the lactose as such has no affordance for me; it is simply one of the things that nourishes me when I ingest milk. This confusion of levels has haunted much of comparative psychology. (See Box 2.)
The fundamental hypothesis of ecological psychology and of this book is that affordances and only the relative availability (or nonavailability) of affordances create selection pressure on the behavior of individual organisms; hence, behavior is regulated with respect to the affordances of the environment for a given animal. This hypothesis has many important implications. One of the most profound is that behavior (in the most general sense, including perception and cognition) is not caused. Affordances are opportunities for action, not causes or stimuli; they can be used and they can motivate an organism to act, but they do not and cannot cause even the behavior that utilizes them.
To say that behavior is not caused has an unscientific ring--especially within psychology, where a positivist intellectual culture still thrives. Yet "the psychological" is, precisely, the uncaused actions and awareness of animals as they encounter their world. An animal's actions and awareness have a rich causal substrate, not just in the animal's nervous system but in the environment surrounding the animal; however, none of these causal factors, either individually or collectively, completely causes psychological states.
Affordances and Resources
The resources encountered by an animal are the affordances of the environment. But no|
animal can encounter all the resources of its environment. An animal encounters other
animals, some plants, and many objects, events, and places, but these are not the entirety
of its environment. An animal that encounters a piece of fruit does not thereby encounter
the fructose or carbohydrates contained in the fruit, even though it ingests them. Although
frugivorous animals appear to develop a taste for combinations of sugars and carbohy-
drates, and maybe even for particular kinds of sugars and carbohydrates, this is still not
quite the same thing as encountering those molecular entities as such. All terrestrial ani-
mals need oxygen, but few have encountered oxygen as such.
The ability to encounter an affordance requires a perceptual system attuned to the use|
of information enabling that affordance to regulate action. Interestingly, there are micro-
organisms that use oxygen concentrations to guide their locomotion, but this is unknown
among the dominant phyla of terrestrial animals (arthropods and vertebrates). Resources--
or, to be more precise, special combinations of resources commonly found in certain habi-
tats--become affordances when natural selection works to evolve such specific regulatory
activity. And it is always an empirical question as to what affordance an animal is aware of
or acting on--whether one considers burrowing worms or thinking humans.