 Up until now, we have been talking primarily about the internal working of systems. But in this section, we will start to present models for understanding systems within the context of the broader environment that they operate in. The first thing we need to discuss is what is called the system's boundary. The system's boundary demarcates a limit to the system's internal components and processes. Internal to its boundary, the system has some degree of integrity, meaning the parts are working together and this integrity gives the system a degree of autonomy. But that is all quite abstract, so let's take some examples. If we take a tree, for example, every part of the tree has been designed in some way to function as a part of the entire system. The bark, the leaves and trunk all serve some function with respect to the whole and thus they are integrated and through this integration, they are able to function independently from other systems in their environment. Thus the leaves in a tree are dependent upon the tree's trunk and all other elements to that tree, but they are independent from the leaves and trunks of other trees. That is to say, the tree as an entirety has a degree of autonomy. A system's boundary is then demarcated by where the nexus of relations that enable it to function, as an integrated and autonomous whole, reach their limit. Beyond this, the system loses its autonomy and has to interact with other systems and its environment. The boundary to a nation-state is another example of this. The nation's border is only a boundary if within this boundary public functions are integrated within the national system as an entirety and by the nation functioning as a whole, it can be autonomous from other nations. If one region of this nation has a different culture from that of its parent nation, instead sharing its heritage with a neighboring country, this will reduce the internal integrity of the nation, its autonomy to act as an entirety and reduce the degree of definition to its boundary. These examples should hopefully help to illustrate that boundaries may have a physical dimension but can't always be defined in physical terms. If we want to be able to achieve sufficient generality to talk about all types of systems, which we should remember is the aim of systems theory, then we need to understand boundaries within this slightly more abstract language of integrity and autonomy. Within the language of systems theory, systems are said to be open or closed. Open systems interface and interact with their environment by receiving inputs and delivering outputs external to their boundary. These boundaries are permeable, meaning that they may permit the exchange of materials, energy, information or ideas. Conversely, closed systems are more prone to resist incorporating new inputs and this resistance at their boundary makes them more strongly defined by the static properties of the boundary. By not adopting inputs, a closed system ceasing to properly serve a function within its environment may become deemed unnecessary to its parent environment and risks atrophy. An isolated system is more restrictive than a closed system as it does not interact with its surroundings in any way. The universe as an entirety might be an example of an isolated system but it is debatable as to whether such a construct could exist in reality. So let's take a few quick examples of open and closed systems to try and cement the idea. A hospital is an example of an open system continuously taking in new patients and discharging others, receiving medical equipment and removing old, hiring new personnel whilst retiring others. This rate of input and output to open systems make them dynamic. They are constantly changing and have to respond to the changes within their environment. An example of a closed system might be a boat on the sea. It is specifically designed not to take in water from the oceanic environment it is part of. Another example might be a group of teenage friends in a public park engrossed within the internal cultural dynamics of their peer group. They are capable of receiving only a very limited input of impressions from their broader environment. Finally, we get to the systems environment. All systems have a boundary and operate within an environment. This environment represents the sum total of other systems and input-output resources that the system interacts with during its operation. Thus, the environment consists of the sum total of resources and systems that lie outside of the boundary of the system of interest and interact with it providing its inputs and consumes its outputs. From this we should note that a systems environment is primarily relative to its functioning. So, a biological system that requires the input and output of natural resources operates within the natural environment. A business or enterprise system that requires the input of economic resources operates within a given market environment. And the political system of a nation operates within the international political environment. We will wrap up here and continue our discussion in the next section where we will be talking about the relations between elements and systems.