This chapter defines information system, which will later serves as the superordinate concept of intelligent system. Some terminologies are introduced, which will be used in the whole book to discuss various types of information systems.
Section 1.1. Defining "information"
An information system is a system whose internal activities and interactions with its environment can be described abstractly in terms of state changes, without specifying the concrete entity and process that carries out the activities and interactions.
When a system is seen as an information system, its internal activity is referred to as information processing, which abstractly describes how the components of the system response to each other. Also, its interaction with its environment is referred to as information transferring.
Information is an abstract description of the state or state change in a system or its environment.
The basic difference between these definitions and the others is: "Information system" and the related concepts belong to a methodology, not to an ontology. The correct question is whether a concrete system can be seen as an information system, not whether it is an information system. Whether a system can be seen as an information system not only depends on the features of the system, but also depends on the purpose of the observer who use the "information system" methodology.
Therefore, when talking about information system, we always assume the existence of an observer with a specify purpose. It is not "objective description".
Section 1.2. What systems are "information systems"
In everyday language, "information system" is mostly used for two kinds of system.
Among naturally formed systems, human beings and some animals can be referred to as information systems. When doing this, what concerned us is how their behaviors change according to the change in the environment.
Among artifacts, typical information systems include computers and various control systems. Again, when they are described as information systems, the actual physical or chemical processes happened is omitted, and the process becomes abstract relations among abstract variables or statues.
What these systems have in common is that their behaviors are all goal-directed, rather than completely determined by the current environment. Intuitively speaking, a goal is a status a system want to keep or approach while the environment is consistently changing.
It is not by accident that "information system" and "goal-directed system" happen to indicate the same group of system. Often a goal is specified abstractly, and the system may have multiple ways to keep/achieve a goal by changing its internal status, or the environment, or both. To describe such a system as an information system can greatly reduce the complexity of the description without losing the essence of the process.
Section 1.3. Inside activity as "information processing"
Every information system can be described by specifying its three major components: goals, actions, and beliefs.
Since information system's behaviors are goal-directed. Therefore, to describe such a system, it is necessary to identify its goal. The second component is the system's actions to change its internal status and external environment, described abstractly, which is what the system can do to achieve its goals. Finally, there is the beliefs of the system that link specific actions to specific goals achievable by the action.
Goal serves as the internal regulator of a system. Information system's nature is: its behavior is determined both by its goals and its environment. If a system's behavior can be completely explained by its current interaction with its environment, usually it is useless to describe the system as an information system.
The action of a system is the set of operations that the system can perform either on its internal structure or on its external environment. Usually there are basic operations whose internal structure cannot be further analyzed within the current information system framework, and compound operations that are time-space combination of simpler operations.
Beliefs are the internal connections of an information system. Its primary function is to link the action of the system to its goal, therefore the system can use the former to achieve the latter. In the simplest form, a belief directly links a concrete goal to a concrete action that can be used to achieve the goal. In complicated situations, beliefs provide links among goals, actions, and other beliefs, therefore indirectly link goal to action. Beliefs also provide the bond between derived goals and original goals, and between simple actions and composed actions.
Here we do not require the system to be aware of its goals, actions, and beliefs --- they are is in the eye of the observer who describe the system as an information system.
Section 1.4. Outside activity as "information transferring"
When an system's interaction with its environment is described as "information transferring", it means for the current purpose of description, what matters is how the system changes its states as the consequence of state changes in the environment that are recognized by the system. In such a description, the actual underlying process that carries out the interaction is treated as completely irrelevant.
Therefore, for an information system, its environment is nothing but certain recognizable states, with certain recognizable relations among them. The system never knows the environment "as it is", and for different purposes the same environment can be described differently in terms of what states and relations are out there.
When a concrete interaction is described as "receiving a message", the amount of information in the message can be measured, usually in terms of the uncertainty it removes in the system on the state of the environment.
Section 1.5. Simulation vs. replication
To call a system "information system" means, as far as the current purpose of the observer is concerned, the system can be described at an abstract level, with many low-level details omitted from the description.
Even if a system cannot be treated as an information system, it often can be "modeled" or "simulated" in an information system. It means that the system can be described at an abstract level, and another system is built that have the same high-level description, though these two systems are completely different at a lower level, which contains essential features of the system to be modeled.
However, if they system being modeled is such a system that all of its major properties are shown at the "information-system" level, then we no longer call the above process "modeling" or "simulating", but call it "replicating", "reproducing", or "implementing".
Section 1.6. Capabilities and restrictions
At a given moment, each concrete information system has a fixed capability, in terms of the goals the system can achieve. This capability is represented by the existing beliefs, or knowledge, of the system that explicitly link the achievable goals to executable actions, so that the system can directly take the actions whenever the goal appears.
This actual capability should be distinguished from the system's potentual capability, defined by the space of possible states that can be arrived by all valid combinations of actions. Though in some systems some of these potential can become reality, at the moment it is still beyond the system's reach, due to the lack of necessary knowledge.
Another restricting factor is the system's resources supply. Since every concrete system at a given moment always has a constant information processing capacity, and every practical problem has a time requirement attached, a "solution" that demends more resources than the system can afford is not a valid solution, and therefore is not included in the system's capability.