Systems thinking and practice
Systems thinking and practice

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Systems thinking and practice

5.1 Definition of a system

At first sight, a computer system and the body's respiratory system don't seem to have much in common, nor do the world financial system and an ecological system. On the other hand, each of them is called a system, so they must have something in common.

When I am not sure what something means I find it helpful to look for opposites: so what isn't a system? Just looking around I can see a brick, a book, a shelf, a packet of mints and a ticket for a concert. None of them seems to be a system. But the shelf with books on (and the brick to stop them falling off the edge) does look like a system, and I suppose the ticket could be thought of as part of a system of organising concerts.

One difference seems to be something to do with connections. The shelves, the brackets holding them up, the screws holding the brackets up, the books and the brick are all connected: but not simply physically connected. I wouldn't describe the same set of things lying on the floor together, before I put the shelves up, as a system. A second difference is that once they are up they have been put together for purpose. The connections between them have been planned and organised.

The activities which have to happen to put on the particular concert – the hiring of the hall, the rehearsals, the ticket selling – are connected too. Although the connections are not of the same kind as those between the screws and brackets, they have been put together for a purpose; it makes sense to talk of a system for putting on the concert. So my first attempt at a definition is that a system is set of things interconnected for a purpose.

This definition needs a little elaboration. First, the ‘things’ may be physical objects – like the shelf, books and brick – or they may be activities like those needed to put on the concert. They may even be ideas, such as those which make up a system of thought. It is helpful to have a generic word which will cover all these possible ‘things’, and because that word suggests only physical objects, I'm going to use the word ‘components’ instead; so I want to redefine a system as set of components interconnected for a purpose.

Next, I want to look at the idea of ‘purpose’ in the definition. On the one hand, it is natural to use the word system only when a set of components seems to have some purpose that we have ascribed to it – some aim or goal. So, the purpose of a car braking system is to enable us to stop the car, and the purpose of the respiratory system is to enable our bodies to take in oxygen. On the other hand, it may occur to you that there are some things called systems in common speech which don't seem to have a purpose; most people would be lost for words if you asked them to describe the purpose of the solar system. In that case why not just drop this idea of purpose from the definition?

There is a good reason. When you are confronted with a set of components and you want to find ways of working with them, or making them work better, it is always useful to look at them as if they had a purpose. In other words: the interconnected set of components – the system – has been identified by someone as being of particular interest. An urban transport system may have grown up over the past fifty or more years, without any overall purpose; but if you want to replan or redesign it, it will always be helpful to look at it as if it had the purpose of enabling people to move easily around the city. It is possible, to take a different example, to look at the chatter of office gossips as if it had the purpose of spreading information quickly around the organisation. You would then be describing it as an (unofficial) information system, and that might tell you quite a lot about the ways in which the official information system was supplemented or bypassed. In other words, for practical purposes, this pack is only going to be concerned with systems where those sets of interconnected components – whether ideas, objects or activities – can sensibly be described as if they had a purpose because we have an interest in them.

We can now elaborate on our definition of system of interest to include other aspects, namely:

  1. A system is an assembly of components connected together in an organised way.

  2. The components are affected by being in the system and the behaviour of the system is changed if they leave it.

  3. This organised assembly of components does something.

  4. This assembly as a whole has been identified by someone who is interested in it.

This is not the only definition used by systems thinkers. Two other definitions that come from noted writers on systems are:

  1. Ackoff:

    A system is a set of two or more elements that satisfies the following three conditions:

    1. The behaviour of each element has an effect on the behaviour of the whole. […]

    2. The behaviour of the elements and their effects on the whole are interdependent. […]

    3. However subgroups of the elements are formed, each has an effect on the behaviour of the whole and none has an independent effect on it. […]

    A system, therefore, is a whole that cannot be divided into independent parts. […]

    The essential properties of a system taken as a whole derive from the interaction of its parts, not their actions taken separately.

    (Ackoff, 1981, pp. 64–5)

  2. Checkland:

    The central concept ‘system’ embodies the idea of a set of elements connected together which form a whole, this showing properties which are properties of the whole, rather than properties of its component parts.

    (Checkland, 1982, p. 3)


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