Each process is associated with two types of media, inside and outside the system, respectively. There is an input/output device that convert an event from one into the other.
When describing the interactions, we usually separate them into separate modalities or channels, and in each of them, into input and output processes.
When the system is described as an information system, the I/O process is described abstractly, as how state-changes in the system and in the environment are related to each other. Such an abstraction is possible, because as far as the relation between an information system and its environment is under consideration, the same effect usually can be achieved by multiple concrete interactions. Therefore, the abstract description filters out the irrelevant details, and focuses on the invariants in the process, which is usually called "information".
Therefore, for an information system, its environment is nothing but certain recognizable states, with certain recognizable relations among them, while what are recognizable are determined by the nature of I/O devices of the system.
An input action can either be triggered by an internal goal, or by an external signal, or "stimulus". The consequence of the action is knowledge of the system, corresponding to the state or state change in the environment. Roughly speaking, the recognition of the signal is what called "sensation", while the forming of knowledge corresponding to the relations among the signals is "perception".
An output action can also be triggered internally or externally. The consequence of the action is certain state changes inside (in the system) or outside (in the environment), as a spontaneous motion or response to a stimulus, respectively.
When an input action and an output action are considered together, we see a "stimulus-response" cycle or an "motion-feedback" cycle.
The streams of input actions and output actions in a period of time form the system's experience and behavior, respectively.
As mentioned in the previous section, the knowledge the system obtains in experience is not like "There is object X with property Y", but that "If I do X, I will observe Y". In principle, what knowledge the system can get, and how it is understood and used by the system, is largely determined by the system's "body", especially the input/output devices. On this point, this theory agrees with the view point that cognition is "embodied", as argued by George Lakoff and Mark Johnson (1999). However, every information system, as defined in this book, already has a "body", which is the underlying host materials and processes. Whether the system is "embodied" or not, therefore, depends on whether the body plays a role in the system's behavior.
In each basic step of communication, one of the two system, the "sender", takes an output action, while the other, the "receiver", takes an input action. What makes this process different from ordinary interaction is that the "information" received not only depend on the concrete physical (chemical, ...) signal, but also the convention between the systems, on the information carried by the signal.
A "symbol" is an informative signal. The correspondence between signals and their information contents is more or less arbitrary, in the sense that it is not fully determined by the intrinsic properties of the signal (though may be related to it), but by the conventions of the system, formed in their communication history.
Symbols form a "second signal system" for information systems, by which they can communicate to inform or perceive the internal state of each other, share experience, coordinate behaviors, etc. Even symbols are still carried by signals, the details are irrelevant, and the systems only respond to high-level patterns in the signals.
A "language", in a broad sense, is a system of symbols, as well as the conventions on how the symbols should be used in communication. A "language community" is a group of systems that "use a language", that is, communicate with one another with similar symbols under similar convention about their information content.
Overall, communication is a goal-oriented activity for both systems. Via communication, a system attempts to achieve certain goals, with the cooperation of the other system(s). During the communication process, the systems involved can switch between the sender/receiver roles.