A system, always of parts and relations, indicates itself in whole.
Walt Whitman, Leaves of Grass: “Time, always without break, indicates itself in parts.”
- A system is a combination of a set of parts, a set of relations among the parts, and a set of relations between the whole of the parts and its surroundings (that which is not part of the particular system). A system and its surroundings form a domain (universe, world). The mind decides what parts and relations make a particular system. Outside the (individual or collective) mind, there is nothing about a set of parts and relations that objectively says “this is a system” or “this is not a system” – systems are creatures of the mind.* A set of parts and relations does not necessarily have to be thought of as a system.
- Parts may be material or immaterial things, including processes, pieces of information, definitions, living organisms, and so on. Relations may be in the form of flows (matter, energy, information, control, ideas, etc.), logic (associations, dependencies, contingencies, etc.), or a combination thereof. System and surroundings may in time affect (change) parts and relations.
- The system concept is useful for two reasons: (a) a system may be complete enough with respect to the topic of interest and still be less complex than the domain as a whole; (b) the concern about a system’s relations with its surroundings can be temporarily separated from the concern about the relations among the system’s parts.
- Systems and parts have properties (predicates, attributes). As qualia, properties are creatures of the mind. Observation and measurement reduce properties to qualities and quantities. There is no limit on the kinds of properties. Static structure and dynamic behavior are two properties. However, even these two ‘ordinary’ properties are not always germane: for example, the decimal number system (a set of rules for expressing numbers and performing arithmetic) may be viewed as having no structure (there is no order among the rules) and no behavior by itself (the rules may be used but as a whole they do not ‘behave’.)
- A part may be thought of as a system of subsidiary parts and associated relations. The mind decides whether and how to reduce a primary part into a system of subsidiary parts and associated relations. Thinking about a (primary) part as a system of (subsidiary) parts and relations may help in analyzing and synthesizing the properties of the primary part.
- Even with systems as creatures of the mind**, we speak of a physical (material, concrete) system if the creature represents a thing in the physical world. An abstract system represents a thing in the metaphysical world. A mixed system represents a thing in the physical world, plus a thing in the meta-physical world. For example, the International System of Units is a mixed system as it includes the standard kilogram kept in Sèvres, France, in addition to a set of (abstract) definitions.
- We speak of an artificial system if the thing it represents comes into existence through at least some human agency; a natural system represents thing that comes (or has come) into existence without any human agency. Per Simon 1996, an artificial system runs as a thin layer through the natural world. The dividing line between artificial systems and natural systems is not always clear.
- An artificial system may be thought of as existing on two planes: on the plane of design as a system design (“design” in the sense of an outcome, not in the sense of an activity); and on the plane of realizations as one or more realized (instantiated, implemented) systems. In terms of the hierarchy of biological classification, a system design is of the species rank and a realized system is of the individuals rank.
- Drawing on Hevner Mills 1993, the plane of design may be thought of as having three sub-planes: the black-box plane, the state-machine plane, and the clear-box plane. In addition, the plane of design may be thought of as having a fourth sub-plane: the properties plane. On the properties plane, a system exists as a bundle (a box) of properties.
- An artificial system may include rules for the system design and system realization activities, including rules for the use, evolution and disposition of the system.
Hevner Mills 1993: A R Hevner, H D Mills, “Box-structured methods for systems development with objects”, IBM Systems Journal, vol. 32, no. 2, 1993.
Simon 1996: H A Simon, “The Sciences of the Artificial”, MIT Press, 1996
* “I am certain that I can have no knowledge of what is outside me except by means of the ideas I have within me.” René Descartes
** The things to which our words refer exist independently of our conceptions of them. This contradicts the postmodern contention that words and concepts are all we know. Just because we invent words (here, ‘system’) and their meanings, it does not mean we invent the world. (After John Horgan, Was Thomas Kuhn Evil? Scientific American, 2019-03-19)