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 chapterhere.
The planet consists of earth, water, and air; a solid, a liquid, and a gas. When these things come into contact, they form a surface. Solid earth provides the literal ground for perception and action, and those unfold in a medium - either a liquid or a gas that is forming a surface with the ground.The Medium
Life does not occur in abstract space. At the ecological scale, events unfold within a medium. This is either a gas (e.g. the atmosphere) or a liquid (e.g. water), because solid things do not allow events to unfold through them while remaining intact. There are six characteristics of a (gas or liquid) medium. First, they can be a medium for locomotion - a detached solid object can move through it without significant resistance. The details of the medium have consequences (e.g. fish in water need to be streamlined, animals in air do not). Second, the medium is typically transparent, allowing light to be transmitted through it. The medium affords vision. At this point Gibson introduces his notion of what light in an environment is like. It is not images, or rays, or any of the other traditional starting points for vision. Instead, light reverberates through a medium, bouncing back and forth between surfaces very fast and quickly reaching a steady state. That steady state illuminates a medium, and it fills it (the light is ambient to every point in the medium). Ambient light is not the same as radiant light (the light emitted from a source). Third, the medium can transmit vibrations and pressure waves caused by a mechanical event. The medium affords hearing. Fourth, the medium allows volatile chemicals to diffuse widely. The medium affords smelling. Fifth, mediums on earth reliably contain oxygen, and so animals have evolved to breathe via lungs or gills. They have a stable and wide ranging composition - there are no sharp boundaries in a medium (no surfaces yet). This matters for breathing but also vision, hearing, and smelling. Sixth, on Earth, mediums have an intrinsic frame of reference thanks to gravity, which is ever-present and creates the difference between 'up' vs 'down'. The other axes of our three dimensions are also constrained by things like the horizon. The medium has structure, unlike a physical space. So animals can locomote through a medium, and this medium is filled with light, sound, and smells. Every point in the medium is a potential point of observation, surrounded by an ambient array in the medium, and an animal moves along a continuous trajectory of these points as it locomotes. At every point, the array is different (again why it's wrong to talk about space, where all the points are equivalent).Substances
Here Gibson is talking about more solid things - things that do not permit the passage of light or the locomotion of animals. Substances persist but also change, depending on what they are made of and what happens to them. how that impacts the animal depends on what this does to the environment. Substances have a chemical composition, but for the purposes of the environment we are more concerned with properties such as rigidity, viscosity, etc; as with physics, chemistry is not the 'more true' description, and it is in fact the wrong level for the environment. Water is a tricky one; it can act as both substance and medium. It depends on the animal (and this emphasises the fact that animal and environment mutually define each other). Gibson will focus on terrestrial animals such as humans, and so for us water is a substance and not medium (mostly - a theme here is that there is an ebb and flow to these category boundaries that reflects the details of the organism, environment, and the activity at hand).Surfaces
The medium is separated from substances by a surface, and surfaces are where 'most of the action is' (pg 19). Surfaces are where radiant light becomes ambient light, for example; how light is structured in the medium depends on the substance it hits at the surface. Things touch at surfaces, creating the waves that we hear. Surfaces are where the volatile chemicals that we smell come from. Surfaces in an environment are arranged in a layout, which persists to varying degrees and offer some and not other actions. Gibson proposes some ecological laws of surfaces and discusses what they mean for the environment:
Ecological laws of surfaces (pg 19)
Surface layout defines the local environment. Surfaces persist to the extent they can resist deformation, disintegration, etc, and the degree to which they persist affects what they offer with respect to locomotion and other actions. Surfaces have texture (perfectly smooth surfaces are the weird limiting case, not the basic case) and the texture is fine or coarse depending on how the units of the surface are arranged and nested (and from Chapter 1, these units will be relative to a surface and animal). Texture specifies the substance, an important point we will return to in Chapter 5. Surfaces have shape, which is how they are laid out at a larger scale than texture but smaller scaled that the environment. Surfaces are illuminated, which varies over time (as, say, the sun changes location relative to the surface). Environments typically do not involve the complete absence of light. Surfaces then vary in how they interact with that illumination; they can absorb a lot or little of it, and they will reflect it in characteristic ways that have to do with the substance the surface is made of. Gibson then replaces the standard primary qualities of the world (things like form, size, position etc) with the modes or qualities of substantial surfaces (things like more or less illuminated, more or less smooth or rough, more or less hard or soft, etc). Perception will be interested in these features of the environment, not the primary qualities of the world of physics.
