A Structural Approach
According to Merriam Webster Structure is defined as:
- 1 : the action of building : construction
- 2a : something (such as a building) that is constructed
- b : something arranged in a definite pattern of organization – a rigid totalitarian structure— J. L. Hess leaves and other plant structures
- 3 : manner of construction : makeup – Gothic in structure
- 4a : the arrangement of particles or parts in a substance or body – soil structure – molecular structure
- b : organization of parts as dominated by the general character of the whole – economic structure – personality structure
- c : coherent form or organization – tried to give some structure to the children’s lives
- 5 : the aggregate of elements of an entity in their relationships to each other – the structure of a language
It is obvious that the noun refers primarily to an arrangement of interrelated parts, a set-up, configuration, framework etc. which consists of a number of individual elements and can be distinguished agains its environment as a whole. As such, the term is closely related to that of Architecture, which is mainly used to identify a particular class of similarly organized structures (e.g. a Gothic architecture).
At this point I would like to add few caveats I believe are important:
- The elements of the structure must be different from each other in shape and/or nature, otherwise the structure is just a heap;
- A structure must be stable enough to be observed and may be static or dynamic in nature.
- In either case the organization, state and behaviour of a structure is in any given moment completely predictable.
- A structure is also passive, that is, it is not able to pare or evade any particular influence from the environment.
The term Structure is very similar to that of System which is in the same dictionary defined as follows:
- 1 : a regularly interacting or interdependent group of items forming a unified whole – a number system : such as
- a(1) : a group of interacting bodies under the influence of related forces – a gravitational system
- a(2) : an assemblage of substances that is in or tends to equilibrium – a thermodynamic system
- b(1) : a group of body organs that together perform one or more vital functions – the digestive system
- b(2) : the body considered as a functional unit
- c : a group of related natural objects or forces – a river system
- d : a group of devices or artificial objects or an organization forming a network especially for distributing something or serving a common purpose – a telephone system – a heating system – a highway system – a computer system
- e : a major division of rocks usually larger than a series and including all formed during a period or era
- f : a form of social, economic, or political organization or practice – the capitalist system
- 2 : an organized set of doctrines, ideas, or principles usually intended to explain the arrangement or working of a systematic whole – the Newtonian system of mechanics
- 3a : an organized or established procedure – the touch system of typing
- b : a manner of classifying, symbolizing, or schematizing – a taxonomic system – the decimal system
- 4 : harmonious arrangement or pattern : order – bring system out of confusion— Ellen Glasgow
- 5 : an organized society or social situation regarded as stultifying or oppressive : establishment sense 2 —usually used with the
Now, it might become obvious when analyzing this list of definitions that most of them may be applied equally to the previously discussed term of Structure “an arrangement of interrelated elements that can also be identified as a whole“.
The only definitions for system that seems somewhat different from the definition of structure are those that include concepts such as function, being for instead of something or having a purpose, like b(1), b(2) and d.
The best way to see if something can be identified as a system seems to be to ask the question not “what is it made of?“, but rather “what is it for?“. If the answer is not immediately obvious, the thing is probably a structure. Terms like “software system” and “system of systems” do not actually help in identifying the system. The stuff the system is made of is important only when speaking about its structure(s). A system’s structure can be made of material, energic(al), or informational elements. When defining a system, what is important is the function (behaviour, purpose) of the whole and that of its components. The ambiguous term of “software system” could identify both a “software product” (a structure) used in an, for example, “air traffic control system“, or even a “software development system“.
In Kihbernetics we would identify a gravitational or planetary system as a mere dynamical structure, because there is no obvious input-output relationship between, let’s say, the Solar system and its environment (the surrounding void?) or between the elements that are part of such a “system“. The state of this dynamical structure in any given point in time is completely predictable and can be precisely calculated by using Newton’s laws of motion and universal gravitation.
I realize this discussion might look like “splitting hairs“, but after three decades of working in “Systems Engineering” I came to appreciate the difference between developing a “product” and a “system“. In too many occasions I saw the consequences of failing to understand the difference between the two, claiming that we are developing a system while in fact investing a disproportionate amount of attention and resources to the development of the product, and neglecting the other two necessary components of the, so called, “Socio-technical system (STS)“, people and process.
With an increased interest in “Systems Thinking“, various “Systems Approaches” and “Complex Systems Theory” it is important to agree on the meaning of fundamental terms such as system and structure. A “Kihbernetic approach“, primarily interested in non-linear “dynamical systems with memory” sometimes identified as “autonomous agents“, “autopoietic” or “living” systems may help with this.
An Operational Approach
As described in the section above one of the main differences between a structure and a system is that a structure does not have a clear input-output relationship with the environment like a system does.