The environment is the context in which the system we are interested in is found. Strictly speaking, it is whatever is not included in our definition of the system. In developing a systems perspective we describe a system in relationship to its environment, and how changes in the environment affect the system.
When describing a system in the context of its environment we speak about the "response" and the "behavior" of the system. The response of a system is how it changes when the environment changes in a particular way. The more general description of a system in the context of diverse conditions is its "behavior".
The concepts of adaptation and evolution are intimately related to the way a system, or a collection of similar systems, respond and change in the context of their environment. Specifically, the concepts of adaptation and evolution refer to changes in a system that affect how it responds to repeated instances of similar or related conditions. These concepts have a biological or social flavor. However, the response of a physical system can also change as a result of environmental forces.
Most of the properties that are of interest in the study of complex systems arise from the flows of energy, matter or information through a system. (This statement can be made more exact by considering that systems that do not have flows through them tend to equilibrium according to the second law of thermodynamics). These flows involve the system, but are also properties of the environment.
The role of the environment is also important when considering the basic idea of observation. Observing a system typically involves the response of a system to its environment. Even when we describe a system's shape or color, we make use of how the system responds to light.
Since the systems we usually consider are a small part of the universe, the environment refers to most of the universe. Completely describing the state of the environment is, in principle, a much more difficult task than describing the state of the system. However, it is generally possible to restrict the aspects of the environment that are described to those forces which are most directly relevant to the system. Part of the process of describing (or modeling) a system's behavior involves identifying the aspects of the environment which are sufficiently important to consider.
In general, when we think about a model of a system, the properties of the environment are treated in a simpler way than the system itself. The environment may even be treated as featureless or unchanging. Alternatively, it may be treated as random, which means its properties have no relationship to the properties of the system itself. Such simplifications are part of the traditional reductionist approach which ignores the relationships or interdependencies of the system with the environment. Realizing that these simplifications are not always applicable doesn't mean they are not useful, it just means that we have to study when they can and cannot be applied, and know how to improve upon them when they cannot.
One useful example is how we apply responsibility or blame for a person's behavior. This is relevant whether we are talking about attitudes toward a successful individual or to a criminal. Some people idolize a successful individual while others look to family or social context for the reasons of success. Some people would assign criminal responsibility to an individual person, and advocate punishment or therapy, while others would assign the responsibility to the environment, and advocate changing societal context or conditions. The more difficult problem of recognizing the interplay of system and environment is part of a systems approach.
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