System

A system is a collection of interacting parts. The parts alone are not enough; the interactions between the parts are equally part of what the system is. Whatever can be observed as a coherent whole, with components that influence one another, is a system.

The basic difference between a set and a system is the interactions. A set is just the parts gathered together, with no interaction; a system includes the parts and their interactions. Take a bicycle. As a set, it is wheels, frame, chain, pedals, and seat, all in a pile. As a system, it is the same parts arranged so they work together. The arrangement, the interaction, is what makes one a bike and the other a heap.

A system has a boundary, a set of components inside that boundary, and a network of communication among the components. The components can be counted and put into categories. The interactions can be tracked as flows of matter, energy, or information. What we call the macroscopic approach to chemistry is, in the end, the systems approach: a focus on the materials and the interactions among them, rather than on the smallest units that make them up.

In the systems approach the whole is greater than the sum of its parts because of the relationships between those parts. A list of parts is a list. A description of the parts together with their interactions is a system. The list cannot tell you what the system does; only the system can. This is the heart of why chemistry, treated systemically, says more than chemistry treated as a parts list.