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!

The Optical Information for Self-Perception (Gibson, 1979, Chapter 7)

So far we have considered what the world looks like to a point of observation that may or may not be occupied. It was important to Gibson that he establish that the optic array available at such a point is public, not private, and so can be shared across individuals (see Chapter 1 for the problem and 3 for the solution). In this chapter, Gibson asks what is seen by an individual when they occupy a point of observation, and in this case there is some privately available information that specifies things about the self. 

Events and the Information for Perceiving Events (Gibson, 1979, Chapter 6)

In the previous chapter, Gibson introduced the notion of the ambient optic array and discussed the kinds of information it can have (flow, and invariants). The focus there was what is revealed by a moving observer. This chapter is about what is revealed as the environment changes; when events occur. 

Sabrina blogged this chapter here and here. 

The Ambient Optic Array (Gibson, 1979, Chapter 5)

In the previous chapter, Gibson laid the foundations for an ecological optics by distinguishing between stimulation and stimulus information. In this chapter, he lays out the details of how to analyse the source of visual stimulus information, as an ambient optic array that contains both invariant and perspective (variant) structure. 

Sabrina blogged this chapter here, here, and here; there is a lot going on in this chapter!

The Relationship Between Stimulation and Stimulus Information (Gibson, 1979, Chapter 4)

The first three chapters describe the world to be perceived, at the ecological scale. This chapter opens the section on how we visually perceive that world - via information. Remember, a key part of the ecological analysis is doing things in this order (see the Introduction). 

The work of the next few chapters is to lay out a theory of ecological optics. Lots of science studies light, but as with the physical world/environment distinction, Gibson will insist on distinguishing between the physics of light and the ecology of it; only the latter will be relevant to a theory of perception. 

Sabrina also blogged this chapter here. 

The Meaningful Environment (Gibson, 1979, Chapter 3)

In Chapter 1, Gibson identified the animal and it's environment as the two mutually defining parts of an ecological system. In Chapter 2, he developed a vocabulary for describing an environment (as opposed to the physical world). Now, in Chapter 3, he will use that vocabulary to identify that the environment of an animal is meaningful, literally full of meaning, and this will mean that meaning is there to be discovered, rather than constructed (as in every theory of perception so far). 

Sabrina also blogged this chapter here. 

Medium, Substances, Surfaces (Gibson, 1979, Chapter 2)

In this chapter, Gibson continues to lay out the rules of the environment (vs the physical world). In physics, there are objects in spaces, but this simply does not work as the basis of perception (see the chapters about Helmholtz and the limits of unconscious inference in the Turvey book). Instead, environments are made of medium, substances, and surfaces, and this chapter defines these at the ecological scale.

This chapter introduces a lot of vocabulary for talking about the environment: I have reviewed some of it, but see the chapter for the full set because it is going to be needed as we described the environment to be perceived. 

Note: Sabrina also blogged this chapter here.