In the previous chapter, Gibson established the importance of the environment to perception. In this chapter, he goes into more detail about the properties of the environment that are relevant to perceivers. Gibson argues that although classical physics talks about the world in terms of bodies in space, this doesn't mean that this is a good way to talk about the environment as it applies to perception. In studying perception, it’s better to think about our environment in terms of mediums, substances, and surfaces. Two things occurred to me as I read this chapter. The first was, “Wow, I’ve never before heard the environment described in this way.” The second was, “Wow, this is so unbelievably straightforward and comprehensive, why don’t all psychologists talk about the environment in this way?”
A surface occurs wherever two states of matter (e.g. gas & solid) meet. The majority of relevant surfaces to humans occur when gaseous air meets solid ground. Animals can move through gases and liquids, so these afford locomotion. They are mediums for locomotion. Light can move through gases and liquids, but light bounces off solids. Mediums have to both transmit and reverberate light. Reverberation causes illumination, which fills a medium with ambient light. Sound, as well as light, can move through gases and liquids. This allows us to hear sounds (be sensitive to vibrations). Volatile or soluble molecules quickly diffuse through gases and liquids, allowing us to smell (to detect a source at a distance).
“Animal locomotion is not usually aimless but is guided or controlled – by light if the animal can see, by sound if the animal can hear, by odour if the animal can smell...The medium thus contains information about things that reflect light, vibrate, or are volatile. By detecting this information, the animal guides and controls locomotion.” p. 17
As an animal moves around, the sights, sounds, and smells change. Each point of observation is unique.
Mediums are relatively homogeneous. The composition of air has been pretty constant for millennia, and the proportion of oxygen in it is pretty constant around the world. Water has somewhat more variability (e.g., saltwater differs from fresh water), but the transitions between types of water is fairly gradual. This homogeneity means that animals can breathe. It also means that waves emanating from a source do so with regularity. Mediums have absolute vertical reference points. Gravity pulls down. Sunlight comes from above. These properties (permitting breathing, locomotion, illumination, detection of vibrations, and detection of diffusing molecules) are invariant. Gibson refers to these opportunities as affordances.
Substances do not transmit light or odour, and don’t afford locomotion. Substances are usually rigid. Compared to mediums, they are heterogeneous. Substances are usually aggregates of different chemical compounds. Animals have to be able to distinguish between substances. For instance, animals have to be able to detect whether something is edible or inedible. It’s preferable to be able to detect this at a distance (e.g., by using vision).
Substances differ in terms of physical properties (e.g., hardness, viscosity), but this is not necessarily the level at which animals distinguish between substances. Substances also differ in terms of the degree to which they reflect light. Many substances, for all intents and purposes, do not change. This means that substances in the environment are often dependable entities. For terrestrial animals, water is a substance. For aquatic animals, water is a medium.
If a substance persists, then the surface interfacing it with the medium will persist. Ambient light makes surfaces potentially visible. All the cool stuff happens on surfaces. Light is absorbed, touch occurs, diffusion occurs. P. 23 lays out the “ecological laws of surfaces”:
1) Persistent substances have surfaces, which have layouts
2) Surfaces resist deformation. Substances that deform easily might not permit locomotion to heavy animals like us. We can move some substances, but not others, out of the way.
3) Surfaces resist disintegration. We can smush clay, but we have to cut wood.
4) Surfaces have both layout textures and pigment textures. Surface texture influences optical texture.
5) Surfaces have shapes
6) Surfaces may be in light or in shade
7) Surfaces may absorb a little or a lot of light
8) Surfaces may reflect a little or a lot of light
9) Surfaces have particular reflectance ratios of different wavelengths of light (i.e., different colours)
The level at which Gibson characterises the environment is very different from the level at which it is described by physics. Instead of talking about bodies in space, Gibson talks about mediums, through which animals can move, subtances, which do not afford locomotion and which are relatively stable, and surfaces, which permit detection of structure in light, vibrations, or volatile molecules. This characterisation describes the world with respect to perceivers. Important aspects of the environment depend on whether we (or some other animal) can detect them. For instance, ultraviolet and infrared are possible colours, as defined by the world of physics, but these are not relevant properties of the environment for humans, who are unable to see light with wavelengths outside a narrow band. Like I said, it's straightforward and comprehensive.