We will use the term lifeworld to mean an environment described in terms of the customary ways of structuring the activities that take place within it -- the conventional uses of tools and materials, the ``loop invariants'' that are maintained within it by conventional activities, and so on. The term originally comes from phenomenological sociology [36], where it refers to the familiar world of everyday life, and specifically to that world as described in the terms that make a difference for a given way of life. Cats and people, for example, can be understood as inhabiting the same physical environment but different lifeworlds. Kitchen cupboards, window sills, and the spaces underneath chairs have different significances for cats and people, as do balls of yarn, upholstery, television sets, and other cats. Similarly, a kitchen affords a different kind of lifeworld to a chef than to a mechanic, though clearly these two lifeworlds may overlap in some ways as well. A lifeworld, then, is not just a physical environment, but the patterned ways in which a physical environment is functionally meaningful within some activity.
This idea is similar to Gibson's theory of perception [14], but the two theories also differ in important ways. Whereas Gibson believes that the perception of worldly affordances is direct, we believe that the perceptual process can be explained in causal terms. Also, whereas Gibson treated the categories of perception as essentially biological and innate, we regard them as cultural and emergent.
In analyzing a lifeworld, one attempts to draw out the individual structures within it that facilitate its customary activities. For example, lifeworlds typically contain artifacts such as tools that have been specifically evolved to support those activities. These tools are also arranged in the world in ways that simplify life and reduce the cognitive burden on individuals: cups are typically found in cupboards, food in refrigerators and grocery stores. No one needs to remember where butter is found in a specific grocery store because butter in all grocery stores is found in a well-defined dairy section, usually along a wall, which can be recognized from a distance; once the dairy section is in view, the butter will be visible in a definite area. Artifacts are also designed to make their functional properties perceptually obvious. Handles are perceptibly suited for picking up, knobs are perceptibly suited for turning, forks are perceptibly suited for impaling things, and so on [9, 43]. Contrarily, it can generally be assumed that artifacts that provide no readily perceptible grounds for drawing functional distinctions are in fact interchangeable. Usually, when some functionally significant property of an object is not obvious, the lifeworld provides some alternate way of marking it. If you see a record player in my house, for example, then you will assume that it is mine unless you have some specific reason not to. These aspects of lifeworlds tend to make it easy to perform particular kinds of activities within them without having to remember too many facts or reinvent the screwdriver from first principles.
Lifeworlds contain networks of interacting conventions and practices that simplify specific aspects of specific activities. The practices relieve agents of the burden of solving certain problems on the spot and diffuse their solutions throughout the activity of the agent or of many agents. For example, a hospital might try to get along without maintaining sterile conditions. People always have germs, so technically they are always infected. The problem is making sure that those infections never get out of control. The most direct solution would be constantly to monitor patients, assess their degree of infection and treat them when it becomes severe. Since this is undesirable for any number of reasons, a hospital instead tries to prevent infections in patients by maintaining sterile conditions. They might do this, for example, by looking for contaminated objects and surfaces and disinfecting them. Unfortunately, sterility is not a visible surface characteristic. Instead, hospitals solve the problem by structuring space and activity. Different locations are kept more or less sterile depending on their conventional uses: operating rooms are more sterile than hallway floors. Objects that can generate germs (people) are washed, masked, and gloved. Critical instruments that come in contact with them are specially sterilized after use. Tongue depressors are assumed to be dirty when they are in the trash can (or biohazard bag) and clean when they are wrapped in paper. All objects and surfaces are periodically disinfected regardless of their level of contamination. These practices are maintained regardless of the immediate need for them. If a hospital were (for some reason) to temporarily find itself without any patients, its workers would not stop washing their hands or disinfecting the bathrooms.