A system is basically a perspective that looks at elements and their relationships under specific conditions. This perspective enables us to understand how something will affect a system under varying conditions.
Generally, a system can be divided into just about anything, e.g., objects, processes, functions, flows, constants, or variables . This explosion of a system into elements is called analysis. The need to put all the elements together to understand their interactions is known as synthesis.
A systems perspective requires both analysis and synthesis to truly understand how something works. Too often, the emphasis has been on analysis, not synthesis, reducing appreciation of the importance of seeing elements as part of an overall system. Russell Ackoff, one of the early proponents of systems theory, says that a system has two or more parts that cannot, ironically, be divided into independent parts . Otherwise, such a division will affect overall performance. 
Capra agrees, particularly in regards to systems that involve living organisms, and observes that systems theory acknowledges the interrelationship and interdependence of phenomena. This holistic view means that an integrated whole cannot be exploded into parts without having some negative impact, because a system can consist of subsystems. 
There is, of course, tremendous pressure to emphasize the analytical side, due largely to our educational and scientific heritage, and this has caused us considerable trouble and angst, despite the accompanying benefits. Yet, as Peter Senge notes, people learn to "fragment" problems and, indeed, the world. While this fragmentation may add simplicity, it can have serious impacts, e.g., not seeing the consequences of behavior. In an effort to increase understanding, great effort is made to reassemble the parts into some framework, but the effort proves impractical because something is lost in the reconstruction. 
Systems in general and systems thinking in particular, therefore, require a holistic perspective that looks beyond the mere assembly of parts. Only by thinking holistically can people determine the impact that a decision or action will have on an entire system.
How a system is constructed , however, is not objective. It is a very subjective effort even though it helps us to understand what is happening within and to a particular system.
The reason is that people cannot extricate themselves from their environment and its influences. They are continuously tied emotionally, psychologically, and physically in terms of time, space, and milieu. Hence, they view phenomena in a way that reflects the prevailing paradigm, mental model, or perceptual model to which they subscribe. The patterns that exist in our minds surface in the patterns around us and reflect our feelings and values.
 Russell L. Ackoff, Systems thinking and thinking systems, Systems Dynamic Review , p. 175, Summer-Fall 1994.
 Fritjof Capra, The Turning Point , Bantam Books, Toronto, 1988, p. 43.
 Peter M. Senge, The Fifth Discipline , Currency Doubleday, New York, 1990, p. 68.