of behaviours; throwing, cutting, striking, trace-making and more. They afford what they do because of what they are made of, but the affordance is what is perceived, not what is figured out based on perceiving those lower-order properties. (This connects to the sidebar, 'To perceive an affordance is not to classify an object'; you don't need to know something is a chair to be able to perceive what it affords. Turvey et al (1981) talk about this point in their section on the marsh periwinkle). Then people and animals afford even more complex social and cultural behaviours, while the places of the environment afford hiding, predation, etc. Note that social behaviours entail perceiving affordances for others. 

A key to note is that affordances all have meaning to the organism, and that meaning can be positive or negative. As usual, the boundaries are blurry; for example fire affords both warming and burning. This also connects to the 'classify' sidebar. 

Gibson then spends some time talking about some developments that fed into but contrast with the concept of affordances. He talks about the Gestalt psychologists, and their notion of how things have invitation character and valance. They were on the right track; they correctly rejected the Cartesian assumptions of all other theories. But they never quite figured out an alternative, and preserved a dualism of the real vs the phenomenal objects, and it was the latter object of experience that did all the psychological work of inviting and having meaning. Gibson rejects any such dualism; things 'seem to be perceived directly because they are perceived directly' (pg 131). 

Finally, Gibson talks about the optical information for affordances, because 'The central question for the theory of affordances is not whether they exist and are real but whether information is available in ambient light for perceiving them.' (pg. 132). They are only psychologically interesting if they can be perceived as such; otherwise they are just a nice idea. He has no specifics here; but he notes that affordances are higher-order properties composed of lower-order properties, some of which themselves my be higher-order than their constituent parts. The information for an affordance may therefore need to be a compound invariant; an invariant composed of other invariants. But, like affordances and like invariants to begin with, these are not different in kind, they are simply higher-order, and will be experienced as a piece. He finally discusses the misperception of affordances; a clear glass door that lies about being an aperture, for example. Things in the natural world typically look the way they are; but there can be missing information and that can throw us off. 

There is a lot in this chapter, but also it's vague enough to explain why we still argue about what affordances are. This is ok; this chapter isn't the end of the theory of affordances, it's the beginning. But Gibson still sets out some key features that affordances must have; they must exist independent of the organism, and there must be information for them.